<!doctype linuxdoc system>
<article>
<title> Serial HOWTO
<author>David S.Lawyer
  <tt><htmlurl url="mailto:dave@lafn.org" name="dave@lafn.org"></tt>
  original by Greg Hankins
<date> v2.27 February 2011 

<!--  Change log:
2.27 Feb. 2011: The serial port is still widely used in embedded
systems, etc., and is not really obsolete.  Better definition of
input, output.  Deleted devfs names such as tts.
2.26 Nov. 2010: Changed EIA-232 to RS-232.  PCI-e bus is serial.  No
serial port on new PCs since it's obsolete and this HOWTO is now
mainly of historical interest.
2.25 Jan. 2007 picocom.  devfs is obsolete. ser2net.  Revised parts on
drivers as modules vs. built into kernel.  Serial Programming
wikibook.
2.24 Feb. 2006: Serial Laplink HOWTO (connecting 2 PCs via the
serial line), Fixed typos found by Charles Brockman.  Messages with
"ttyS01" now show it as "ttyS1, etc".
v2.23 Links to Vern's stuff on my website, serlook, use mouse to find port
v2.23 Nov. 2004: typo fixed,  Quick Help added, Serial ports on
 motherboard likely ISA or LPC
v2.22 Dec. 2003: revised Complex Flow Control Example, more on devfs
v2.21 Nov. 2003: Kernel compile USB options for serial ports, revised setserial 
v2.20 Oct. 2003: MAKEDEV is often only in /sbin and not in /dev.
v2.19 Sept. 2003: linux-serial email now at kernel.org, new
section: Servers, pinout diagram
v2.18 May 2003: EIA-485 features not supported by Linux, Flow control
"typos" fixed
v2.17 Feb. 2003 url signum->cendio, Mac port names, clarity when
stopping data flow when printing, ide2 address conflict
v2.16 March 2002 fixed a few broken links.
v2.15 November 2001: mention of MIDI ports, problems with lockfiles 
 for devfs
v2.14 August 2001: major revision of Configuring the Serial Port
 Now it's: Locating ...
v2.13 August 2001: fixed typos: done->down and "is is", USRT chip,
 synchronous defined better
v2.12 July 2001 serial printing under LPRng
v2.11 May 2001: stty 0 => hangup (was ok in v2.08. )
v2.10 Feb 2001: EIA-485, frame errors on networks, gkermit, firewire
v2.09 October 2000: link to Serial Driver homepage, ttySD etc.
v2.08 June 2000: /proc/tty, fixed link to Gary's Encyclopedia.
v2.07 May 2000: locking methods, clarity re uart protocol, sticky parity
v2.06 2 March 2000:  more on multiport, not 3-3 for null modem,
 butter -> buffer,
v2.05 1 January 2000: Vern's & my url, ref to multiport modem cards
v2.04 28 Nov. 1999:  duplicate info removed from setserial
v2.03 Nov. 1999: ttyS minor is 4, not 5
v2.02 Oct. 1999: National Instruments card, removed m4_define TorS, lockfile
 error mesg
v2.01 Aug. 1999. more on HSSI, irq=0, serial module, clarity,
 Computone update, New copyright, interrupts mis-set symptoms,
 m4-include files (PnP stuff, stty, etc.)
v2.00 May 1999 holding reg. to shift reg.
 Added tech info.  Removed modem and terminal info and put into Modem-HOWTO
 and Text-Terminal-HOWTO.  Added info from Modem-HOWTO which is
 only about the serial port.
v1.12 July 1998: reissue of old doc (v1.11).  Added more info on Winmodems.
v1.11 15 November 1997 by Greg Hankins
(Prior to this, a number of versions are not listed and it would take
   some research to determine them)
v1.8.1: 9 Oct. 1995
v1.7: 29 Oct. 1994
v1.0: 6 Jan. 1994: first Serial-HOWTO by Greg Hankins
v1.0: June 1993: was called Serial FAQ, by Greg Hankins
-->
<abstract>
This document is for the UART serial port. This port has mostly
disappeared from desktops and laptops is still used elsewhere such as
for embedded systems.  It covers information other than that which
should be covered by Modem-HOWTO, PPP-HOWTO, Serial-Programming-HOWTO,
or Text-Terminal-HOWTO.  It lists info on multiport serial cards.  It
contains technical info about the serial port itself in more detail
than found in the above HOWTOs and should be best for troubleshooting
when the problem is the serial port itself.  If you are dealing with a
Modem, PPP (used for Internet access on a phone line), or a
Text-Terminal, those HOWTOs should be consulted first.  </abstract>

<toc>
<sect>Introduction

<p> This HOWTO covers basic info on the serial port which is missing
on newer personal computers but is being used in recent
embedded systems as well as for routers, point-of-sale equipment, etc.
It includes multiport serial cards.  It was written when the serial
port was a major port for connecting a PC to modems and printers, etc.
and the style of this article reflects this.  If you're having
problems with the serial port, want to understand how it works, or
need a detailed introduction to it before studying serial programming,
this is one place to find out about it.

This HOWTO is about the original serial port which uses a UART
chip and is sometimes called a "UART serial port" to differentiate it
from the newer types of serial devices: Universal Serial Bus or
Firewire.  It's <ref id="slow_" name="slow">
compared to newer serial devices but can send text several times
faster than you can read it.  Information specific to devices which
use serial ports: analog modems, text-terminals, infrared devices, and
a few printers are found in Modem-HOWTO, Text-Terminal-HOWTO,
Infrared-HOWTO, and Printing-HOWTO.  Info on getty (the program that
runs the login process or the like) has been also moved to other
related HOWTOs since mgetty and uugetty are best for modems while
agetty is best for text-terminals.  If you are dealing with a modem,
text terminal, infrared device, or printer, then you may not need to
consult this HOWTO.  But if you are using the serial port for some
other device, using a multiport serial card, trouble-shooting the
serial port itself, or want to understand more technical details of
the serial port, then you may want to use this HOWTO as well as some
of the other HOWTOs.  (See <ref id="related_howtos" name="Related
HOWTO's">)  This HOWTO lists info on various multiport serial cards.
This HOWTO addresses Linux running on PCs (ISA and/or PCI buses),
although it might be valid for other architectures.

<sect1> Copyright, Disclaimer, & Credits
<sect2>Copyright
<p>
Copyright (c) 1993-1997 by Greg Hankins, (c) 1998-2005 by David S.
Lawyer <url url="mailto:dave@lafn.org">
<!-- license.D begin -->

Please freely copy and distribute (sell or give away) this document
in any format.  Send any corrections and comments to the document
maintainer.  You may create a derivative work and distribute it
provided that you:

<enum>
<item> If it's not a translation: Email a copy of your derivative work
 (in a format LDP accepts) to the author(s) and maintainer (could be
 the same person).  If you don't get a response then email the LDP
 (Linux Documentation Project): submit@en.tldp.org.
<item>License the derivative work in the spirit of this license or use
  GPL.  Include a copyright notice and at least a pointer to the
  license used.
<item>Give due credit to previous authors and major contributors.
</enum>

If you're considering making a derived work other than a
translation, it's requested that you discuss your plans with the
current maintainer.

<sect2>Disclaimer
<p> While I haven't intentionally tried to mislead you, there are
likely a number of errors in this document.  Please let me know about
them.  Since this is free documentation, it should be obvious that I
cannot be held legally responsible for any errors.

<sect2>Trademarks.
<p> Any brand names (starts with a capital letter such as MS Windows)
should be assumed to be a trademark).  Such trademarks belong to their
respective owners. <!-- license.D end -->


<sect2>Credits
<p> Most of the original Serial-HOWTO was written by  Greg Hankins.
<url url="mailto:gregh@twoguys.org">
He also rewrote many contributions by others in order to maintain
continuity in the writing style and flow.  He wrote: ``Thanks to
everyone who has contributed or commented, the list of people has
gotten too long to list (somewhere over one hundred).  Special thanks
to Ted Ts'o for answering questions about the serial drivers.''
Approximately half of v2.00 was from Greg Hankins HOWTO and the other
half were additions by David Lawyer.  Ted Ts'o has continued to be
helpful.  In Jan. 2006 "Charles Brockman" reviewed it for typos which
resulted in many typos being fixed.

<sect1> New Versions of this Serial-HOWTO
<p> New versions will be issued perhaps every year or two.  They will be
available to browse and/or download at LDP mirror sites see: <url
url="http://www.tldp.org/mirrors.html">.  Various formats are
available.  If you only want to quickly check the date of the latest
version look at <url
url="http://www.tldp.org/HOWTO/Serial-HOWTO.html"> and compare it to
this version: v2.27 February 2011 .

<sect1>New in Recent Versions <p> For a full revision history going
back to the time I started maintaining this HOWTO, see the source file
(in linuxdoc format): <url
url="http://cvs.tldp.org/go.to/LDP/LDP/howto/linuxdoc/Serial-HOWTO.sgml?view=markup"
name="(cvs) Serial-HOWTO.sgml">
<itemize>
<item> TO do: explain use of udev for serial port for setting names
and permissions.  Fix dead links.
<item>2.27 Feb. 2011: The serial port is still widely used in embedded
systems, etc., and is not really obsolete.  Better definition of
input, output.  Deleted devfs names such as tts.
<item>2.26 Nov. 2010 Changed EIA-232 to RS-232.  PCI-e bus is serial.  No
serial port on new PCs.  I incorrectly wrote in this update that the serial port was obsolete and that this HOWTO is now mainly of historical interest. 
<item>2.25 Jan. 2007 picocom.  devfs is obsolete. ser2net.  Revised
parts on drivers as modules vs. built into kernel.  Serial Programming
wikibook.
</itemize>

<sect1> Related HOWTO's, etc. about the Serial Port <label id="related_howtos">
<p> Modems, Text-Terminals, some printers, and other peripherals often
used the serial port.  Get these HOWTOs from the nearest mirror site as
explained above.

<itemize>
<item><tt>Modem-HOWTO</tt> is about installing and configuring modems
<item><tt>Printing-HOWTO</tt> has info for serial printers using old
 lpr command
<item><tt>LPRng-HOWTO</tt> (not a LDP HOWTO, may come with software)
 has info for serial printing for "Next Generation" lpr
<item> <url url="http://en.wikibooks.org/wiki/Programming:Serial_Data_Communications" name="Serial Programming"> is a wiki-book
 on the Internet
<item><tt>Serial-Programming-HOWTO</tt> helps you write
 C programs that read and write to the serial port
 and/or check/set its state.  A version written by Vern Hoxie but not
 submitted to LDP is at <ref id="vern_" name="Internet section">.
<item><tt>Text-Terminal-HOWTO</tt> is about how they work, how to install
 configure, and repair them.  It includes a section on "Make a
 Terminal the Console" which is useful for using a remote terminal to
 control a server (via the serial port).
<item><tt>Remote-Serial-Console-HOWTO</tt> is about making a
 text-terminal be the console so it can display boot-time messages, etc.
</itemize>

<sect1>Feedback
<p> Please send me any  suggestions, correction or additional
material.  Tell me what you don't understand, or what could be
clearer.  You can reach me via email at <tt><url
url="mailto:dave@lafn.org"></tt>.

<sect1> What is a Serial Port?
<p> The conventional serial port (not the newer USB port, or Firewire
port) is a very old I/O (Input/Output) port.  Until around 2006, most
new desktop PC's had one, and old PC's from the 1990's sometimes had 2
of them.  Most laptops gave up them before the desktops did.  Macs
(Apple Computer) after mid-1998 only had the USB port.  However, it's
possible, to put a conventional serial port device on the USB bus
which is on all modern PCs.

Each serial port has a "file" associated with it in the /dev
directory.  It isn't really a file but it seems like one.  For
example, /dev/ttyS0. Other serial ports are /dev/ttyS1, /dev/ttyS2,
etc.  But ports on the USB bus, multiport cards, etc. have different
names.

The common specification for the conventional serial port is RS-232
(or RS-232).  So it's often called a "RS-232 serial port".  The
connector(s) for the serial port are often seen as one or two 9-pin
connectors (in some cases 25-pin) on the back of a PC.  But the serial
port is more than just connectors.  It includes the associated
electronics which must produce signals conforming to the RS-232
specification.  See <ref id="volt_shape" name="Voltage Waveshapes">.
One pin is used to send out data bytes and another to receive data
bytes.  Another pin is a common signal ground.  The other "useful"
pins are used mainly for signalling purposes with a steady negative
voltage meaning "off" and a steady positive voltage meaning "on".

The UART (Universal Asynchronous Receiver-Transmitter) chip does most
of the work.  Today, the functionality of this chip is usually built
into another chip.  See <ref id="uart_" name="What Are UARTs?"> These
have improved over time and old models (prior to say 1994) are usually
very obsolete.

The serial port was originally designed for connecting external modems
to a PC but it's used to connect many other devices also such as mice,
text-terminals, some printers, etc. to a computer.  You just plug
these devices into the serial port using the correct cable.  Many
internal modem cards have a built-in serial port so when you install
one inside your PC it's as if you just installed another serial port
in your PC.

<sect> Quick Help
<p>This repeats more detailed information found elsewhere.  If your
computer can't seem to find your serial port and you already know
something about hardware resources (addresses like 3F8 and IRQs like
5) then try this:  First, get into the BIOS (often called "setup")
when the computer is powered on by pressing certain keys.  To find out
what keys to press, watch the screen as your PC starts up.  If
the words that flash by on the screen too fast to read, freeze them 
by holding down the "pause" and "shift" keys at the same time.  Then
when read, hit any key to resume (cease pausing) and hold down the
key(s) required to enter the BIOS setup.   You may have to try this
again since there may be more than one screen which you can freeze
with the "pause" key.  Also, look for messages about the serial ports
on these frozen screens.

Once in the BIOS menus, try to find  menus dealing with the serial port.
They could be shown in a menu dealing with Resources, Plug-and-Play,
Peripherals, Ports, etc.  Some old BIOSs setups (before 1995 ?) didn't
deal with the serial ports.  Make sure the ports you need are not
disabled and note how they are configured (like 3F8 IRQ 4).  You may
need to change the configuration to prevent conflicts.  There could be
a shortage of IRQs if the BIOS has reserved some IRQs that it didn't
need to reserve.

For serial ports to be found, either the kernel must have been
compiled with serial support, or serial support must be provided by a
module.  To check this look in the file /boot/config-2.6... and search
for SERIAL.  =m means it's a module and you may check to see the
modules that are being used by typing: lsmod.

<sect> How the Hardware Transfers Bytes <label id="how_hdw_xfers">

<p> Below is an introduction to the topic, but for a more advanced
treatment of it see <ref id="fifo_" name="FIFOs">.

<sect1> Transmitting
<p> Transmitting is sending bytes out of the serial port away from the
computer (output).  Once you understand transmitting, receiving
(input) is easy to understand since it's similar.  The first
explanation given here will be grossly oversimplified.  Then more
detail will be added in later explanations.  When the computer wants
to send a byte out the serial port (to the external cable) the CPU
sends the byte on the bus inside the computer to the I/O (Input
Output) address of the serial port.  I/O is often written as just IO.
The serial port takes the byte, and sends it out one bit at a time (a
serial bit-stream) on the transmit pin of the serial cable connector.
For what a bit (and byte) look like electrically see <ref
id="volt_shape" name="Voltage Waveshapes">.

Here's a replay of the above in a little more detail (but still very
incomplete).  Most of the work at the serial port is done by the UART
chip (or the like).  To transmit a byte, the serial device driver
program (running on the CPU) sends a byte to the serial port"s I/O
address.  This byte gets into a 1-byte "transmit shift register" in
the serial port.  From this shift register bits are taken from the
byte one-by-one and sent out bit-by-bit on the serial line.  Then when
the last bit has been sent and the shift register needs another byte
to send, it could just ask the CPU to send it another byte.  Thus would
be simple but it would likely introduce delays since the CPU might not
be able to get the byte immediately.  After all, the CPU is usually
doing other things besides just handling the serial port.

A way to eliminate such delays is to arrange things so that the CPU
gets the byte before the shift register needs it and stores it in a
serial port buffer (in hardware).  Then when the shift register has
sent out its byte and needs a new byte immediately, the serial port
hardware just transfers the next byte from its own buffer to the shift
register.  No need to call the CPU to fetch a new byte.

The size of this serial port buffer was originally only one byte, but
today it is usually 16 bytes (more in higher priced serial ports).
Now there is still the problem of keeping this buffer sufficiently
supplied with bytes so that when the shift register needs a byte to
transmit it will always find one there (unless there are no more bytes
to send).  This is done by contacting the CPU using an interrupt.

First we'll explain the case of the old fashioned one-byte buffer,
since 16-byte buffers work similarly (but are more complex).  When the
shift register grabs the byte out of the buffer and the buffer needs
another byte, it sends an interrupt to the CPU by putting a voltage on
a dedicated wire on the computer bus.  Unless the CPU is doing
something very important, the interrupt forces it to stop what it was
doing and start running a program which will supply another byte to
the port's buffer.  The purpose of this buffer is to keep an extra
byte (waiting to be sent) queued in hardware so that there will be no
gaps in the transmission of bytes out the serial port cable.

Once the CPU gets the interrupt, it will know who sent the interrupt
since there is a dedicated interrupt wire for each serial port (unless
interrupts are shared).  Then the CPU will start running the serial
device driver which checks registers at I/0 addresses to find out what
has happened.  It finds out that the serial's transmit buffer is empty
and waiting for another byte.  So if there are more bytes to send, it
sends the next byte to the serial port's I/0 address.  This next byte
should arrive when the previous byte is still in the transmit shift
register and is still being transmitted bit-by-bit.

In review, when a byte has been fully transmitted out the transmit
wire of the serial port and the shift register is now empty the
following 3 things happen in rapid succession:

<enum>
<item> The next byte is moved from the transmit buffer into
the transmit shift register
<item> The transmission of this new byte (bit-by-bit) begins
<item> Another interrupt is issued to tell the device driver to send
yet another byte to the now empty transmit buffer
</enum>

Thus we say that the serial port is interrupt driven.  Each time the
serial port issues an interrupt, the CPU sends it another byte.  Once
a byte has been sent to the transmit buffer by the CPU, then the CPU
is free to pursue some other activities until it gets the next
interrupt.  The serial port transmits bits at a fixed rate which is
selected by the user (or an application program).  It's sometimes
called the baud rate.  The serial port also adds extra bits to each
byte (start, stop and perhaps parity bits) so there are often 10 bits
sent per byte.  At a rate (also called speed) of 19,200 bits per
second (bps), there are thus 1,920 bytes/sec (and also 1,920
interrupts/sec).

Doing all this is a lot of work for the CPU.  This is true for many
reasons.  First, just sending one 8-bit byte at a time over a 32-bit
data bus (or even 64-bit) is not a very efficient use of bus width.
Also, there is a lot of overhead in handing each interrupt.  When the
interrupt is received, the device driver only knows that something
caused an interrupt at the serial port but doesn't know that it's
because a character has been sent.  The device driver has to make
various checks to find out what happened.  The same interrupt could
mean that a character was received, one of the control lines changed
state, etc.

A major improvement has been the enlargement of the buffer size of the
serial port from 1-byte to 16-bytes.  This means that when the CPU
gets an interrupt it gives the serial port up to 16 new bytes to
transmit.  This is fewer interrupts to service but data must still be
transferred one byte at a time over a wide bus.  The 16-byte buffer is
actually a FIFO (First In First Out) queue and is often called a FIFO.
See <ref id="fifo_" name="FIFOs"> for details about the FIFO along
with a repeat of some of the above info.

<sect1> Receiving
<p> Receiving bytes by a serial port is similar to sending them only
it's in the opposite direction.  It's also interrupt driven.  For the
obsolete type of serial port with 1-byte buffers, when a byte is fully
received from the external cable it goes into the 1-byte receive
buffer.  Then the port gives the CPU an interrupt to tell it to pick
up that byte so that the serial port will have room for storing the
next byte which is currently being received.  For newer serial ports
with 16-byte buffers, this interrupt (to fetch the bytes) may be sent
after 14 bytes are in the receive buffer.  The CPU then stops what it
was doing, runs the interrupt service routine, and picks up 14 to 16
bytes from the port.  For an interrupt sent when the 14th byte has
been received, there could be 16 bytes to get if 2 more bytes have
arrived since the interrupt.  But if 3 more bytes should arrive
(instead of 2), then the 16-byte buffer will overrun.  It also may
pick up less than 14 bytes by setting it up that way or due to
timeouts.  See <ref id="fifo_" name="FIFOs"> for more details.

<sect1> The Large Serial Buffers
<p> We've talked about small 16-byte serial port hardware
buffers but there are also much larger buffers in main memory.  When
the CPU takes some bytes out of the receive buffer of the hardware, it
puts them into a much larger (say 8k-byte) receive buffer in main
memory.  Then a program that is getting bytes from the serial port
takes the bytes it's receiving out of that large buffer (using a
"read" statement in the program).

A similar situation exists for bytes that are to be transmitted.  When
the CPU needs to fetch some bytes to be transmitted it takes them out
of a large (8k-byte) transmit buffer in main memory and puts them into
the small 16-byte transmit buffer in the hardware.  And when a program
wants to send bytes out the serial port, it writes them to this large
transmit buffer.

Both of these buffers are managed by the serial driver.  But the
driver does more than just dealing with these buffers.  It also does
limited filtering (minor modifications) of data passing thru these buffers
and listens for control certain characters.  All this is configurable
using the <ref id="stty_" name="Stty"> program.

<sect> Serial Port Basics <label id="basics_">
<!-- basics.D  begin   <sect> Serial Port and Modem Basics
 or <sect> Serial Port Basics  In SS and MM -->
<!-- Change log:
Nov. '99: 2 serial drivers concurrently NG
Sept. '00: data flow diagram
Dec. '00 flow control +
July '01 done -> down
Feb. '02 4k buffer -> 8k
Feb. '03: UARTs with auto flow control.  Rewrite of Flow Control.
Mar. '03: Flip buffer (the 4th buffer)
Apr. '03: flow control clarity, RTS -> CTS re modems
Nov. '04: Better clarity re Data Flow, Winmodem support
Jan. '07: typos: fill->full, "a to"->"to a", date->date, and->""
-->


<!-- ifdef SERIAL_ -->
<p> You don't have to understand the basics to use the serial port But
understanding it may help to determine what is wrong if you run into
problems.  This section not only presents new topics but also repeats
some of what was said in the previous section <ref id="how_hdw_xfers"
name="How the Hardware Transfers Bytes"> but in greater detail.

<sect1> What is a Serial Port ?
<sect2> Intro to Serial
<p> The UART serial port (or just "serial port for short" is an I/O (Input/Output) device.

<!-- ifdef SERIAL_ -->
An I/O device is just a way to get data into and out of a computer.
There are many types of I/O devices such as the older serial ports and
parallel ports, network cards, universal serial buses (USB), and
firewire etc.
<!-- ifdef SERIAL_ end -->
Most pre-2007 PC's have a serial port or two (on older PC's).  Each
has a 9-pin connector (sometimes 25-pin) on the back of the computer.
Computer programs can send data (bytes) to the transmit pin (output)
and receive bytes from the receive pin (input).  The other pins are
for control purposes and ground.

The serial port is much more than just a connector.  It converts the
data from parallel to serial and changes the electrical representation
of the data.  Inside the computer, data bits flow in parallel (using
many wires at the same time).  Serial flow is a stream of bits over a
single wire (such as on the transmit or receive pin of the serial
connector).  For the serial port to create such a flow, it must
convert data from parallel (inside the computer) to serial on the
transmit pin (and conversely).

Most of the electronics of the serial port is found in a computer chip
(or a part of a chip) known as a UART.  For more details on UARTs
see the section
<ref id="uart_" name="What Are UARTS?"> But you may want to finish this section first so that you will
hopefully understand how the UART fits into the overall scheme of
things.

<sect2> Pins and Wires
<p> Old PC's used 25 pin connectors but only about 9 pins were
actually used so later on most connectors were only 9-pin.  Each of
the 9 pins usually connects to a wire.  Besides the two wires used for
transmitting and receiving data, another pin (wire) is signal ground.
The voltage on any wire is measured with respect to this ground.  Thus
the minimum number of wires to use for 2-way transmission of data is
3.  Except that it has been known to work with no signal ground wire
but with degraded performance and sometimes with errors.

There are still more wires which are for control purposes (signalling)
only and not for sending bytes.  All of these signals could have been
shared on a single wire, but instead, there is a separate dedicated
wire for every type of signal.  Some (or all) of these control wires
are called "modem control lines".  Modem control wires are either in
the asserted state (on) of +5 volts or in the negated state (off) of
-5 volts.  One of these wires is to signal the computer to stop
sending bytes out the serial port cable.  Conversely, another wire
signals the device attached to the serial port to stop sending bytes
to the computer.  If the attached device is a modem, other wires may
tell the modem to hang up the telephone line or tell the computer that
a connection has been made or that the telephone line is ringing
(someone is attempting to call in).  See section
<ref id="pinout_" name="Pinout and Signals"> for more details.



<!-- ifdef SERIAL_ -->
<sect2> RS-232  (EIA-232, etc.)
<p> The serial port (not the USB) is usually a RS-232-C, EIA-232-D, or
EIA-232-E.  These three are almost the same thing.  The original RS
(Recommended Standard) prefix officially became EIA (Electronics
Industries Association) and later EIA/TIA after EIA merged with TIA
(Telecommunications Industries Association).  The EIA-232 spec
provides also for synchronous (sync) communication but the hardware to
support sync is almost always missing on PC's.  The RS designation is
was intended to become obsolete but is still widely used and will be
used in this howto for RS-232.  Other documents may use the full
EIA/TIA designation, or just EIA or TIA.  For info on other
(non-RS-232) serial ports see the section <ref id="non_rs232"
name="Other Serial Devices (not async RS-232)">
<!-- ifdef SERIAL_ end -->

<sect1> IO Address & IRQ
<p> Since the computer needs to communicate with each serial port, the
operating system must know that each serial port exists and where it
is (its I/O address).  It also needs to know which wire (IRQ number)
the serial port must use to request service from the computer's CPU.
It requests service by sending an interrupt voltage on this wire.
Thus every serial port device must store in its non-volatile memory
both its I/O address and its Interrupt ReQuest number: IRQ.  See <ref
id="interrupt_" name="Interrupts">.  The PCI bus has its own system of
interrupts.  But since the PCI-aware BIOS sets up these PCI interrupts
to map to IRQs, it seemingly behaves just as described above.  Except
that sharing of PCI interrupts is allowed (2 or more devices may use
the same IRQ number).

I/O addresses are not the same as memory addresses.  When an I/O
addresses is put onto the computer's address bus, another wire is
energized.  This both tells main memory to ignore the address and
tells all devices which have I/O addresses (such as the serial port)
to listen to the address sent on the bus to see if it matches the
device's.  If the address matches, then the I/O device reads the data
on the data bus.

The I/O address of a certain device (such as ttyS2) will actually be a
range of addresses.  The lower address in this range is the base
address.  "address" usually means just the "base address".

<sect1> Names: ttyS0, ttyS1, etc.
<p> The serial ports are named ttyS0, ttyS1, etc. (and usually
correspond respectively to COM1, COM2, etc. in DOS/Windows).  The /dev
directory has a special file for each port.  Type "ls /dev/ttyS*" to
see them.  Just because there may be (for example) a ttyS3 file,
doesn't necessarily mean that there exists a physical serial port
there.

Which one of these names (ttyS0, ttyS1, etc.) refers to which
physical serial port is determined as follows.  The serial driver
(software) maintains a table showing which I/O address corresponds to
which ttyS.  This mapping of names (such as ttyS1) to I/O addresses
(and IRQ's) may be both set and viewed by the "setserial" command.
See <ref id="set_serial" name="What is Setserial">.  This does
<tt/not/ set the I/O address and IRQ in the hardware itself (which is
set by jumpers or by plug-and-play software).  Thus which physical port
corresponds to say ttyS1 depends both on what the serial driver thinks
(per setserial) and what is set in the hardware.  If a mistake has
been made, the physical port may not correspond to any name (such as
ttyS2) and thus it can't be used.  See <ref id="ttySN_" name="Serial
Port Devices /dev/ttyS2, etc."> for more details.

<sect1> Interrupts <label id="interrupt_">
<p>

When the serial port receives a number of bytes (may be set to 1, 4,
8, or 14) into its FIFO buffer, it signals the CPU to fetch them by
sending an electrical signal known as an interrupt on a certain wire
normally used only by that port.  Thus the FIFO waits until it has
received a number of bytes and then issues an interrupt.

However, this interrupt will also be sent if there is an unexpected
delay while waiting for the next byte to arrive (known as a timeout).
Thus if the bytes are being received slowly (such as from someone
typing on a terminal keyboard) there may be an interrupt issued for
every byte received.  For some UART chips the rule is like this: If 4
bytes in a row could have been received in an interval of time, but
none of these 4 show up, then the port gives up waiting for more bytes
and issues an interrupt to fetch the bytes currently in the FIFO.  Of
course, if the FIFO is empty, no interrupt will be issued.

Each interrupt conductor (inside the computer) has a number (IRQ) and
the serial port must know which conductor to use to signal on.  For
example, ttyS0 normally uses IRQ number 4 known as IRQ4 (or IRQ 4).  A
list of them and more will be found in "man setserial" (search for
"Configuring Serial Ports").  Interrupts are issued whenever the
serial port needs to get the CPU's attention.  It's important to do
this in a timely manner since the buffer inside the serial port can
hold only 16 incoming bytes.  If the CPU fails to remove such received
bytes promptly, then there will not be any space left for any more
incoming bytes and the small buffer may overflow (overrun) resulting
in a loss of data bytes.

There is no <ref
id="flow_control" name="Flow Control"> to prevent this.

Interrupts are also issued when the serial port has just sent out all
of its bytes from its small transmit FIFO buffer out the external
cable.  It then has space for 16 more outgoing bytes.  The interrupt
is to notify the CPU of that fact so that it may put more bytes in the
small transmit buffer to be transmitted.  Also, when a modem control
line changes state, an interrupt is issued.

The buffers mentioned above are all hardware buffers.  The serial port
also has large buffers in main memory.  This will be explained later

Interrupts convey a lot of information but only indirectly.  The
interrupt itself just tells a chip called the interrupt controller
that a certain serial port needs attention.  The interrupt controller
then signals the CPU.  The CPU then runs a special program to service
the serial port.  That program is called an interrupt service routine
(part of the serial driver software).  It tries to find out what has
happened at the serial port and then deals with the problem such a
transferring bytes from (or to) the serial port's hardware buffer.
This program can easily find out what has happened since the serial
port has registers at IO addresses known to the serial driver
software.  These registers contain status information about the serial
port.  The software reads these registers and by inspecting the
contents, finds out what has happened and takes appropriate action.





<!-- ifdef SERIAL_ -->
<sect1> Data Flow (Speeds)
<p> Data (bytes representing letters, pictures, etc.) flows into and
out of your serial port.  Flow rates (such as 56k (56000) bits/sec)
are (incorrectly) called "speed".  But almost everyone says "speed"
instead of "flow rate".

It's important to understand that the average speed is often less than
the specified speed.  Waits (or idle time) result in a lower average
speed.  These waits may include long waits of perhaps a second due to
<ref id="flow_control" name="Flow Control">.  At the other extreme
there may be very short waits (idle time) of several micro-seconds
between bytes.  If the device on the serial port (such as a modem)
can't accept the full serial port speed, then the average speed must
be reduced.
<!-- ifdef SERIAL_ end -->

<sect1> Flow Control <label id="flow_control">
<p> Flow control means the ability to slow down the flow of bytes in a
wire.  For serial ports this means the ability to stop and then
restart the flow without any loss of bytes.  Flow control is needed
for modems and other hardware to allow a jump in instantaneous flow rates.

<sect2> Example of Flow Control
<p> For example, consider the case where you connect a 33.6k external
modem via a short cable to your serial port.  The modem sends and
receives bytes over the phone line at  33.6k bits per second (bps).
Assume it's not doing any data compression or error correction.  You
have set the serial port speed to 115,200 bits/sec (bps), and you are
sending data from your computer to the phone line.  Then the flow from
the your computer to your modem over the short cable is at 115.2k bps.
However the flow from your modem out the phone line is only 33.6k bps.
Since a faster flow (115.2k) is going into your modem than is coming
out of it, the modem is storing the excess flow (115.2k -33.6k = 81.6k
bps) in one of its buffers.  This buffer would soon overrun (run out
of free storage space) unless the high 115.2k flow is stopped.

But now flow control comes to the rescue.  When the modem's buffer is
almost full, the modem sends a stop signal to the serial port.  The
serial port passes on the stop signal on to the device driver and the
115.2k bps flow is halted.  Then the modem continues to send out data
at 33.6k bps drawing on the data it previous accumulated in its
buffer.  Since nothing is coming into this buffer, the number of bytes
in it starts to drop.  When almost no bytes are left in the buffer,
the modem sends a start signal to the serial port and the 115.2k flow
from the computer to the modem resumes.  In effect, flow control
creates an average flow rate in the short cable (in this case 33.6k)
which is significantly less than the "on" flow rate of 115.2k bps.
This is "start-stop" flow control.

In the above simple example it was assumed that the modem did no data
compression.  This could happen when the modem is sending a file
which is already compressed and can't be compressed further.  Now
let's consider the opposite extreme where the modem is compressing the
data with a high compression ratio.  In such a case the modem might
need an input flow rate of say 115.2k bps to provide an output (to the
phone line) of 33.6k bps (compressed data).  This compression ratio is
3.43 (115.2/33.6).   In this case the modem is able to compress the
115.2 bps PC-to-modem flow and send the same data (in compressed form)
out the phone line at 33.6bps.  There's no need for flow control here
so long as the compression ratio remains higher than 3.43.  But the
compression ratio varies from second to second and if it should drop
below 3.43, flow control will be needed 

In the above example, the modem was an external modem.  But the same
situation exists (as of early 2003) for most internal modems.  There is
still a speed limit on the PC-to-modem speed even though this flow
doesn't take place over an external cable.  This makes the internal
modems compatible with the external modems.

In the above example of flow control, the flow was from the computer to
a modem.  But there is also flow control which is used for the
opposite direction of flow: from a modem (or other device) to a
computer.  Each direction of flow involves 3 buffers: 1. in the modem
2. in the UART chip (called FIFOs) and 3. in main memory managed by the
serial driver.  Flow control protects all buffers (except the FIFOs)
from overflowing.  

Under Linux, the small UART FIFO buffers are not protected by flow
control but instead rely on a fast response to the interrupts they
issue.  Some UART chips can be set to do hardware flow control to
protect their FIFOs but Linux (as of early 2003) doesn't seem to
support it.  FIFO stand for "First In, First Out" which is the way it
handles bytes in a queue.  All the 3 buffers use the FIFO rule but
only the one in the UART is named "FIFO".   This is the essence of flow
control but there are still some more details.



<!-- ifdef SERIAL_ -->
<sect2> Symptoms of No Flow Control
<p> Understanding flow-control theory can be of practical use.  The
symptom of no flow control is that chunks of data missing from files
sent without the benefit of flow control.  When overflow happens,
often hundreds or even thousands of bytes get lost, and all in
contiguous chunks.
<!-- ifdef SERIAL_ end -->

<sect2> Hardware vs. Software Flow Control
<p> If feasible, it's best to use "hardware" flow control that uses two
dedicated "modem control" wires to send the "stop" and "start"
signals.  Hardware flow control at the serial port works like this:
The two pins, RTS (Request to send) and CTS (Clear to send) are used.
When the computer is ready to receive data it asserts RTS by putting a
positive voltage on the RTS pin (meaning "Request To Send to me").
When the computer is not able to receive any more bytes, it negates
RTS by putting a negative voltage on the pin saying: "stop sending to
me".  The RTS pin is connected by the serial cable to another pin on
the modem, printer, terminal, etc.  This other pin's only function is
to receive this signal.

For the case of a modem, this "other" pin will be the modem's RTS pin.
But for a printer, another PC, or a non-modem device, it's usually a
CTS pin so a "crossover" or "null modem" cable is required.  This
cable connects the CTS pin at one end with the RTS pin at the other
end (two wires since each end of the cable has a CTS pin).  For a
modem, a straight-thru cable is used.

For the opposite direction of flow a similar scheme is used.  For a
modem, the CTS pin is used to send the flow control signal to the CTS
pin on the PC.  For a non-modem, the RTS pin sends the signal.  Thus
modems and non-modems have the roles of their RTS and CTS pins
interchanged.  Some non-modems such as dumb terminals may use other
pins for flow control such as the DTR pin instead of RTS.

Software flow control uses the main receive and transmit data wires to
send the start and stop signals.  It uses the ASCII control characters
DC1 (start) and DC3 (stop) for this purpose.  They are just inserted
into the regular stream of data.  Software flow control is not only
slower in reacting but also does not allow the sending of binary data
unless special precautions are taken.  Since binary data will likely
contain DC1 and DC3 characters, special means must be taken to
distinguish between a DC3 that means a flow control stop and a DC3
that is part of the binary code.  Likewise for DC1. <!-- ifdef MODEM_
--> 

<sect1> Data Flow Path; Buffers 
<p>It's been mentioned that there are 3 buffers for each direction of
flow (3 pairs altogether): 16-byte FIFO buffers (in the UART), a pair
of larger buffers inside a device connected to the serial port (such
as a modem), and a pair of buffers (say 8k) in main memory.  When an
application program sends bytes to the serial port they first get
stashed in the transmit serial port buffer in main memory.  The other
member of this pair consists of a receive buffer for the opposite
direction of byte-flow.  Here's an example diagram for the case of
browsing the Internet with a browser.  Transmit data flow is left to
right while receive flow is right to left.  There is a separate buffer
for each direction of flow.

<verb>
application	8k-byte		16-byte	       1k-byte	      tele-	
BROWSER ------- MEMORY -------- FIFO --------- MODEM -------- phone
program		buffer		buffer	       buffer	      line
</verb>

For the transmit case, the serial device driver takes out say 15 bytes
from this transmit buffer (in main memory), one byte at a time and
puts them into the 16-byte transmit buffer in the serial UART for
transmission.  Once in that transmit buffer, there is no way to stop
them from being transmitted.  They are then transmitted to the
modem or (other device connected to the serial port) which also has a
fair sized (say 1k) buffer.  When the device driver (on orders from
flow control sent from the modem) stops the flow of outgoing bytes
from the computer, what it actually stops is the flow of outgoing
bytes from the large transmit buffer in main memory.  Even after this
has happened and the flow to the modem has stopped, an application
program may keep sending bytes to the 8k transmit buffer until it
becomes full.  At the same time, the bytes stored in the FIFO continue
to be sent out.  Bytes stored in the modem will continue to be sent
out the phone line unless the modem has gotten a modem-to-modem flow
control stop from the modem at the other end of the phone line.

When the memory buffer gets full, the application program can't send
any more bytes to it (a "write" statement in a C-program blocks) and
the application program temporarily stops running and waits until some
buffer space becomes available.  Thus a flow control "stop" is
ultimately able to stop the program that is sending the bytes.  Even
though this program stops, the computer does not necessarily stop
computing since it may switch to running other processes while it's
waiting at a flow control stop.

The above was a little oversimplified in three ways.  First, some UARTs
can do automatic hardware flow control which can stop the transmission
out of the FIFO buffers if needed (not yet supported by Linux).
Second, while an application process is waiting to write to the
transmit buffer, it could possibly perform other tasks.  Third, the
serial driver (located between the memory buffer and the FIFO) has
it's own small buffer (in main memory) used to process characters.



<!-- ifdef SERIAL_ -->
<sect1> Complex Flow Control Example
<p> For many situations, there is a transmit path involving several
links, each with its own flow control.  For example, I type at a
text-terminal connected to a PC and the PC (under my control) dials
out to another computer using a modem.  Today, a "text-terminal" is
likely to be just another PC emulating a text-terminal.  The main
(server) PC, in addition to serving my text-terminal, could also have
someone else sitting at it doing something else.  Note that calling
this PC a "server" is not technically correct but it does serve the
terminal.

The text-terminal uses a command-line interface with no graphical
display.  Every letter I type at the text-terminal goes over the
serial cable to my main PC and then over the phone line to the
computer that I've dialed out to.  To dial out, I've used the
communication software: "minicom" which runs on my PC.

This sounds like a simple data path.  I hit a key and the byte that
key generates flows over just two cables (besides the keyboard cable):
1. the cable from my text-terminal to my PC and 2. the telephone line
cable to some other computer.  Of course, the telephone cable is
actually a number of telephone system cables and includes switches
and electronics so that a single physical cable can transmit many
phone calls.  But I can think of it like one cable (or one link).

Now, let's count the number and type of electronic devices each
keystroke-byte has to pass thru.  The terminal-to-PC cable has a
serial port at each end.  The telephone cable has both a serial port
and a modem at each end.  This adds up to 4 serial ports and 2 modems.
Since each serial port has 2 buffers, and each modem one buffer, that
adds up to 10 buffers.  And that's just for one direction of flow.
Each byte also must pass thru the minicom software as well.

While there's just 2 cables in the above scenario, if external modems
were used there would be an additional cable between each modem and
it's serial port.  This makes 4 cables in all.  Even with internal
modems it's like there is a "virtual cable" between the modem and its
serial port.  On all these 4 links (or cables), flow control takes
place.

Now lets consider an example of the operation of flow control.
Consider the flow of bytes from the remote computer at the other end
of the phone line to the screen on the text-terminal that I'm sitting
at.  A real text-terminal has a limit to the speed at which bytes can
be displayed on its screen and issues a flow control "stop" from time
to time to slow down the flow.  This "stop" propagates in a direction
opposite to the flow of bytes it controls.  What happens when such a
"stop" is issued?  Let's consider a case where the "stop" waits long
enough before canceling it with a "start", so that it gets thru to
the remote computer at the other end of the phone line.  When it gets
there it will stop the program at the remote computer which is sending
out the bytes.

Let's trace out the flow of this "stop" (which may be "hardware" on
some links and "software" on others).  First, suppose I'm "capturing"
a long file from the remote computer which is being sent
simultaneously to both my text-terminal and to a file on my hard-disk.
The bytes are coming in faster than the terminal can handle them so it
sends a "stop" out its serial port to a serial port on my PC.  The
device driver detects it and stops sending bytes from the 8k PC serial
buffer (in main memory) to the terminal.  But minicom still keeps
sending out bytes for the terminal into this 8k buffer.

When this 8k transmit buffer (on the first serial port) is full,
minicom must stop writing to it.  Minicom stops and waits.  But this
also causes minicom to stop reading from the 8k receive buffer on the
2nd serial port connected to the modem.  Flow from the modem continues
until this 8k buffer too fills up and sends a different "stop" to the
modem.  Now the modem's buffer ceases to send to the serial port and
also fills up.  The modem (assuming error correction is enabled) sends
a "stop signal" to the other modem at the remote computer.  This modem
stops sending bytes out of its buffer and when its buffer gets full,
another stop signal is sent to the serial port of the remote computer.
At the remote computer, the 8-k (or whatever) buffer fills up and the
program at the remote computer can't write to it anymore and thus
temporarily halts.

Thus a stop signal from a text terminal has halted a program on a
remote computer.  What a long sequence of events!  Note
that the stop signal passed thru 4 serial ports, 2 modems, and one
application program (minicom).  Each serial port has 2 buffers (in one
direction of flow): the 8k one and the hardware 16-byte one.  The
application program may have a buffer in its C_code.  This adds up to
11 different buffers the data is passing thru.  Note that the small
serial hardware buffers do not participate directly in flow control.
Also note that the two buffers associated with the text-terminal's
serial port are going to be dumping their contents to the screen
during this flow control halt.  This leaves 9 other buffers that may
be getting filled up during the flow control halt.

If the terminal speed limitation is the bottleneck in the flow from
the remote computer to the terminal, then its flow control "stop" is
actually stopping the program that is sending from the remote computer
as explained above.  But you may ask: How can a "stop" last so long
that 9 buffers (some of them large) all get filled up?   It seldom
happens, but it can actually happen this way if all the buffers were
near their upper limits when the terminal sent out the "stop".

But if you were to run a simulation on this you would discover that it's
usually more complicated than this.  At an instant of time some links
are flowing and others are stopped (due to flow control).  A "stop"
from the terminal seldom propagates back to the remote computer neatly as
described above.  It may take a few "stops" from the terminal to
result in one "stop" at the remote computer, etc.  To understand what
is going on you really need to observe a simulation which can be done
for a simple case with coins on a table.  Use only a few buffers and
set the upper level for each buffer at only a few coins.

Does one really need to understand all this?  Well, understanding this
explained to me why capturing text from a remote computer was loosing
text.  The situation was exactly the above example but modem-to-modem
flow control was disabled.  Chunks of captured text that were supposed
to also get to my hard-disk never got there because of an overflow at
my modem buffer due to flow control "stops" from the terminal.  Even
though the remote computer had a flow path to the hard-disk without
bottlenecks, the same flow also went to a terminal which issued flow
control "stops" with disastrous results for the branch of the flow
going to a file on the hard-disk.  The flow to the hard-disk passed
thru my modem and since the overflow happened at the modem, bytes
intended for the hard-disk were lost.

<!-- ifdef SERIAL_ end -->

<sect1> Serial Driver Module
<p> The device driver for the serial port is the software that
operates the serial port.  It is now provided as a serial module.
From kernel 2.2 on, this module will normally get loaded automatically
if it's needed.  In earlier kernels, you had to have <tt/kerneld/
running in order to do auto-load modules on demand.  Otherwise the
serial module needed to be explicitly listed in /etc/modules.  Before
modules became popular with Linux, the serial driver was usually built
into the kernel (and sometimes still is).  If it's built-in don't let
the serial module load or else you will have two serial drivers
running at the same time.  With 2 drivers there are all sorts of
errors including a possible "I/O error" when attempting to open a
serial port.  Use "lsmod" to see if the module is loaded.

When the serial module is loaded it displays a message on the screen
about the existing serial ports (often showing a wrong IRQ).  But once
the module is used by <tt/setserial/ to tell the device driver the
(hopefully) correct IRQ then you should see a second display similar
to the first but with the correct IRQ, etc.  See
<ref id="ser_module" name="Serial Module">
See <ref id="set_serial" name="What is Setserial"> for more info on
<tt/setserial/.

<!-- basics.D end  -->


<sect1> The Serial Port is Now Obsolete on PCs <p> The serial port is
today (2011) sort of obsolete on PCs (and often called a "legacy"
device) but it is still in use on some older computers and is used in
imbedded systems, for communication with routers and point-of-sale
equipment, etc.  The serial port is <ref id="slow_" name="slow">
and after about 2005 most new PC's no longer had them.  Most laptops
and Macs discontinued them even earlier.  During the era when some new
PC's came with serial ports and others didn't, the PC's that didn't
have serial ports were euphemistically called "legacy-free".  However,
while PC's today no longer have serial ports, some do have them built
into a "legacy" modem (which plugs into a telephone line).  Such a serial port
only works with the modem and can't be used for any other device.  The
reason they have such a "built in" serial port is that analog modems
are designed to only work thru a serial port. 

The physical serial port on the back of a PC (including the chip its
connected to inside the PC), must pass data between the computer and
an external cable.  Thus it has two interfaces: the serial-port-to
cable and the serial-port-to-computer-bus.  Both of these interfaces
are slow.  First we'll consider the interface via external cable to
the outside world.

<sect1> RS-232 Cable Is Low Speed & Short Distance <label id="slow_">
<p> The conventional RS-232 serial port is inherently low speed and
is severely limited in distance.  Ads often read "high speed" but it
can only work at "high speed" over very short distances such as to a
modem located right next to the computer.  Compared to a network card,
even this "high speed" is actually low speed.  All of the RS-232
serial cable wires use a common ground return wire so that
twisted-pair technology (needed for high speeds) can't be used without
additional hardware.  More modern interfaces for serial ports exist
but they are not standard on PC's like the RS-232 is.  See <ref
id="non_232" name="Successors to RS-232">.  Some multiport serial
cards support them.

It is somewhat tragic that the RS-232 standard from 1969 did not use
twisted pair technology which could operate about a hundred times
faster.  Twisted pairs have been used in telephone cables since the
late 1800's.  In 1888 (over 120 years ago) the "Cable Conference"
reported its support of twisted-pair (for telephone systems) and
pointed out its advantages.  But over 80 years after this approval by
the "Cable Conference", RS-232 failed to utilize it.   Since RS-232
was originally designed for connecting a terminal to a low speed modem
located nearby, the need for high speed and longer distance
transmission was apparently not recognized.  The result was that since
the serial port couldn't handle high speeds, new types of serial
interfaces were devised that could: Ethernet, USB, Firewire, etc.
The final outcome was the demise of the serial port which due to it's
ancient technology became obsolete for high speed uses.

<sect1> Inefficient PCI Interface to the Computer (in some cases)

<p> The serial port communicates with the computer via the PCI bus,
the LPC bus, X-bus, or ISA bus.  The PCI bus is now 32 or 64 bits wide,
but the serial port only sends a byte at a time (8 bits wide) which is
a waste of PCI bus bandwidth.  Not so for the LPC bus which has only a
4-bit wide bus and thus provides an efficient interface.  The ISA bus
is usually 16-bits wide and the efficiency is intermediate as compared
to efficient LPC and inefficient PCI.

<sect> Multiport Serial Boards/Cards/Adapters
<sect1> Intro to Multiport Serial

<p> Multiport serial cards install in slots in a PC on the ISA or PCI
bus.  They are also called "...  adapters" or "... boards".  Each such
card provides you with many serial ports.  Today they are commonly
used for the control of external devices (including automation for
both industry and the home).  They can connect to computer servers for
the purpose of monitoring/controlling the server from a remote
location.  They were once mainly used for connecting up many dumb
terminals and/or modems to serial ports.  Today, use of dumb terminals
has declined, and several modems (or digital modems) can now be built
into an internal card.  So multiport serial cards are not as
significant as they once were.

Each multiport card has a number of external connecters (DB-25 or
RJ45) so that one may connect up a number of devices (modems,
terminals, etc.).  Each such physical device would then be connected
to its own serial port.  Since the space on the external-facing part
of the card is limited there is often not enough room for all the
serial port connectors.  To solve this problem, the connectors may be
on the ends of cables which come out (externally) from the card
(octopus cable).  Or they may be on an external box (possibly rack
mountable) which is connected by a cable to a multiport card.


<sect1> Dumb vs. Smart Cards
<p>Dumb multiport cards are not too much different than ordinary serial
ports.  They are interrupt driven and the CPU of the computer does
most all the work servicing them.  They usually have a system of
sharing a single interrupt for all the ports.  This doesn't decrease
the load on the CPU since the single interrupt will be sent to the CPU
each time any one port needs servicing.  Such devices usually require
special drivers that you must either compile into the kernel or use as
a module.

Smart boards may use ordinary UARTs but handle most interrupts from
the UARTs internally within the board.  This frees the CPU from the
burden of handling  all these interrupts.  The board may save up bytes
in its large internal FIFOs and transfer perhaps 1k bytes at a time to
the serial buffer in main memory.  It may use the full bus width of 32
bits for making data transfers to main memory (instead of transferring
only 8-bit bytes like dumb serial cards do).  Not all "smart" boards
are equally efficient.  Many boards today are Plug-and-Play.

<sect1> Getting/Enabling a Driver
<sect2> Introduction
<p>For a multiport board to work, a special driver for it must be used.
This driver may either be built into the kernel source code or
supplied as a module.  For the 2.6 kernels on, most drivers are
supplied both ways: as a module or it can be built into the kernel.
Take care not to both build support into the kernel and force the
module to load for a certain serial card.  For older kernels, there
were often no modules for dumb serial multiport boards so support was
built into the kernel.

<sect2> Build (compile) support into the kernel?
<p>A pre-compiled kernel may not have a driver for your multiport card
built in.  So then you must either compile the kernel yourself and
build in the right driver, or insure that the module is available and
loads.  Of course if the driver doesn't come both ways (as a
compile-time option and as a module) you have no such choice.

If you want to see what has already been compiled into an existing
working kernel, go to the /boot directory (or wherever the compiled
kernel(s) reside) and look in the config... file.

In the 2.6 kernel there are many options to select from in the
configuration file for compiling.  Adding support for certain
multiport cards is listed under the headings "Character devices" or
"Serial drivers".   Old multiport cards had support as part of the
serial driver and are found under "Serial Drivers".  More advanced
cards have their own driver found under "Character devices"

For compiling kernel 2.6 you should select "CONFIG_SERIAL_8250_EXTENDED".  (or
just "CONFIG_SERIAL_EXTENDED" for 2.4).  Then you will be asked more
questions about your serial ports with more options to select.  If the
resulting configuration is not quite right, then you may need to edit
the kernel configuration file manually.

<sect2> Using module support <label id="modules_">
<p>A pre-compiled kernel may come with a pre-compiled module for the
board so that you don't need to recompile the kernel.  This module
must be loaded in order to use it and if there is installation
software for the driver, it should also set up Linux to load the
module (probably at boottime).  Some of the modules to load at
boottime are listed in /etc/modules or /etc/modules.conf
Also certain parameters may need to be passed
to the driver via entries in these files or via lilo's "append"
command or via grub's "kernel" command.  For kernel 2.6 (and 2.4) the
(unloaded) modules should be found in 
<tt>/lib/modules/.../kernel/drivers/char.</tt>

<sect2> Getting info on multiport boards
<p> The board's manufacturer should have info on their website.
Unfortunately, info for old boards is sometimes not there but might be
found somewhere else on the Internet (including discussion groups).
You might also want to look at the kernel documentation in
/usr/share/doc/linux-doc... (formerly kernel-doc in pre 2.6 kernels).
For configuring the kernel or modules prior to compiling see:
Configure.help and search for "serial", etc.  There are also kernel
documentation files for certain boards including computone, hayes-esp,
moxa-smartio, riscom8, specialix, stallion, and sx (specialix).

<sect1> Multiport Devices in the /dev Directory,
<p>
The serial ports your multiport board uses depends on what kind of
board you have.  Some have their own device names like /dev/ttyE27
(Stallion) or /dev/ttyD2 (Digiboard), etc.  For various other brands,
see devices.txt in the kernel documentation.  Some use the
standard names like /dev/ttyS14 and may be found in configuration
files that used as arguments to <tt>setserial</tt>.  Such files may be
included in a setserial or serial package.

<sect1> Making Legacy Multiport Devices in the /dev Directory
<label id="make_multi">
<p> An installation script may do this for you.  But if not, here's
some examples of how to create a device name in the /dev directory.
If you use udev, MAKEDEV will <em/not create devices in the device
directory since this directory is only in memory and will be lost when
you turn off the computer.  Instead it will create the device in
/dev/.static/dev directory.

For the names and numbers of other types of serial ports other
than ttyS.. See devices.txt in the kernel documentation.  Either use
the <tt/mknod/ command, or the <tt/MAKEDEV/ script.  Typing "man
makedev" may show instructions on using it.

Using the <tt/MAKEDEV/ script, you would first become the superuser
(root) and type (for example) either:

<tscreen><verb>
linux# MAKEDEV ttyS17
</verb></tscreen>

Or if the above doesn't work cd to /dev before giving the above
command>.  Substitute whatever your port is for ttyS17.

Using <tt/mknod/ is a more complicated option since you need to know
the major and minor device numbers.  These numbers are in the
"devices" file in the kernel documentation.  For ttyS serial ports the
minor number is: 64 + port number (=81 for the example below).  Note
the "major" number is always 4 for ttyS devices (and 5 for the
obsolete cua devices).  So, if you wanted to create a device for
<tt>ttyS17</tt> using <tt/mknod/, you would type:

<tscreen><verb>
linux# mknod -m 666 /dev/ttyS17 c 4 81
</verb></tscreen>

<sect1>Standard PC Serial Cards

<p> In olden days, PCs came with a serial card installed.  Later on,
the serial function was put on the hard-drive interface card.  In the
1990s and early 2000s one or two serial ports were usually built into
the motherboard (on-board). Most of them (as of 2002) use a 16550 but
some use 16650 (32-byte FIFOs).  But one may still buy the individual
PC serial cards if they need more serial ports.  They can be used to
connect external serial devices (modems, serial mice, etc...).  Only a
tiny percentage of retail computer stores carry such cards.  But one
can purchase them on the Internet.  Before getting one for the PCI
bus, make sure Linux supports it.

Here's a list of a few popular brands:
<itemize>
<item>Byte Runner (may order directly, shows prices) <url
 url="http://www.byterunner.com">
<item> SIIG <url url="http://www.siig.com/products/io/">
<item> Dolphin <url url="http://www.dolphinfast.com/sersol.html">
</itemize>
<p>
Note: due to address conflicts, you may not be able to use /dev/ttyS3
with a IBM8514 video card (and some others) simultaneously.  See <ref
id="video_8514" name="Avoiding IO Address Conflicts with Certain Video
Boards">

<sect1>Dumb Multiport Serial Boards (with standard UART chips)
<p> They are also called "serial adapters".  Each port has its own
address.  They often have a special method of sharing interrupts which
requires that you compile support for them into the kernel.<newline>

* =>  The file that ran setserial in Debian shows some details of
configuring<newline>
# => See note below for this board
<itemize>
<item>AST FourPort and clones (4 ports) * #
<item>Accent Async-4 (4 ports) *
<item>Arnet Multiport-8 (8 ports)
<item>Bell Technologies HUB6 (6 ports)
<item>Boca BB-1004 (4 ports), BB-1008 (8 ports), BB-2016 (16 ports;
 See the Boca mini-howto revised in 2001) * #
<item>Boca IOAT66 or? ATIO66 (6 ports, Linux doesn't support its IRQ
 sharing ??  Uses odd-ball 10-cond RJ45-like connectors)
<item>Boca 2by4 (4 serial ports, 2 parallel ports)
<item>Byte Runner <url url="http://www.byterunner.com">
<item>Computone ValuePort V4-ISA (AST FourPort compatible) *
<item>Digi PC/8 (8 ports) #
<item>Dolphin <url url="http://www.dolphinfast.com/sersol/">
<item>Globetek <url url="http://www.globetek.com/">
<item>GTEK BBS-550 (8 ports; See the mini-howto)
<item>Hayes ESP (after kernel 2.1.15)
<item>HUB-6 See Bell Technologies.
<item>Longshine LCS-8880, Longshine LCS-8880+ (AST FourPort compatible) *
<item>Moxa C104, Moxa C104+ (AST FourPort compatible) *
<item><url
url="http://digital.natinst.com/manuals.nsf/web%2Fbyproductcurrent?OpenView&amp;Start=1&amp;Count=500&amp;Expand=15.1#15.1" name="NI-SERIAL"> by National Instruments
<item>NetBus (2 ports)<url url="http://www.netbus.com"> using patch from 
<url url="http://lists.insecure.org/linux-kernel/2001/Feb/2809.html">
Discontinued.
<item>PC-COMM (4 ports) <item><url url="http://www.sealevel.com"
  name="Sealevel Systems">
 COMM-2 (2 ports), COMM-4 (4 ports) and COMM-8 (8 ports)
<item>SIIG I/O Expander 2S IO1812 (4 ports) #
<item>STB-4COM (4 ports)
<item>Twincom ACI/550
<item>Usenet Serial Board II (4 ports) *
<item>VScom (uses same driver as ByteRunner)
</itemize>
<p>
In general, Linux will support any serial board which uses a 8250,
16450, 16550, 16550A, 16650, 16650V2, 16654, 16750, 16850, 16950, and
16954.  UART.  See the latest man page for "setserial" for a more
complete list.

Notes:

AST Fourport: You might need to specify <tt/skip_test/ in <tt/rc.serial/.

BB-1004 and BB-1008 do not support DCD and RI lines, and thus are not
usable for dialin modems.  They will work fine for all other purposes.

Digi PC/8 Interrupt Status Register is at 0x140.

SIIG IO1812 manual for the listing for COM5-COM8 is
wrong.  They should be COM5=0x250, COM6=0x258, COM7=0x260, and
COM8=0x268.

<sect1>Intelligent Multiport Serial Boards
<p>
Make sure that a Linux-compatible driver is available and read the
information that comes with it.  These boards use special devices (in
the /dev directory), and not the standard ttyS ones.  This information
varies depending on your hardware.  If you have updated info which
should be shown here please email it to me.

Names of Linux driver modules are *.ko (*.o prior to kernel 2.6) but
these may not work for all models shown.  See <ref id="modules_"
name="Modules (mostly for smart boards)"> The needed module may have
been supplied with your Linux distribution.  Also, parameters (such as
the io and irq often need to be given to the module so you need to
find instructions on this (possibly in the source code tree).

There are many different brands, each of which often offers many
different cards.  No attempt is currently being made to list all the
cards here (and many listed are obsolete and have bad internet links
to them which need to be fixed).  But all major brands and websites
should be shown here so it something is missing let me know.  Go to
the webpage shown for more information.  These websites often also
have info (ads) on related hardware such as modem pools, remote access
servers (RASs), and terminal servers.  Where there is no webpage, the
cards are likely obsolete.  If you would like to put together a better
list, let me know.

<itemize>
<item>Chase Research, now Perle Systems Ltd (UK based, ISA/PCI cards)<newline>
webpage: <tt><url url="http://www.perle.com"></tt><newline>
driver status: included in kernel 2.4+ for PCI only; otherwise supported by
Perle<newline>
driver and manual location: <url
url="http://www.perle.com/downloads/multi_port.shtml"> <newline>

<item>Comtrol RocketPort (36MHz ASIC; 4, 8, 16, 32, up to 128 ports)<newline>
 webpage: <tt><htmlurl url="http://www.comtrol.com"
 name="http://www.comtrol.com"></tt><newline>
 driver status: supported by Comtrol.  rocket.o<newline>
        driver location: <tt><htmlurl
 url="ftp://tsx-11.mit.edu/pub/linux/packages/comtrol"
 name="ftp://tsx-11.mit.edu/pub/linux/packages/comtrol"></tt>

<item>Computone IntelliPort II (ISA, PCI and EISA busses up to 64
 ports)<newline>
 webpage: <url url="http://www.computone.com"><newline>
 driver location: old patch at <url
  url="http://www.wittsend.com/computone/linux-2.2.10-ctone.patch.gz"><newline>
 mailing list: <url url="mailto:majordomo@lazuli.wittsend.com"> with
  "subscribe linux-computone" in body<newline>
 note: Old ATvantage and Intelliport cards are not supported by Computone

<item> Connecttech<newline>
 website: <tt><url url="http://www.connecttech.com/"></tt><newline>
 driver location: <url url="ftp://ftp.connecttech.com/pub/linux/">

<item>Cyclades<newline>
 Cyclom-Y (Cirrus Logic CD1400 UARTs; 8 - 32 ports),<newline>
 Cyclom-Z (MIPS R3000; 8 - 64 ports)<newline>
 website: <tt><url
 url="http://www.cyclades.com/products/svrbas/zseries.php"></tt><newline>
 driver status: supported by Cyclades<newline>
 driver location: <tt><htmlurl url="ftp://ftp.cyclades.com/pub/cyclades"
 name="ftp://ftp.cyclades.com/pub/cyclades"></tt> and included in Linux
 kernel since version 1.1.75: cyclades.o

<item>Decision PCCOM (2-8 ports; ISA and PCI; aka PC COM)<newline>
 ISA:<newline>
 contact: <tt><url url="mailto:info@cendio.se"></tt><newline>
 driver location: (dead link) <tt><htmlurl url="ftp://ftp.cendio.se/pub/pccom8"
 name="ftp://ftp.cendio.se/pub/pccom8"></tt><newline>
 PCI:<newline>
 drivers: <url url="http://www.decision.com.tw"><newline>
 driver status: Support in serial driver 5.03.  For an earlier driver,
 there exists a patch for kernel 2.2.16 at <url
 url="http://www.qualica.com/serial/"> and for kernels 2.2.14-2.2.17
 at<url url="http://www.pccompci.com/mains/installing_pci_linux1.html">

<item>Digi PC/Xi (12.5MHz 80186; 4, 8, or 16 ports),<newline>
 PC/Xe (12.5/16MHz 80186; 2, 4, or 8 ports),<newline>
 PC/Xr (16MHz IDT3041; 4 or 8 ports),<newline>
 PC/Xem (20MHz IDT3051; 8 - 64 ports)<newline>
 website: <tt><url url="http://www.dgii.com"></tt><newline>
 driver status: supported by Digi<newline>
 driver location: <tt><htmlurl
 url="ftp://ftp.dgii.com/drivers/linux"
 name="ftp://ftp.dgii.com/drivers/linux"></tt> and
 included in Linux kernel since version 2.0. epca.o

<item>Digi COM/Xi (10MHz 80188; 4 or 8 ports)<newline>
 contact: Simon Park, <tt><htmlurl url="mailto:si@wimpol.demon.co.uk"
 name="si@wimpol.demon.co.uk"></tt><newline>
 driver status: ?<newline>
 note: Simon is often away from email for months at a time due to
 his job.  Mark Hatle, <url url="mailto:fray@krypton.mankato.msus.edu">
 has graciously volunteered to make the driver available if you need
 it.  Mark is not maintaining or supporting the driver.

<item>Equinox SuperSerial Technology (30MHz ASIC; 2 - 128 ports)<newline>
 website: <tt><htmlurl url="http://www.equinox.com"
 name="http://www.equinox.com"></tt><newline>
 driver status: supported by Equinox<newline>
 driver location: <tt><htmlurl
 url="ftp://ftp.equinox.com/library/sst"
 name="ftp://ftp.equinox.com/library/sst"></tt>

<item>Globetek<newline>
 website: <url url="http://www.globetek.com/products.shtml"><newline>
 driver location: <url url="http://www.globetek.com/media/files/linux.tar.gz">

<item>GTEK Cyclone (16C654 UARTs; 6, 16 and 32 ports),<newline>
 SmartCard (24MHz Dallas DS80C320; 8 ports),<newline>
 BlackBoard-8A (16C654 UARTs; 8 ports),<newline>
 PCSS (15/24MHz 8032; 8 ports)<newline>
 website: <tt><htmlurl url="http://www.gtek.com" name="http://www.gtek.com">
 </tt><newline>
 driver status: supported by GTEK<newline>
 driver location: <tt><htmlurl url="ftp://ftp.gtek.com/pub"
 name="ftp://ftp.gtek.com/pub"></tt>

<item>Hayes ESP (COM-bic; 1 - 8 ports)<newline>
 website: <tt><htmlurl url="http://www.nyx.net/&tilde;arobinso"
 name="http://www.nyx.net/&tilde;arobinso"></tt><newline>
 driver status: Supported by Linux kernel (1998) since v. 2.1.15.
 esp.o.  Setserial 2.15+ supports. Also supported by author<newline>
 driver location: <tt><htmlurl url="http://www.nyx.net/&tilde;arobinso"
 name="http://www.nyx.net/&tilde;arobinso"></tt>

<item>Intelligent Serial Interface by Multi-Tech Systems<newline>
 PCI: 4 or 8 port.  ISA 8 port. DTE speed 460.8k.  Discontinued<newline>
 webpage: <url url="http://www.multitech.com/en_US/products/">

<item>Maxpeed SS (Toshiba; 4, 8 and 16 ports)<newline>
 website: <tt><htmlurl url="http://www.maxpeed.com"
 name="http://www.maxpeed.com"></tt><newline>
 driver status: supported by Maxpeed<newline>
 driver location: <tt><htmlurl url="ftp://maxpeed.com/pub/ss"
 name="ftp://maxpeed.com/pub/ss"></tt>

<item> Microgate SyncLink ISA and PCI high speed multiprotocol
 serial.  Intended for synchronous HDLC.<newline>
 website: <url
 url="http://ww/microgate.com/products/sllinux/hdlcapi.htm"><newline>
 driver status: supported by Microgate: synclink.o

<item>Moxa C218 (12MHz 80286; 8 ports),<newline>
 Moxa C320 (40MHz TMS320; 8 - 32 ports)<newline>
 website: <tt><htmlurl url="http://www.moxa.com"
 name="http://www.moxa.com"></tt><newline>
 driver status: supported by Moxa<newline>
 driver locations: <tt><url
 url="http://www.moxa.com/support/download/download.php3>"></tt>
 <tt><url url="ftp://ftp.moxa.com/drivers/linux" ></tt>
 (also from Taiwan at www.moxa.com.tw/...) where ... is the same as
 above)

<item>SDL RISCom/8 (Cirrus Logic CD180; 8 ports)<newline>
 website: <tt><htmlurl url="http://www.sdlcomm.com"
 name="http://www.sdlcomm.com"></tt><newline>
 driver status: supported by SDL<newline>
 driver location: <tt><htmlurl url="ftp://ftp.sdlcomm.com/pub/drivers"
 name="ftp://ftp.sdlcomm.com/pub/drivers"</tt>

<item> Specialix SX (25MHz T225; 8? - 32 ports),<newline>
 SIO/XIO (20 MHz Zilog Z280; 4 - 32 ports)<newline>
 webpage: Old link is broken.  Out of business?<newline>
 driver status: Was supported by Specialix<newline>
 driver location: <url url="http://www.BitWizard.nl/specialix/"><newline>
 old driver location: <url
 url="ftp://metalab.unc.edu/pub/Linux/kernel/patches/serial">

<item>Stallion EasyIO-4 (4 ports), EasyIO-8 (8 ports), and<newline>
 EasyConnection (8 - 32 ports) - each with
 Cirrus Logic CD1400 UARTs,<newline>
 Stallion (8MHz 80186 CPU; 8 or 16 ports),<newline>
 Brumby (10/12 MHz 80186 CPU; 4, 8 or 16 ports),<newline>
 ONboard (16MHz 80186 CPU; 4, 8, 12, 16 or 32 ports),<newline>
 EasyConnection 8/64 (25MHz 80186 CPU; 8 - 64 ports)<newline>
 contact: <tt><htmlurl url="mailto:sales@stallion.com"
 name="sales@stallion.com"></tt> or
 <tt><htmlurl url="http://www.stallion.com"
 name="http://www.stallion.com"></tt><newline>
 driver status: supported by Stallion<newline>
 driver location: <tt><htmlurl
 url="ftp://ftp.stallion.com/drivers/ata5/Linux"
 name="ftp://ftp.stallion.com/drivers/ata5/Linux"></tt> and
 included in linux kernel since 1.3.27.  Moved: it's now at ?.

<item>System Base
 website: <url url="http://www.sysbas.com/">
</itemize>		

<p>A review of Comtrol, Cyclades, Digi, and Stallion products was
printed in the June 1995 issue of the <EM/Linux Journal/.  The article
is available at <url url= "http://m.linuxjournal.com/article/1097"
name="Review: Intelligent Multiport Serial Boards"> Besides the
listing of various brands of multiports found above in this HOWTO
there is <url url="http://eupedia.org/serialcards.html" name="Gary's
Encyclopedia - Serial Cards">.  It's not as complete, but may have
some different links.

<sect1> Unsupported Multiport Boards
<p> The following brands that formerly made boards for with Linux
support don't mention any Linux support as of 1 Jan.  2000.  Let me
know if this changes.
<itemize>
<item> <url url="http://sie-cs.com/en/browse/product/serial-io"
name="Aurora (PCI only)">
</itemize>


<sect>Servers for Serial Ports
<p>A computer that has many serial ports (with many serial cables
connected to it) is often called a server.  Of course, most servers
serve other functions besides just serving serial ports, and many do
not serve serial ports at all (although they likely have a serial port
on them).  For example, a "serial server" may have serial cables,
each of which runs to a different (non-serial) server.  The serial
server (perhaps called a "console server") controls, via a console,
all the other servers.  The console may be physically located remote
from the serial server, communicating with the server over a network.

There are two basic types of serial servers.  One type is just an
ordinary computer (perhaps rack mounted) that uses multiport cards on
a PCI bus (or the like).  The other type is a proprietary server that
is a dedicated computer that serves a special purpose.  Servers of
both types may be called: serial servers, console servers, print
servers, or terminal servers.  They are not the same.

The terminal server was originally designed to provide many serial
ports, each connected to a dumb text-terminal.  Today, a terminal
server often connects to graphic terminals over a fast network and
doesn't use serial ports since they are too slow.  One network cable
takes the place of many serial cables and each graphic terminal uses
far more bandwidth than the text-terminals did.  However, graphic
terminals may be run in text mode to reduce the bandwidth required.  A
more detailed discussion of terminal servers (serial port) is in
Text-Terminal-HOWTO.  For networked terminal servers (not serial port)
see <url url="http://www.ltsp.org/index.php" name="Linux Terminal
Server Project (LTSP)">

(To-do: Discuss other types of serial servers, but the author knows
little about them.)

<sect>Configuring Overview
<p>In most cases configuring is automatically taken care of and you
have nothing to do.  Linux should detect your serial ports, and load
driver modules if needed.  Then the driver should insure that IRQ and
address space resources have been allocated.  Finally, the application
program which uses the serial port should set the port speed, etc.

For any of this to work, serial support must either be compiled into
the kernel (by you or by whoever compiled your kernel) or provided by
modules that are loaded into the kernel when you start to use the
serial port.  In most cases, if it hasn't been compiled into the
kernel, a module(s) will do the job and Linux should hopefully
automatically find and load the correct modules.

But if you have more than 4 (or perhaps 2) serial ports, then the kernel
must be told this as it doesn't seem to do it automatically.  See <ref
id="nr_ports" name="Number of Serial Ports Supported">, <ref
id="kernel_conf" name="Kernel Configuration"> and. <ref
id="ser_module" name="Serial Modules">.

Once the proper support is in your kernel and modules, then The rest
of the configuring of the serial port should happen automatically.
This is done by the serial driver software often with help from your
application software.  But sometimes it doesn't get configured right
and then you need to do it yourself.  Or perhaps you need to configure
it in a special way, etc.  This HOWTO only covers configuration of the
serial port itself and not the configuring of any devices attached to
the port (such as a modem or printer).

Resource allocation (locating the hardware or low-level configuring)
is assigning each port an IO address, IRQ, and name (such as ttyS2).
This IO-IRQ pair must be set in both the hardware and 
become known to the serial driver.  We might just call this "io-irq"
configuring for short.  The "setserial" program is sometimes used to
tell the driver io-irq info that an administrator has put in a
configuration file or given as parameters to the setserial command.
PnP methods, jumpers, etc, are used to set the IO and IRQ in the
hardware.  Details will be supplied later.  If you need to configure
but don't understand certain details it's easy to get into trouble.
See <ref id="locate_port" name="Locating the Serial Port: IO address
IRQs"> <ref id="set_serial" name="What is Setserial">

The second part (high-level configuring) is assigning it a speed (such
as 115.2k bits/sec), selecting flow control, etc.  This is often
initiated by communication programs such as wvdial, PPP, minicom,
picocom or by getty (which you may run on the port so that others may
log into your computer from an old-fashioned dumb terminal connected
to the port).  However you will need to tell these programs what speed
you want, etc.  by using a menu or a configuration file.  This
high-level configuring may also be done manually with the <tt/stty/
program.  <tt/stty/ is also useful to view the current status if
you're having problems.  See the section <ref id="stty_" name="Stty">

<sect>Locating the Serial Port: IO address, IRQs <label id="locate_port">
<!-- locate.D begin (in MM, SS)
<sect>Configuring the Serial Port
Change-log:
Aug. 2001: removed mention of patch to convert Linux to a PnP OS;
Better explanation of PCI
July 2001: Improve PCI part.  Remove "card" since serial ports are
often built into the motherboard.
Feb. 2003: Removed request to send info to Ted Tso.
Dec. 2003: May need to create /dev/ttyS4 even if its auto detected
Nov. 2004: Added: What Bus is my Serial Port On?  LPC: More work
Mar. 2006: Fixed typos, etc.
Jan. 2007: Fixed typos
 needed.
-->

<sect1>What Bus is my Serial Port On?
<p>If you need to find a serial port it often helps if you know what
bus it's on.  If the serial port is on a card, you may know what bus
the card inserts into such as a PCI slot.  But if the serial port is
built into the motherboard it may not be clear what bus it's on.  For
old motherboards that have ISA bus slots, it's likely on the ISA bus
and may not even be Plug-and-Play.  But even if all your slots are
PCI, the serial port is likely to be on either an ISA bus or LPC bus
(also called a "LPC interface").  LPC is common on laptop computers.
Type "lspci" to see if it shows "LPC".  Unfortunately, the LPC bus has
no standard Plug-and-Play method for low-level configuring devices on
it.  But according to the specs, the BIOS is supposed to do such
configuring (using ACPI ??).  To see if you have a LPC bus, type
"lspci" and look for a LPC bridge or the like.

<sect1> IO & IRQ Overview
<p> For a serial port to work properly it first must be given both an
IO address and an IRQ.  For old hardware (of mid 1990s), jumpers on a
card or a saved BIOS setting does it.  For newer hardware the BIOS
or Linux must set them at boot-time, and the new hardware doesn't
remember how it was set once it's powered down.  Enabling the hardware
(by using software) gives it both an IRQ and an IO address.  Without
an IO address, it can't be used.  Without an IRQ it will need to use
inefficient polling methods for which one must set the IRQ to 0 in the
serial driver.  Using digital signals sent to the hardware by the BIOS
or Linux, it all should get configured automatically (provided the
BIOS has not been previously set up to disable it).  So you only
need to read this if you're having problems or if you want to
understand how it works.

The driver must of course know both the IO address and IRQ so that it
can talk to the serial port chip.   Modern serial port drivers (kernel
2.4) try to determine this by PnP methods so one doesn't normally need
to tell the driver (by using "setserial").   A driver may also set an
IO address or IRQ in the hardware.  But unfortunately, there is some
PCI serial port hardware that the driver doesn't recognize so you
might need to enable the port yourself.  See <ref id="pci_enabled"
name="PCI: Enabling a disabled port">

For the old ISA bus, the driver also probes likely serial port
addresses to see if there are any serial ports there.  This works for
the case of jumpers and sometimes works for a ISA PnP port when the
driver doesn't do ISA PnP (prior to kernel 2.4).

Locating the serial port by giving it an IRQ and IO address is
low-level configuring.  It's often automatically done by the serial
driver but sometimes you have to do it yourself.  What follows repeats
what was said above but in more detail.

The low-level configuring consists of assigning an IO address, IRQ,
and names (such as ttyS2) .  This IO-IRQ pair must be set in
both the hardware and told to the serial driver.  And the driver needs
to call this pair a name (such as ttyS2).  We could call this "io-irq"
configuring for short.  The modern way to do this is for the driver to
use PnP methods to detect/set the IO/IRQ and then remember what it
did.  An easy way for a driver to do this is for the driver to ask the
kernel to enable the device and then the kernel tells the driver what
IO/IRQ it has used.  The old way is using the "setserial" program to
tell the driver.  For jumpers, there is no PnP but the driver might
detect the port if the jumpers are set to the usual IO/IRQ.  If you
need to configure but don't understand certain details it's easy to
get into trouble.

When Linux starts, an effort is made to detect and configure
(low-level) the serial ports.  Exactly what happens depends on your
BIOS, hardware, Linux distribution, kernel version, etc.  If the
serial ports work OK, there may be no need for you to do any more
low-level configuring.

If you're having problems with the serial ports, then you may need to
do low-level configuring.  If you have kernel 2.2 or lower, 
then you need to do it if you:

<itemize>
<item> Plan to use more than 2 ISA serial ports
<item> Are installing a new serial port (such as an internal modem)
<item> One or more of your serial ports have non-standard IRQs or IO
  addresses
</itemize>

Starting with kernel 2.2 you may be able to use more than 2 serial
ports without doing any low-level configuring by sharing interrupts.
All PCI ports should support this but for ISA it only works for some
hardware.  It may be just as easy to give each port a unique interrupt
if they are available.  See <ref id="int_share-2.2" name="Interrupt
sharing and Kernels 2.2+">

The low-level configuring (setting the IRQ and IO address) seems to
cause people more trouble than the high-level stuff, although for many
it's fully automatic and there is no configuring to be done.  Until
the port is enabled and the serial driver knows the correct IRQ and IO
address, the port will not usually not work at all.  

A port may be disabled, either by the BIOS or by failure of Linux to
find and enable the port.  For modern ports (provided the BIOS hasn't
disabled them) manual PnP tools such as lspci should be able to find
them.  Applications, and utilities such as "setserial" and "scanport"
(Debian only ??) only probe IO addresses, don't use PnP tools, and
thus can't detect disabled ports. 

Even if an ISA port can be found by the probing done by the serial
driver it may work extremely slow if the IRQ is wrong.  See <ref
id="slow_" name="Extremely Slow: Text appears on the screen slowly
after long delays">.  PCI ports are less likely to get the IRQ wrong.

IO address, IRQs, etc. are called "resources" and we are thus
configuring certain resources.  But there are many other types of
"resources" so the term has many other meanings.  In summary, the
low-level configuring consists of enabling the device, giving it a
name (ttyS2 for example) and putting two values (an IRQ number and IO
address) into two places:

<enum>
<item> The device driver (done by PnP or "<tt/setserial/")
<item> Configuration registers of the serial port hardware itself,
done by PnP software (or jumpers on legacy hardware).
</enum>

You may watch the start-up (= boot-time) messages.  They are usually
correct.  But if you're having problems, your serial port may not show
up at all or if you do see a message from  "setserial" it may not
show the true configuration of the hardware (and it is not necessarily
supposed to).  See <ref id="boot_mesgs" name="I/O Address & IRQ:
Boot-time messages">.

<sect1> PCI Bus Support <label id="PCI_ser_conf">
<sect2>Introduction
<p>


If you have kernel 2.4 or better, then there should be support for PnP
for both the PCI and ISA buses (either built-in or by modules).  Some
PCI serial ports can be automatically detected and low-level
configured by the serial driver.  Others may not be.  

While kernel 2.2 supported PCI in general, it had no support for PCI
serial ports (although some people got them working anyway).  Starting
with kernel 2.4, the serial driver will read the id number digitally
stored in the serial hardware to determine how to support it (if it
knows how).  It should assign an I/O address to it, determine its
IRQ, etc.  So you don't need to use "setserial" for it.  

There is a possible problem if you don't use the device filesystem.
The driver may assign the port to say tt/ttyS4 per a boot-time
message (use <tt/dmesg/ to see it).  But if you don't have a "file"
/dev/ttyS4 then the port will not work.  So you will then need to
create it, using<newline>
<tt>cd /dev; ./MAKEDEV ttyS4</tt><newline>

<!--
<sect2> Requesting that future drivers support your serial port
<p>If you have a

PCI modem card you should be looking at Modem-HOWTO and not this
Serial-HOWTO.  If you just have a PCI serial port card (with no modem
on the card) but it will not work because the latest serial driver doesn't support
it, you can help in attempting to create a driver for it.  To do this
you'll need to contact the maintainer of the serial driver, Theodore
(Ted) Y. Ts'o.
 

Look at <url url="http://serial.sourceforge.net" name="Ted Ts'o's
site"> for the details of what you need to do.  Here's a summary of
what you need to do to help him.  You will need to email Ted Ts'o a
copy of the output of "lspci -vv" with full information about the
model and manufacturer of the PCI modem (or serial port).  Then he
will try to point you to a test driver which might work for it.  You
will then need to get it, compile it and possibly recompile your
kernel.  Then you will test the driver to see if it works OK for you
and report the results to Ted Ts'o.  If you are willing to do all the
above (and this is the latest version of this HOWTO) then email the
needed info to him at: <url url="mailto:tytso@mit.edu">.
-->
<sect2>More info on PCI
<p>PCI ports are not well standardized.  Some use main memory for
communication with the PC.  Some require special enabling of the IRQ.
The output of "lspci -vv" can help determine if one can be supported.
If you see a 4-digit IO port, the port might work by just telling
"setserial" the IO port and the IRQ.  
For example, if lspci shows IRQ 10, I/O at 0xecb8 and you decide to
name it ttyS2 then the command is:

setserial /dev/ttyS2 irq 10 port 0xecb8 autoconfig

Note that the boot-time message "Probing PCI hardware" means reading
the PnP configuration registers in the PCI devices which detects info
about all PCI cards and on-board PCI devices.  This is different than
the probing of IO addresses by the serial driver which means reading
certain IO addresses to see if what's read looks like there's a serial
port at that address.

<sect1> Common mistakes made re low-level configuring
<p> Here are some common mistakes people make:
<itemize>
<item>setserial command: They run it (without the "autoconfig" and
 auto_irq options) and think it has checked the hardware to see if
 what it shows is correct (it hasn't).
<item>setserial messages:  They see them displayed on the screen at
 boot-time (or by giving the setserial command) and erroneously think
 that the result always shows how their hardware is actually
 configured.
<item>/proc/interrupts: When their serial device isn't in use they
 don't see its interrupt there, and erroneously conclude that their
 serial port can't be found (or doesn't have an interrupt set).
<item>/proc/ioports and /proc/tty/driver/serial: People think this
 shows the actual hardware configuration when it only shows about the
 same info (possibly erroneous) as setserial.
</itemize>

<sect1> IRQ & IO Address Must be Correct <label id="what_is_io_irq">
<p>There are really two answers to the question "What is my IO and
IRQ?" 1. What the device driver thinks has been set (This is what
setserial usually sets and shows.).  2. What is actually set in the
hardware.  Both 1. and 2. above should be the same.  If they're not it
spells trouble since the driver has incorrect info on the physical
serial port.  In some cases the hardware is disabled so it has no IO
address or IRQ.

If the driver has the wrong IO address it will try to send data to a
non-existing serial port --or even worse, to some other device.  If it
has the wrong IRQ the driver will not get interrupt service requests
from the serial port, resulting in a very slow or no response.  See
<ref id="slow_" name="Extremely Slow: Text appears on the screen
slowly after long delays">.  If it has the wrong model of UART there
is also apt to be trouble.  To determine if both IO-IRQ pairs are
identical you must find out how they are set in both the driver and
the hardware.

<sect1>  What is the IO Address and IRQ per the driver ?
<sect2> Introduction
<p>What the driver thinks is not necessarily how the hardware is
actually set.  If everything works OK then what the driver thinks is
likely correct (set in the hardware) and you don't need to investigate
(unless you're curious or want to become a guru).  Ways to determine
what the driver thinks include: boot-time messages <ref
id="boot_mesgs" name="I/O Address & IRQ: Boot-time messages">, the
/proc directory "files" <ref id="proc_dir" name="The /proc directory
and setserial">, and the "setserial" command.


<sect2>  I/O Address & IRQ: Boot-time messages <label id="boot_mesgs">
<p> In many cases your ports will automatically get low-level
configured at boot-time (but not always correctly).  To see what is
happening, look at the start-up messages on the screen.  Don't neglect
to check the messages from the BIOS before Linux is loaded (no
examples shown here).  These BIOS messages may be frozen by pressing
the Pause key (while holding down shift).  It's often tricky to freeze
them and you may need to hit Ctrl-Alt-Del while Linux is booting to
start rebooting and try again.  What these messages display may change
as booting progresses and it's often tricky to freeze it at exactly the
right words.

Use Shift-PageUp to scroll back to the messages after they have
flashed by.  Shift-PageDown will scroll in the opposite direction.
The <tt/dmesg/ command (or looking at logs in /var/log) will show only
the first of these two messages.  Here's an example of the start-up
messages (as of 2004, almost the same as for 1999).  Note that in
older versions ttyS1 was displayed in these messages as ttyS01, etc.

At first you see what was detected (but the irq is only a wild guess):

<tscreen><verb>
Serial driver version 4.27 with no serial options enabled
ttyS0 at 0x03f8 (irq = 4) is a 16550A
ttyS1 at 0x02f8 (irq = 3) is a 16550A
ttyS2 at 0x03e8 (irq = 4) is a 16550A
ttyS4 at port 0xeff0 (irq = 10) is a 16550A
</verb></tscreen>

Note that ttyS0-ttyS2 were detected by probing the standard addresses
while ttyS4 is a PCI port detected by probing the PCI configuration.
Later setserial shows you  what was saved in a configuration file
(which you may edit), but it's not necessarily correct either:

<tscreen><verb>
Loading the saved-state of the serial devices...
/dev/ttyS1 at 0x02f8 (irq = 3) is a 16550A
/dev/ttyS2 at 0x03e8 (irq = 5) is a 16550A
</verb></tscreen>
 
Note that the configuration file only had ttyS1-2 in it so that ttyS0
and ttyS4 were not affected by it.  There is also a slight
discrepancy: The first message shows ttyS2 at irq=4 while the second
shows it at irq=5.  In most cases the second message is the correct
one.  But if you're having trouble, it may be misleading.  Before reading
the explanation of all of this complexity in the rest of this section,
you might just try using your serial port and see if it works OK.  If
so it may not be essential to read further.

The second message is from the <tt/setserial/ program being run at
boot-time from a script in the /etc directory tree.  It shows what the
device driver thinks is the correct configuration.  But this too could
be wrong.  For example, the irq could actually be set to irq=8 in the
hardware (both messages wrong).  The irq=5 could be there because the
configuration file is incorrect.  

With old jumper-set serial ports Linux sometimes gets IRQs wrong
because it doesn't by default probe for IRQs.  It just assumes the
"standard" ones (first message) or accepts what is in a configuration
file (second message).  Neither of these is necessarily correct.  If
the serial driver has the wrong IRQ, the serial port is very slow or
doesn't seem to work at all.

The first message is a result of Linux probing the ISA serial port
addresses but it doesn't probe for IRQs.  If a port shows up here it
exists but the IRQ may be wrong.  Linux doesn't check IRQs because
doing so is not foolproof.  It just assumes the IRQs are as shown
because they are the "standard" values.  You may check them manually
with <tt/setserial/ using the <tt/autoconfig/ and <tt/auto_irq/
options but this isn't guaranteed to be correct either.

The data shown by the BIOS messages (which you see at first before
Linux is booted) are what is initially set in the hardware.  If your
serial port is Plug-and-Play (PnP) then it's possible that "isapnp" or
"setpci" will run and change these settings.  Look for messages about
this after Linux starts.  The last serial port message shown in the
example above should agree with the BIOS messages (as possibly
modified by isapnp or setpci).  If they don't agree then you either
need to change the setting in the port hardware or use setserial to
tell the driver what is actually set in the hardware.

Also, if you have Plug-and-Play (PnP) serial ports, they can only be
found by PnP software unless the IRQ and IO has been set inside the
hardware by Plug-and-Play software.  Prior to kernel 2.4 this was a
common reason why the start-up messages did not show a serial port
that physically exists.  A PnP BIOS may automatically low-level
configure them.  PnP configuring will be explained later.

<sect2> The /proc directory and setserial <label id="proc_dir">	
<p> Type "setserial -g /dev/ttyS*".   There are some other
ways to find this info by looking at "files" in the /proc directory.
Be warned that there is no guarantee that the same is set in the
hardware.

<tt>/proc/ioports</tt> will show the IO addresses that the drivers are using.
<tt>/proc/interrupts</tt> shows the IRQs that are used by drivers of
currently running processes (that have devices open).  It shows how
many interrupts have actually be issued.
<tt>/proc/tty/driver/serial</tt> shows much of the above, plus the
number of bytes that have been received and sent (even if the device
is not now open).

Note that for the IO addresses and IRQ assignments, you are only seeing
what the driver thinks and not necessarily what is actually set in the
hardware.  The data on the actual number of interrupts issued and
bytes processed is real however.  If you see a large number of
interrupts and/or bytes then it probably means that the device is (or
was) working.  But the interrupts might be from another device.  If
there are no bytes received (rx:0) but bytes were transmitted (tx:3749
for example), then only one direction of flow is working (or being
utilized).

Sometimes a showing of just a few interrupts doesn't mean that the
interrupt is actually being physically generated by any serial port.
Thus if you see almost no interrupts for a port that you're trying to
use, that interrupt might not be set in the hardware.  To view
/proc/interrupts to check on a program that you're currently running
(such as "minicom") you need to keep the program running while you
view it.

<sect1>What is the IO Address & IRQ of my Serial Port Hardware?
 <label id="io-irq_in_hdw">
<sect2>Introduction
<p>If it's PCI or ISA PnP then what's set in the hardware has been done
by PnP methods.  Even if nothing has been set or the port disabled,
PnP ports may still be found by using "lspci -v" or "isapnp
--dumpregs".  Ports disabled by jumpers (or hardware failures) are
completely lost.  See <ref id="isa_pnp_dump" name="ISA PnP ports">,
<ref id="pci_io_irq" name="PCI: What IOs and IRQs have been set?">,
<ref id="pci_enabled" name="PCI: Enabling a disabled port">

PnP ports don't store their configuration in the hardware when the
power is turned off.  This is in contrast to Jumpers (non-PnP) which
remain the same with the power off.  That's why a PnP port is more
likely to be found in a disabled state than an old non-PnP one.

<sect2> PCI: What IOs and IRQs have been set? <label id="pci_io_irq">
<p> For PCI, the BIOS almost always sets the IRQ and may set the IO
address as well.  To see how it's set use "lspci -vv" (best) or look
in /proc/bus/pci (or for kernels &lt;2.2 /proc/pci).  The modem's
serial port is often called a "Communication controller".  Look for
this.  If lspci shows "I/O ports at ... [disabled]" then the serial
port is disabled and the hardware has no IO address so it's lost and
can't be used.  See <ref id="pci_enabled" name="PCI: Enabling a
disabled port"> for how to enable it.

If more than one IO address is shown, the first one is more likely to
be it.  You can't change the IRQ (at least not with "setpci")   This
is because if one writes an IRQ it it's hardware register no action is
taken on it.  It's the BIOS that should actually set up the IRQs and
then write the correct value to this register for lspci to view.  If
you must, change the IO address with "setpci" by changing the
BASE_ADDRESS_0 or the like.  

<sect2> PCI: Enabling a disabled port <label id="pci_enabled">
<p>If the port communicates via an IO address then "lspci -vv" should
show "Control: I/O+ ..." with + meaning that the IO address is
enabled.  If it shows "I/O-" (and "I/O ports at ... [disabled]") then
you may need to use the setpci command to enable it.  For example
"setpci -d 151f:000 command=101".  151f is the vendor id, and 000 is
the device id both obtained from "lspci -n -v" or from /proc/bus/pci
or from "scanpci -v".  The "command=101" means that 101 is put into
the command register which is the same as the "Control" register
displayed by "lspci".  The 101h sets two bits: the 1 sets I/O to + and
the 100 part keeps SERR# set to +.   In this case only the SERR# bit
of the Control register was initially observed to be + when the lspci
command was run.  So we kept it enabled to + by setting bit 8 (where
bit 0 is I/O) to 1 by the first 1 in 101.  Some serial cards don't use
SERR# so if you see SERR#- then there's no need to enable it so then
use: command=1.  Then you'll need to set up "setserial" to tell the
driver the IO and IRQ.

Bit 8 is actually the 9th bit since we started counting bits from 0.
Don't be alarmed that lspci shows a lot of - signs showing that the
card doesn't have many features available (or enabled).  Serial ports
are relatively slow and don't need these features.

Another way to enable it is to let the BIOS do it by telling the BIOS
that you don't have a plug-and-play operating system.  Then the BIOS
should enable it when you start your PC. If you have MS Windows9x on
the same PC then doing this might cause problems with Windows (see
Plug-and-Play-HOWTO).

<sect2>ISA PnP ports <label id="isa_pnp_dump">
<p>For an ISA Plug-and-Play (PnP) port one may try the <tt/pnpdump/
program (part of <tt/isapnptools/).  If you use the --dumpregs option
then it should tell you the actual IO address and IRQ set in the port.
It should also find an ISA PnP port that is disabled.  The address it
"trys" is not the device's IO address, but a special address used for
communicating with PnP cards.

<sect2>Finding a port that is not disabled (ISA, PCI, PnP, non-PnP)
<p> Perhaps the BIOS messages will tell you some info before Linux
starts booting.  Use the shift-PageUp key to step back thru the
boot-time messages and look at the very first ones which are from the
BIOS.  This is how it was before Linux started.  Setserial can't
change it but isapnp or setpci can.  Starting with kernel 2.4, the
serial driver can make such changes for many (but not all) serial
ports.

Using "scanport" (Debian only ??) will probe all I/O ports and will
indicate what it thinks may be serial port.  After this you could try
probing with setserial using the "autoconfig" option.  You'll need to
guess the addresses to probe at (using clues from "scanport").  See
<ref id="set_serial" name="What is Setserial">.

For a port set with jumpers, the IO ports and IRQs are set per the
jumpers. If the port is not Plug-and-Play (PnP) but has been setup by
using a DOS program, then it's set at whatever the person who ran that
program set it to.

<sect2>Exploring via MS Windows (a last resort)
<p>For PnP ports, checking on how it's configured under DOS/Windows
may (or may not) imply how it's under Linux.  MS Windows stores its
configuration info in its Registry which is not used by Linux so they
are not necessarily configured the same.  If you let a PnP BIOS
automatically do the configuring when you start Linux (and have told
the BIOS that you don't have a PnP operating system when starting
Linux) then Linux should use whatever configuration is in the BIOS's
non-volatile memory.  Windows also makes use of the same non-volatile
memory but doesn't necessarily configure it that way.

<sect1>Choosing Serial IRQs <label id="choose_IRQ">

<p> If you have Plug-and-Play ports then either a PnP BIOS or a
serial driver may configure all your devices for you so then you may
not need to choose any IRQs.  PnP software determines what it thinks
is best and assigns them (but it's not always best).  But if you
directly use isapnp (ISA bus) or jumpers then you have to choose.  If
you already know what IRQ you want to use you could skip this section
except that you may want to know that IRQ 0 has a special use (see the
following paragraph).

<sect2> IRQ 0 is not an IRQ
<p> While IRQ 0 is actually the timer (in hardware) it has a special
meaning for setting a serial port with setserial.  It tells the driver
that there is no interrupt for the port and the driver then will use
polling methods.  Such polling puts more load on the CPU but can be
tried if there is an interrupt conflict or mis-set interrupt.  The
advantage of assigning IRQ 0 is that you don't need to know what
interrupt is set in the hardware.  It should be used only as a
temporary expedient until you are able to find a real interrupt to
use.

<sect2> Interrupt sharing,  Kernels 2.2+ <label id="int_share-2.2">
<p> Sharing of IRQs is where two devices use the same IRQ.  As a
general rule, this wasn't allowed for the ISA bus.   The PCI bus may
share IRQs but one can't share the same IRQ between the ISA and the
PCI bus.  Most multi-port boards may share IRQs.  Sharing is not as
efficient since every time a shared interrupt is given a check must be
made to determine where it came from.  Thus if it's feasible, it's
nicer to allocate every device its own interrupt.

Prior to kernel 2.2, serial IRQs could not be shared with each other
except for most multiport boards.  Starting with kernel 2.2 serial
IRQs may be sometimes shared between serial ports.  In order for
sharing to work in 2.2 the kernel must have been compiled with
CONFIG_SERIAL_SHARE_IRQ, and the serial port hardware must support
sharing (so that if two serial cards put different voltages on the
same interrupt wire, only the voltage that means "this is an
interrupt" will prevail).  Since the PCI bus specs permit sharing, any
PCI card should allow sharing.

<sect2> What IRQs to choose?

<p> The serial hardware often has only a limited number of IRQs.  Also
you don't want IRQ conflicts.  So there may not be much of a choice.
Your PC may normally come with <tt/ttyS0/ and <tt/ttyS2/ at IRQ 4, and
<tt/ttyS1/ and <tt/ttyS3/ at IRQ 3.  Looking at
<tt>/proc/interrupts</tt> will show which IRQs are being used by
programs currently running.  You likely don't want to use one of
these.  Before IRQ 5 was used for sound cards, it was often used for a
serial port.

Here is how Greg (original author of Serial-HOWTO) set his up in
/etc/rc.d/rc.serial.  rc.serial is a file (shell script) which runs at
start-up (it may have a different name or location).  For versions of
"setserial" after 2.15 it's not always done this way anymore but this
example does show the choice of IRQs.

<tscreen><verb>
/sbin/setserial /dev/ttyS0 irq 3	# my serial mouse
/sbin/setserial /dev/ttyS1 irq 4	# my Wyse dumb terminal
/sbin/setserial /dev/ttyS2 irq 5	# my Zoom modem
/sbin/setserial /dev/ttyS3 irq 9	# my USR modem
</verb></tscreen>
<p>
Standard IRQ assignments:
<verb>
	IRQ  0    Timer channel 0 (May mean "no interrupt".  See below.)
	IRQ  1    Keyboard
	IRQ  2    Cascade for controller 2
	IRQ  3    Serial port 2
	IRQ  4    Serial port 1
	IRQ  5    Parallel port 2, Sound card
	IRQ  6    Floppy diskette
	IRQ  7    Parallel port 1
	IRQ  8    Real-time clock
	IRQ  9    Redirected to IRQ2
	IRQ 10    not assigned
	IRQ 11    not assigned
	IRQ 12    not assigned
	IRQ 13    Math co-processor
	IRQ 14    Hard disk controller 1
	IRQ 15    Hard disk controller 2
</verb>
<p>

There is really no Right Thing to do when choosing interrupts.  Try to
find one that isn't being used by the motherboard, or any other
boards.  2, 3, 4, 5, 7, 10, 11, 12 or 15 are possible choices.  Note
that IRQ 2 is the same as IRQ 9.  You can call it either 2 or 9, the
serial driver is very understanding.  If you have a very old serial
board it may not be able to use IRQs 8 and above.

Make sure you don't use IRQs 1, 6, 8, 13 or 14!  These are used by
your motherboard.  You will make her very unhappy by taking her IRQs.
When you are done you might want to double-check
<tt>/proc/interrupts</tt> when programs that use interrupts are being
run and make sure there are no conflicts.

<sect1> Choosing Addresses --Video card conflict with ttyS3
<label id="choose_address">
<p> Here's a problem with some old serial cards.  The IO address of
the IBM 8514 video board (and others like it) is allegedly 0x?2e8
where ? is 2, 4, 8, or 9.  This may conflict with the IO address of
<tt/ttyS3/ at 0x02e8.  Your may think that this shouldn't happen since
the addresses are different in the high order digit (the leading 0 in
02e8).  You're right, but a poorly designed serial port may ignore the
high order digit and respond to any address that ends in 2e8.  That is
bad news if you try to use <tt/ttyS3/ (ISA bus) at this IO address.

For the ISA bus you should try to use the default addresses shown
below.  PCI cards use different addresses so as not to conflict with
ISA addresses.  The addresses shown below represent the first address
of an 8-byte range.  For example 3f8 is really the range 3f8-3ff.
Each serial device (as well as other types of devices that use IO
addresses) needs its own unique address range.  There should be no
overlaps (conflicts).  Here are the default addresses for commonly
used serial ports on the ISA bus:

<tscreen><verb>
ttyS0 address 0x3f8
ttyS1 address 0x2f8
ttyS2 address 0x3e8
ttyS3 address 0x2e8
</verb></tscreen>

Suppose there is an address conflict (as reported by <tt>setserial -g
/dev/ttyS*</tt>) between a real serial port and another port which
does not physically exist (and shows UART: unknown).  Such a conflict
shouldn't cause problems but it sometimes does in older kernels.  To
avoid this problem don't permit such address conflicts or delete
/dev/ttySx if it doesn't physically exist.

<sect1> Set IO Address & IRQ in the hardware (mostly for PnP)
 <label id="io-irq_methods">

<p> After it's set in the hardware don't forget to insure that it also
gets set in the driver by using <tt/setserial/.  For non-PnP serial
ports they are either set in hardware by jumpers or by running a DOS
program ("jumperless") to set them (it may disable PnP).  The rest of
this subsection is only for PnP serial ports.  Here's a list of the
possible methods of configuring PnP serial ports:

<itemize>
<item> Using a PnP BIOS CMOS setup menu
 (usually only for external
  devices
 on ttyS0 (Com1) and ttyS1 (Com2))
<item> Letting a PnP BIOS automatically configure a PnP serial port
 See <ref id="bios_conf" name="Using a PnP BIOS to IO-IRQ Configure">
<item> Doing nothing if the serial driver recognized your card OK
<item> Using <tt/isapnp/ for a PnP serial port (non-PCI)
<item> Using setpci (pciutils or pcitools) for the PCI bus
</itemize>

The IO address and IRQ must be set (by PnP) in their registers each
time the system is powered on since PnP hardware doesn't remember how
it was set when the power is shut off.  A simple way to do this is to
let a PnP BIOS know that you don't have a PnP OS and the BIOS will
automatically do this each time you start.  This might cause problems
in Windows (which is a PnP OS) if you start Windows with the BIOS
thinking that Windows is not a PnP OS.  See Plug-and-Play-HOWTO.

Plug-and-Play (PnP) was designed to automate this io-irq configuring,
but for Linux it initially made life much more complicated.  In modern
Linux (2.4 kernels --partially in 2.2 kernels), each device driver has
to do its own PnP (using supplied software which it may utilize).
There is unfortunately no centralized planning for assigning IO
addresses and IRQs as there is in MS Windows.   But it usually works
out OK in Linux anyway.

<sect2> Using a PnP BIOS to IO-IRQ Configure <label id="bios_conf">
<p> While the explanation of how to use setpci or isapnp for io-irq
configuring should come with such software, this is not the case if
you want to let a PnP BIOS do such configuring.  Not all PnP BIOS can
do this.  The BIOS usually has a CMOS menu for setting up the first
two serial ports.  This menu may be hard to find.  For an "Award"
BIOS it was found under "chipset features setup"  There is often
little to choose from.  For ISA serial ports, the first two ports
normally get set at the standard IO addresses and IRQs.  See <ref
id="dev_nos" name="More on Serial Port Names">

Whether you like it or not, when you start up a PC, a PnP BIOS starts
to do PnP (io-irq) configuring of hardware devices.  It may do the job
partially and turn the rest over to a PnP OS (which Linux is in some
sense) or if thinks you don't have a PnP OS it may fully configure all
the PnP devices but not configure the device drivers.

If you tell the BIOS that you don't have a PnP OS, then the PnP BIOS
should do the configuring of all PnP serial ports --not just the first
two.  An indirect way to control what the BIOS does (if you have
Windows 9x on the same PC) is to "force" a configuration under
Windows.  See Plug-and-Play-HOWTO and search for "forced".  It's
easier to use the CMOS BIOS menu which may override what you
"forced" under Windows.  There could be a BIOS option that can set or
disable this "override" capability.

If you add a new PnP device, the BIOS should PnP configure it.  It
could even change the io-irq of existing devices if required to avoid
any conflicts.  For this purpose, it keeps a list of non-PnP devices
provided that you have told the BIOS how these non-PnP devices are
io-irq configured.  One way to tell the BIOS this is by running a
program called ICU under DOS/Windows.

But how do you find out what the BIOS has done so that you set up the
device drivers with this info?  The BIOS itself may provide some info,
either in its setup menus of via messages on the screen when you turn
on your computer.  See <ref id="io-irq_in_hdw" name="What is set in my
serial port hardware?">.  Other ways of finding out is to use lspci for
the PCI bus or isapnp --dumpregs for the ISA bus.  The cryptic results
it shows you may not be clear to a novice.

<sect1> Giving the IRQ and IO Address to Setserial
<p> Once you've set the IRQ and IO address in the hardware (or arranged
for it to be done by PnP) you also need to insure that the "setserial"
command is run each time you start Linux.  See the subsection <ref
id="sets_boot_time" name="Boot-time Configuration">
<!-- configure.D end-->


<sect> Configuring the Serial Driver (high-level) "stty"
<label id="config_stty">
<sect1>Overview
<p>See the section <ref id="stty_" name="Stty">.  The "stty" command
sets many things such as flow control, speed, and parity.  The only
one discussed in this section is flow control.

<sect1> Flow Control
<p> Configuring Flow Control: Hardware Flow Control is Usually Best
See <ref id="flow_control" name="Flow Control"> for an explanation of
it.  It's usually better to use hardware flow control rather than
software flow control using Xon/Xoff.  To use full hardware flow
control you must normally have two dedicated wires for it in the cable
between the serial port and the device.  If the device is on a card or
the motherboard, then it should always be possible to use hardware
flow control.

Many applications (and the getty program) give you an option
regarding flow control and will set it as you specify or it might
enable hardware flow control by default if you don't set it.  It must
be set both in the serial driver and in the hardware connected to the
serial port.  How it's set into this hardware is hardware dependent.
Sometimes there is a certain "init string" you send to the hardware
device via the serial port from your PC.  For a modem, the
communication program should set it in both places.

If a program you use doesn't set flow control in the serial driver,
then you may do it yourself using the <tt/stty/ command.  Since the
driver doesn't remember the setting after you stop Linux, you could
put the stty command in a file that runs at start-up or when you login
(such as /etc/profile for the bash shell).  Here's what you would add
for hardware flow control for port ttyS2:

<tscreen><verb>
stty -F /dev/ttyS2 crtscts
</verb></tscreen>
or for old stty versions < 1.17:
<tscreen><verb>
stty crtscts < /dev/ttyS2
</verb></tscreen>

<tt/crtscts/ stands for a Control setting to use the RTS and CTS pins of
the serial port for hardware flow control.  Note that RTS+CTS almost
spells: <tt/crtscts/ and the initial "c" means "control".

<sect> Serial Port Devices /dev/ttyS2, etc. <label id="ttySN_">
<!-- serial_names.D begin
in Modem and Serial HOWTOs
Nov. 2003: rewrite
May 2003: PCI uses higher numbers
Nov. 2004: All-PCI slot motherboards may not use PCI serial ports
Apr. 2006: ttyS2 may be used on PCI cards instead of ttyS4 or ttyS14.
Aug. 2006: udev uses old names
Dec. 2006: cleaned up on above
Feb. 2010: moved devfs info to obsolete section of Serial-Howto
<sect> Serial Port Devices /dev/ttyS2, etc. -->

<sect1>Serial Port Names: ttyS4, etc
<p>Common serial port names are /dev/ttyS0, /dev/ttyS1, etc.  Then
around the year 2000 came the USB bus with names like /dev/ttyUSB0 and
/dev/ttyACM1 (for the ACM modem on the USB bus).  Multiport serial
card used somewhat differnt names (depending on the brand) such as
/dev/ttyE5.

<sect1>The PCI Bus
<p>Since DOS provided for 4 serial ports on the old ISA bus:
COM1-COM4, or ttyS0-ttyS3 in Linux, most serial ports on the newer PCI
bus used higher numbers such as ttyS4 or ttyS14 (prior to kernel
2.6.13).  But since most PCs only came with one or two serial ports,
ttyS0 and possibly ttyS1 (for the second port) the PCI bus can now use
ttyS2 (kernel 2.6.15 on).  All this permits one to have both ISA
serial ports and PCI serial ports on the same PC with no name
conflicts.  0-1 (or 0-3) are reserved for the old ISA bus (or the
newer LPC bus) and 2-upward (or 4-upward or 14-upward) are used for
PCI, where older schemes are shown in parentheses .  It's not required
to be this way but it often is.

If you're using udev (which puts only the device you have on your
computer into the /dev directory at boottime) then there's an easy way
to change the device names by editing files in /etc/udev/.  For
example, to change the name of what the kernel detects as ttyS3 to
what you want to name it: ttyS14, add a line similar to this to
/etc/udev/udev.rules<newline> <em/BUS=="pci" KERNEL=="ttyS3",
NAME="ttyS14"/

On-board serial ports on motherboards which have both PCI and ISA
slots are likely to still be ISA ports.  Even for all-PCI-slot
motherboards, the serial ports are often not PCI.  Instead, they are
either ISA, on an internal ISA bus or on a LPC bus which is intended
for slow legacy I/O devices: serial/parallel ports and floppy drives.


<sect1>Serial Port Device Names & Numbers 
<p>Devices in Linux have major and minor numbers.  The serial port
ttySx (x=0,1,2, etc.) is major number 4.  You can see this (and the
minor numbers too) by typing: "ls -l ttyS*" in the /dev directory.  To
find the device names for various devices, see the "devices" file in
the kernel documentation.

There formerly was a "cua" name for each serial port and it behaved
just a little differently.  For example, ttyS2 would correspond to
cua2.  It was mainly used for modems.  The cua major number was 5 and
minor numbers started at 64.  You may still have the cua devices in
your /dev directory but they are now deprecated.  For details see
Modem-HOWTO, section: cua Device Obsolete.

For creating the old devices in the device directory see:

<ref id="create_dev" name="Creating Devices In the /dev directory">

<sect1>More on Serial Port Names <label id="dev_nos">
<p>Dos/Windows use the COM name while the messages from the serial driver
use ttyS00, ttyS01, etc.  Older serial drivers (2001 ?) used just
tty00, tty01, etc.  

<p>The tables below shows some examples of serial device names.  The
IO addresses are the default addresses for the old ISA bus (not for
the newer PCI and USB buses).

<tscreen><verb>
dos     common                  IO       USB-BUS ( ACM => acm modem )
name     name     major minor address || common name      common name
COM1   /dev/ttyS0  4,  64;   3F8      || /dev/ttyUSB0  |  /dev/ttyACM0
COM2   /dev/ttyS1  4,  65;   2F8      || /dev/ttyUSB1  |  /dev/ttyACM1
COM3   /dev/ttyS2  4,  66;   3E8      || /dev/ttyUSB2  |  /dev/ttyACM2
COM4   /dev/ttyS3  4,  67;   2E8      || /dev/ttyUSB3  |  /dev/ttyACM3
 -     /dev/ttyS4  4,  68;   various
</verb></tscreen>

<sect1> USB (Universal Serial Bus) Serial Ports
<p> For more info see the usb subdirectory in the kernel documentation
directory for files: usb-serial, acm, etc.

<sect1> Link ttySN to /dev/modem
<p> On some installations, two extra devices will be created,
<tt>/dev/modem</tt> for your modem and <tt>/dev/mouse</tt> for a
mouse.  Both of these are symbolic links to the appropriate
device in <tt>/dev</tt>.

Historical note: Formerly (in the 1990s) the use of
<tt>/dev/modem</tt> (as a link to the modem's serial port) was
discouraged since lock files might not realize that it was really say
<tt>/dev/ttyS2</tt>.  The newer lock file system doesn't fall into
this trap so it's now OK to use such links.

<!-- serial_names.D end -->


<sect1> Which Connector on the Back of my PC is ttyS1, etc? <label
id="identify_ttyS">
<sect2> Inspect the connectors
<p> Inspecting the connectors may give some clues but is often not
definitive.  The serial connectors on the back side of a PC are
usually DB connectors with male pins.  9-pin is the most common but
some are 25-pin (especially older PCs like 486s).  There may be one
9-pin (perhaps ttyS0 ??) and one 25-pin (perhaps ttyS1 ??).  For two
9-pin ones the top one might be ttyS0.

If you only have one serial port connector on the back of your PC,
this may be easy.  If you also have an internal modem, a program like
wvdial may be able to tell you what port it's on (unless it's a PnP
that hasn't been enabled yet).  A report from setserial (at
boot-time or run by you from the command line) should help you
identify the non-modem ports.

If you have two serial ports it may be more difficult.  You could have
only one serial connector but actually have 2 ports, one of which
isn't used (but it's still there electronically).  First check manuals
(if any) for your computer.  Look at the connectors for meaningful
labels.  You might even want to take off the PC's cover and see if
there are any meaningful labels on the card where the internal ribbon
serial cables plug in.  Labels (if any) are likely to say something like
"serial 1", "serial 2" or A, B.  Which com port it actually is will
depend on jumper or PnP settings (sometimes shown in a BIOS setup
menu).  But 1 or A are more likely to be ttyS0 with 2 or B ttyS1.

<sect2> Send bytes to the port
<p> Labels are not apt to be definitive so here's another method.  If
the serial ports have been configured correctly per setserial, then
you may send some bytes out a port and try to detect which connector
(if any) they are coming out of.  One way to send such a signal is to
copy a long text file to the port using a command like: cp
my_file_name /dev/ttyS1.  A voltmeter connected to the DTR pin (see
Serial-HOWTO for Pinout) will display a positive voltage as soon as
you give the copy command.

The transmit pin should go from several volts negative to a voltage
fluctuating around zero after you start sending the bytes.  If it doesn't
(but the DTR went positive) then you've got the right port but it's
blocked from sending.  This may be due to a wrong IRQ, -clocal being
set, etc.  The command "<tt>stty -F /dev/ttyS1 -a</tt>" should show
clocal (and not -clocal).  If not, change it to clocal.

Another test is to jumper the transmit and receive pins (pins 2 and 3
of either the 25-pin or 9-pin connector) of a test serial port.  Then
send something to each port (from the PCs keyboard) and see if it gets
sent back.  If it does it's likely the port with the jumper on it.
Then remove the jumper and verify that nothing gets sent back.  Note
that if "echo" is set (per stty) then a jumper creates an infinite
loop.  Bytes that pass thru the jumper go into the port and come right
back out of the other pin back to the jumper.  Then they go back in
and out again and again.  Whatever you send to the port repeats itself
forever (until you interrupt it by removing the jumper, etc.).	This
may be a good way to test it as the repeating test messages halt when
the jumper is removed.

As a jumper you could use a mini (or micro) jumper cable (sold in some
electronic parts stores) with mini alligator clips.  A small scrap of
paper may be used to prevent the mini clips from making electrical
contact where it shouldn't.  Metal paper clips can sometimes be bent
to use as jumpers.  Whatever you use as a jumper take care not to bend
or excessively scratch the pins.  To receive something from a port,
you can go to a virtual terminal (for example Alt-F2 and login) and
type something like "cp /dev/ttyS2 /dev/tty".  Then at another virtual
terminal you may send something to ttyS2 (or whatever) by "echo
test_message > /dev/ttyS2".  Then go back to the receive virtual
terminal and look for the test_message.  See <ref id="ser_elect_test"
name="Serial Electrical Test Equipment"> for more info.

<sect2> Connect a device to the connector
<p> Another way to try to identify a serial port is to connect some
physical serial device to it and see if it works.  But a problem here
is that it might not work because it's not configured right.  A serial
mouse might get detected at boot-time if connected.

You may put a device, such as a serial mouse (use 1200 baud), on a port
and then use minicom or picocom to communicate with that port.  Then
by clicking on the mouse, or otherwise sending characters with the
device, see if they get displayed.  It not you may have told picocom
the wrong port (such as ttyS0 instead of ttyS1) so try again.

<sect2> Missing connectors
<p> If the software shows that you have more serial ports than you
have connectors for (including an internal modem which counts as a
serial port) then you may have a serial port that has no connector.
Some motherboards come with a serial port with no cable or external
serial DB connector.  Someone may build a PC from this and decide not
to use this serial port.  There may be a "serial" connector and label
on the motherboard but no ribbon cable connects to its pins.  To use
this port you must get a ribbon cable and connector.  I've seen
different wiring arrangements for such ribbon cables so beware.

<sect1>Creating Devices In the /dev directory <label id="create_dev">
<p>
If you don't use devfs (which automatically creates such devices) and
don't have a device "file" that you need, you will have to create it.
Use the <tt/mknod/ command or with the MAKEDEV shell script.
Example, suppose you needed to create <tt/ttyS0/:

<tscreen><verb>
linux# mknod -m 666 /dev/ttyS0 c 4 64
</verb></tscreen>
The MAKEDEV script is easier to use.
See the man page for it.  For example, if you needed to make the
device for <tt>ttyS0</tt> you would just type:

<tscreen><verb>
linux# MAKEDEV ttyS0
</verb></tscreen>

If the above command doesn't work (and you are the root user), look
for the MAKEDEV script in the /dev directory and run it.

This handles the devices creation and should set the correct permissions.
For making multiport devices see <ref id="make_multi" name="Making
multiport devices in the /dev directory">.

<sect>Interesting Programs You Should Know About
<p> Most info on getty has been moved to Modem-HOWTO with a little info on
the use of getty with directly connected terminals now found in
Text-Terminal-HOWTO.

<sect1> Serial Monitoring/Diagnostics Programs <label id="serial_mon">
<p> A few Linux programs (and one "file") will monitor various modem
control lines and indicate if they are positive (1 or green) or
negative (0 or red).
<itemize>
<item> <url url="http://sourceforge.net/projects/serlook/files/" 
 name="serlook"> can snoop on serial line traffic (via a wiretap) and
 also send/receive on a serial line.
<item> The "file": /proc/tty/driver/serial lists those that are
 asserted (positive voltage)
<item> modemstat (Only works correctly on Linux PC consoles.)  Status
 monitored in a tiny window.  Color-coded and compact.  Must kill
 it (a process) to quit.
<item> statserial (Info displayed on entire screen)
<item> serialmon (Doesn't monitor RTS, CTS, DSR but logs other
  functions)
</itemize>
As of June 1998, I know of no diagnostic program in Linux for the
serial port.

<sect1> Changing Interrupt Priority
<p>
<itemize>
<item> <tt/irqtune/ will give serial port interrupts higher
 priority to improve performance.
<item> <tt/hdparm/ for hard-disk tuning may help some more.
</itemize>

<sect1>What is Setserial ? <label id="set_serial">
<!-- setserial.D  begin  (in MM TT SS)
<sect1>What is Setserial ? <label id="set_serial">
Change Log:
May 2000: <sect2> IRQs near end ttyS0 -> ttyS1 + clarity
Nov. 2000: auto_irq may work on the 2nd try
Dec. 2000: saving state of serial module
June 2001 OK to use setserial with Laptops
Nov. 2002 Debian's /var/lib/serial.conf
Nov. 2003 Major revision.  Plug-and-play dominates
May 2004 Old Debian 1999 bug reported by me removed (fixed in 1999)
Feb.2005 Where config files reside
 /var/lib/setserial/autoserial.conf
-->
<p> This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal.  There
are some minor differences, depending on which HOWTO it appears in.

<sect2>Setserial problems with linmodems, laptops
<p> 
If you have a Laptop (PCMCIA) don't use <tt/setserial/ until you
read <ref id="laptops_" name="Laptops: PCMCIA">.

<sect2> Introduction
<p><tt/setserial/ is a program used for the user to communicate with
the serial device driver.  You normally never need to use it, provided
that you only use the one or two serial ports that come as standard
equipment with a PC.  Even in other cases, most extra serial ports
should be auto-detected by modern kernels.  Except you'll need to use
setserial if you have an old ISA serial port set by jumpers on the
physical hardware or if your kernel (such as 2.2 or older) doesn't
both detect and set your add-on PCI serial ports.

<tt/setserial/ allows you (or a shell script) to talk to the serial
software.  But there's also another program, tt/stty/, that also deals
with the serial port and is used for setting the port speed, etc.

<tt/setserial/ deals with the lower-level configuring of the serial
port, such as dealing with IRQs (such as 5), port addresses (such as
3f8), and the like.  A major problem with it is that it can't
set or configure the serial port hardware: It can't set the IRQ or
port addresses into the hardware.  Furthermore, when it seemingly
reports the configuration of the hardware, it's sometimes wrong since
it doesn't actually probe the hardware unless you specifically tell it
to.  Even then, it doesn't do the modern type of bus probing and some
hardware may never be found by it.  Still, what it shows is right most
all the time but if you're having trouble getting a serial port to
work, then there's a fair chance it's wrong.

In olden days, when the IRQ and port address was set by jumpers on the
serial card, one would use <tt/setserial/ to tell the driver how these
jumpers were set.  Today, when plug-and-play methods detect how the
jumperless serial port is set, <tt/setserial/ is not really needed
anymore unless you're having problems or using old hardware.
Furthermore, if the configuration file used by <tt/setserial/ is
wrong, then there's trouble.  In this case, if you use <tt/setserial/
to try to find out how the port is configured, it may just repeat the
incorrect information in the configuration file.

<tt/setserial/ can sometimes be of help to find a serial port.  But 
it's only of use if you know the port address and use the right
options.  For modern ports, there's usually better ways to look for
them by plug-and-play methods.

Thus the name <tt/setserial/ is somewhat of a misnomer since it
doesn't set the I/O address nor IRQ in the hardware, it just "sets"
them in the driver software.  And the driver naively believes that
what <tt/setserial/ tells it, even if it conflicts with what the driver
has found by using plug-and-play methods.  Too bad that it fails to
at least issue a warning message for such a conflict.  Since the
device driver is considered to be part of the kernel, the word
"kernel" is often used in other documentation with no mention made of
any "serial driver". 

Some distributions (and versions) set things up so that <tt/setserial/
is run at boot-time by an initialization shell script (in the
/etc directory tree).   But the configuration file which this script
uses may be either in the /etc tree or the /var tree.  In some cases,
if you want <tt/setserial/ to run at boot-time, you may have to take
some action.  <tt/setserial/will not work without either serial
support built into the kernel or loaded as a module.  The module may
get loaded automatically if you (or a script) attempt to use
<tt/setserial/.

While <tt/setserial/ can be made to probe the hardware IO port
addresses to try to determine the UART type and IRQ, this has
severe limitations.  See <ref id="probing_ss" name="Probing">.  It
can't set the IRQ or the port address in the hardware of PnP or PCI
serial ports (but the plug-and-play features of the serial driver may
do this).  It also can't directly read the PnP data stored in
configuration registers in the hardware.  But since the device driver
can read these registers and setserial tells you what the device
driver thinks, it might be correct.  Or it could be telling you what
<tt/setserial/ had previously (and perhaps erroneously) told the
driver.  There's no way to know for sure without doing some other
checks.

The serial driver (for Linux Kernel 2.4+) looks for a few "standard"
legacy serial ports, for PnP ports on the ISA bus, and for all
supported port hardware on the PCI bus.  If it finds your ports
correctly, then there's no need to use <tt/setserial/.  The driver
doesn't probe for the IRQs of old ISA serial ports set with jumpers on
the card and may get these wrong.

Besides the man page for <tt/setserial/, check out info in
<tt>/usr/doc/setserial.../</tt> or <tt>/usr/share/doc/setserial</tt>.
This should tell you how setserial is handled for your distribution of
Linux.  While <tt/setserial/ behaves the same in all distributions,
the scripts for running it, how to configure such scripts (including
automatic configuration), and the names and locations of the script
files, etc., are all distribution-dependent.  

<sect2>Serial module unload
<p>If a serial module gets unloaded, the changes previously made by
<tt/setserial/ will be forgotten by the driver.  But while the driver
forgets it, a script provided by the distribution may save it in a
file somewhere so that it can the restored if the module is reloaded.

<sect2>Slow baud rates of 1200 or less
<p>There once was a problem with slow serial printers (especially the
old ones of the 1980s).  The printing program would close the serial
port at the "end" of printing well before all the characters from the
large serial buffer (in main memory) were sent to the printer.  The
result was a truncated print job that didn't print the last paragraph
or last page, etc.

But the newer lprng print program (and possibly other printing
programs) keeps the port open until printing is finished so "problem
solved", even if you're using an antique printer.  Setserial can
modify the time that the port will keep operating after it's closed
(in order to output any characters still in its buffer in main RAM).
This is done by the "closing_wait" option per the <tt/setserial/ man
page.  For "bad" software that closes the port too soon, it might also
be needed at speeds above 1200 if there are a lot of "flow control"
waits.  

<sect2>Giving the <tt/setserial/ command
<p>Remember, that <tt/setserial/ can't set any I/O addresses or IRQs
in the hardware.  That's done either by plug-and-play software (run by
the driver) or by jumpers for legacy serial ports.  Even if you give
an I/O address or IRQ to the driver via <tt/setserial/ it will not set
such values and assumes that they have already been set.  If you give
it wrong values, the serial port will not work right (if at all).

For legacy ports, if you know the I/O address but don't know the IRQ
you may command setserial to attempt to determine the IRQ.

You can see a list of possible commands by just typing <tt/setserial/
with no arguments.  This fails to show you the one-letter options such
as -v for verbose which you should normally use when troubleshooting.
Note that setserial calls an IO address a "port".  If you type:
<tscreen><verb>
setserial -g /dev/ttyS*
</verb></tscreen>
You'll see some info about how the device driver is configured for
your ports.  In many cases you'll see some ports displayed with what
appears at first glance to be erroneous IRQs and addresses.  But if
you also see: <tt>"UART: unknown"</tt> just ignore the entire line
since no serial port exists at that address. 

If you add -a to the option -g you will see more info although few
people need to deal with (or understand) this additional info since
the default settings you see usually work fine.  In normal cases the
hardware is set up the same way as "setserial" reports.  But if you are
having problems there is a good chance that <tt/setserial/ has it wrong.
In fact, you can run "setserial" and assign a purely fictitious I/O
port address, any IRQ, and whatever uart type you would like to have.
Then the next time you type "setserial ..." it will display these
bogus values you've supplied to the driver.  They will also be officially
registered with the kernel as displayed (at the top of the screen) by
the "scanport" command (Debian).  Of course the serial
port driver will not work correctly (if at all) if you attempt to use
such a port.  Thus, when giving parameters to <tt/setserial/, "anything
goes".  Well almost.  If you assign one port a base address that is
already assigned (such as 3e8) it may not accept it.  But if you use
3e9 it will accept it.  Unfortunately 3e9 is actually assigned since it
is within the range starting at base address 3e8.  Thus the moral of
the story is to make sure your data is correct before assigning
resources with setserial.

<sect2>Configuration file
<p>While assignments made by setserial are lost when the PC is powered
off, a configuration file may restore them when the PC is started
up again.  In newer versions, what you change by setserial might get
automatically saved to a configuration file.  When <tt/setserial/ runs
it uses the info from the configuration file.

Where this configuration file resides depends on your distribution.
Look at the start-up scripts somewhere in the /etc/ tree (such as
/etc/init.d/ or /etc/rc.d/) and read the startup script for "serial"
or "setserial" or the like.  It should show where the configuration
file(s) reside.  In Debian there are 4 options for use of this
configuration file:

<enum>
<item>Don't use this file at all.  At each boot, the serial driver
alone detects the ports and setserial doesn't ever run.  ("kernel" option)
<item>Save what <tt/setserial/ reports when the system is first
shutdown and put it in the configuration file.  After that, don't ever
make any changes to the configuration file, even if someone has made
changes by running the <tt/setserial/ command on the command line and
then shuts down the system. ("autosave-once" option)
<item>At every shutdown, save whatever <tt/setserial/ detects to the
configuration file.  ("autosave" option)
<item>Manually edit the configuration file to set the configuration.
Don't ever do any automatic saves to it. ("manual" option)
</enum>

In olden days (perhaps before 2000), there wasn't any configuration
file and the configuration was manually set (hard coded) inside the
shell script that ran <tt/setserial/.  See <ref id="old_sets_script"
name="Edit a script (prior to version 2.15)">.

<sect2> Probing <label id="probing_ss">
<p>You probe for a port with <tt/setserial/ only when you suspect that
it has been enabled (by PnP methods, the BIOS, jumpers, etc.).
Otherwise <tt/setserial/ probing will never find it since its address
doesn't exist.  A problem is where the software looks for a port at
specified I/O addresses.  Prior to probing with "setserial", one may
run the "scanport" (Debian) command to check all possible ports in one
scan.  It makes crude guesses as to what is on some ports but doesn't
determine the IRQ.  It's a fast first start.  It may hang your PC but
so far it's worked fine for me.  Note that non-Debian distributions
don't seem to supply "scanport".  Is there another scan program?

With appropriate options, <tt/setserial/ can probe (at a given I/O
address) for a serial port but you must guess the I/O address.  If you
ask it to probe for /dev/ttyS2 for example, it will only probe at the
address it thinks ttyS2 is at (2F8).  If you tell setserial that ttyS2
is at a different address, then it will probe at that address, etc.
See <ref id="probing_ss" name="Probing">

The purpose of such probing is to see if there is a uart there, and if
so, what its IRQ is.  Use <tt/setserial/ mainly as a last resort as
there are faster ways to attempt it such as wvdialconf to detect
modems, looking at very early boot-time messages, or using <tt>pnpdump
--dumpregs</tt>, or lspci -vv.  But if you want to detect hardware
with <tt/setserial/ use for example :<newline> <tt>setserial
/dev/ttyS2 -v autoconfig</tt><newline>
If the resulting message shows a uart type such as 16550A, then you're
OK.  If instead it shows "<tt/unknown/" for the uart type, then there
is supposedly no serial port at all at that I/O address.  Some cheap
serial ports don't identify themselves correctly so if you see
"<tt/unknown/" you still might have a serial port there.

Besides auto-probing for a uart type, setserial can auto-probe for
IRQ's but this doesn't always work right either.  In one case it first
gave the wrong irq but when the command was repeated it found the
correct irq.  In versions of setserial >= 2.15, the results of your
last probe test could be automatically saved and put into a
distribution-specific configuration file such as
<tt>/etc/serial.conf</tt> or <tt>/etc/sysconfig/serial</tt> or
<tt>/var/lib/setserial/autoserial.conf</tt> for Debian.  This will be
used next time you start Linux.  

It may be that two serial ports both have the same IO address set in
the hardware.  Of course this is not normally permitted for the ISA
bus but it sometimes happens anyway.  Probing detects one serial port
when actually there are two.  However if they have different IRQs,
then the probe for IRQs may show IRQ = 0.  For me, it only did this if
I first used <tt/setserial/ to give the IRQ a fictitious value.

<sect2>Boot-time Configuration <label id="sets_boot_time">
<p>While <tt/setserial/ may run via an initialization script,
something akin to <tt/setserial/ also runs earlier when the serial
module is loaded (or when the kernel starts the built-in serial driver
if it was compiled into the kernel).  Thus when you watch the start-up
messages on the screen it may look like it ran twice, and in fact it
has. 

If the first message is for a legacy port, the IRQs shown may be wrong
since it didn't probe for IRQs.  If there is a second report of serial
ports, it may the result of a script such as /etc/init.d/setserial.
It usually does no probing and thus could be wrong about how the
hardware is actually set.  It only shows configuration data that got
saved in a configuration files.  The old method, prior to setserial
2.15, was to manually write such data directly into the script.

When the kernel loads the serial module (or if the "module equivalent"
is built into the kernel) then all supported PnP ports are detected.
For legacy (non-PnP) ports, only <tt/ttyS{0-3}/ are auto-detected
and the driver is set to use only IRQs 4 and 3 (regardless of what
IRQs are actually set in the hardware).  No probing is done for IRQs
but it's possible to do this manually.  You see this as a boot-time
message just as if <tt/setserial/ had been run.

To correct possible errors in IRQs (or for other
reasons) there may be a script file somewhere that runs
<tt/setserial/.  Unfortunately, if this file has some IRQs wrong, the
kernel will still have incorrect info about the IRQs.  This file is
usually part of the initialization done at boot-time.  Whether it
runs or not depends on how you (and/or your distribution) have set
things up.  It may also depends on the runlevel.  

Before modifying a configuration file, you can test out a "proposed"
<tt/setserial/ command by just typing it on the command line.  In some
cases the results of this use of <tt/setserial/ will automatically get
saved somewhere such as /etc/serial.conf (or autoserial.conf or
serial) when you shutdown.  So if it worked OK (and solved your
problem) then there's no need to modify any configuration file.  See
<ref id="config_file" name="Configuration method using
/etc/serial.conf, etc.">.

<sect2> Edit a script (required prior to version 2.15)
 <label id="old_sets_script">
<p> This is how it was done prior to <tt/setserial/ 2.15 (1999)
The objective was to modify (or create) a script file in the /etc
tree that runs setserial at boot-time.  Most distributions provided
such a file (but it may not have initially resided in the /etc tree).

So prior to version 2.15 (1999) it was simpler.  All you did was edit
a script.  There was no /etc/serial.conf file (or the like) to
configure setserial.   Thus you needed to find the file that runs
"setserial" at boot time and edit it.  If it didn't exist, you needed
to create one (or place the commands in a file that ran early at
boot-time).  If such a file was currently being used it's likely
was somewhere in the /etc directory-tree.  But Redhat <6.0 has supplied it
in /usr/doc/setserial/ but you need to move it to the /etc tree before
using it.

The script <tt>/etc/rc.d/rc.serial</tt> was commonly used in the past.
The Debian distribution used <tt>/etc/rc.boot/0setserial</tt>.
Another file once used was <tt>/etc/rc.d/rc.local</tt> but it's may
not have run early enough.  It was reported that other processes may
try to open the serial port before rc.local ran resulting in serial
communication failure.  Later on it most likely was found in
/etc/init.d/ but wasn't normally intended to be edited.

If such a file was supplied, it likely contained a number of
commented-out examples.  By uncommenting some of these and/or
modifying them, you could set things up correctly.  It was important
use a valid path for <tt/setserial/, and a valid
device name.  You could do a test by executing this file manually
(just type its name as the super-user) to see if it works right.
Testing like this was a lot faster than doing repeated reboots to get
it right.

For versions >= 2.15 (provided your distribution implemented the
change, Redhat didn't at first) it may be more tricky to do since the
file that runs setserial on startup, /etc/init.d/setserial or the like
was not intended to be edited by the user.  See <ref id="config_file"
name="Configuration method using /etc/serial.conf, etc.">.

An example line in such a script was:
<tscreen><verb>
/sbin/setserial /dev/ttyS3 irq 5 uart 16550A  skip_test
</verb></tscreen>

or, if you wanted setserial to automatically determine the uart and the
IRQ for ttyS3 you would have used something like this:

<tscreen><verb>
/sbin/setserial  /dev/ttyS3 auto_irq skip_test autoconfig
</verb></tscreen>

This was done for every serial port you wanted to auto configure,
using a device name that really does exist on your machine.  In some
cases it didn't work right due to the hardware. 

<sect2> Configuration method using /etc/serial.conf, etc.
<label id="config_file">
<p> Prior to setserial version 2.15 (1999), the way to configure
setserial was to manually edit the shell-script that ran setserial at
boot-time.  See <ref id="old_sets_script" name="Edit a script (before
version 2.15)">.  This was simple, but the simple and clear method has
been changed to something that is unnecessarily complex.  Today the
script and configuration file are two different files instead of one.
This shell-script is not edited but gets its data from a configuration
file such as <tt>/etc/serial.conf</tt> (or
<tt>/var/lib/setserial/autoserial.conf</tt>).

Furthermore you may not even need to edit serial.conf (or the like)
because using the "setserial" command on the command line may
automatically cause serial.conf to be edited appropriately.  This was
done so that you may not need to edit any file in order to set up (or
change) what setserial does each time that Linux is booted.

What often happens is this:  When you shut down your PC the script
that ran "setserial" at boot-time is run again, but this time it only
does what the part for the "stop" case says to do:  It uses
"setserial" to find out what the current state of "setserial" is, and
it puts that info into the serial configuration file such as
<tt>serial.conf</tt>.  Thus when you run "setserial" to change
the serial.conf file, it doesn't get changed immediately but only when
and if you shut down normally.

Now you can perhaps guess what problems might occur.  Suppose you
don't shut down normally (someone turns the power off, etc.) and the
changes don't get saved.  Suppose you experiment with "setserial" and
forget to run it a final time to restore the original state (or make a
mistake in restoring the original state).  Then your "experimental"
settings are saved.  And worst of all, unless you know which options
were set in the configuration file, you don't know what will happen.
One option in Debian (and likely other distributions) is known as
"AUTOSAVE-ONCE" which saves changes only for the first time you make
them with the setserial command.

If the option "###AUTOSAVE###" is set and you manually edit
serial.conf, then your editing is destroyed when you shut down because
it gets changed back to the state of setserial at shutdown.  There is
a way to disable the changing of serial.conf at shutdown and that is
to remove "###AUTOSAVE###" or the like from first line of serial.conf.
In the Debian distribution, the removal of "###AUTOSAVE###" from the
first line was once automatically done after the first time you
shutdown just after installation.  To retain this effect the
"AUTOSAVE-ONCE" option was created which only does a save when time
the system is shut down for the first time (just after you install or
update the setserial program).

The file most commonly used to run setserial at boot-time (in
conformance with the configuration file) is now /etc/init.d/setserial
(Debian) or /etc/init.d/serial (Redhat), or etc.,  but it should not
normally be edited.  For 2.15, Redhat 6.0 just had a file
/usr/doc/setserial-2.15/rc.serial which you have to move to
/etc/init.d/ if you want setserial to run at boot-time.

To disable a port, use <tt/setserial/ to set it to "uart none".  This
will not be saved.  The format of /etc/serial.conf appears to be just
like that of the parameters placed after "setserial" on the command
line with one line for each port.  If you don't use autosave, you may
edit /etc/serial.conf manually.

In order to force the current settings set by setserial to be saved to
the configuration file (serial.conf) without shutting down, do what
normally happens when you shutdown: Run the shell-script
<tt>/etc/init.d/{set}serial stop</tt>.  The "stop" command will save
the current configuration but the serial ports still keep working OK.

In some cases you may wind up with both the old and new configuration
methods installed but hopefully only one of them runs at boot-time.
Debian labeled obsolete files with "...pre-2.15".

<sect2> IRQs

<p> By default, both ttyS0 and ttyS2 will share IRQ 4, while ttyS1 and
ttyS3 share IRQ 3.  But while sharing serial interrupts (using them in
running programs) is OK for the PCI bus, it's not permitted for the
ISA bus unless you: 1. have kernel 2.2 or better, and 2. you've
compiled in support for this, and 3. your serial hardware supports it.
See

<ref id="int_share-2.2" name="Interrupt sharing and Kernels 2.2+">

If you only have two serial ports, ttyS0 and ttyS1, you're still OK
since IRQ sharing conflicts don't exist for non-existent devices.

If you add a legacy internal modem (without plug-and-play) and retain
ttyS0 and ttyS1, then you should attempt to find an unused IRQ and set
it in your serial port (or modem card) and then use setserial to
assign it to your device driver.  If IRQ 5 is not being used for a
sound card, this could be used for a modem.

<sect2> Laptops: PCMCIA <label id="laptops_">
<p>If you have a Laptop, read PCMCIA-HOWTO for info on the serial
configuration.  For serial ports on the motherboard, setserial is used
just like it is for a desktop.  But for PCMCIA cards (such as a modem)
it's a different story.  The configuring of the PCMCIA system should
automatically run setserial so you shouldn't need to run it.  If you
do run it (by a script file or by /etc/serial.conf) it might be
different and cause trouble.  The autosave feature for serial.conf
shouldn't save anything for PCMCIA cards (but Debian did until
2.15-7).  Of course, it's always OK to use setserial to find out how
the driver is configured for PCMCIA cards.

<!-- setserial.D  end -->


<sect1> Stty <label id="stty_">
<!-- stty.D begin <sect1> Stty <label id="stty_">
In Serial and Text-Terminal
Feb. 2005: Put redirection info into Obsolete section
-->
<sect2> Introduction
<p> <tt/stty/ does much of the configuration of the serial port but
since application programs (and the getty program) often handle this,
you may not need to use it much.  It's handy if you're having problems
or want to see how the port is set up.  Try typing ``stty -a'' at your
terminal/console to see how it's now set.  Also try typing it without
the -a (all) for a short listing which shows how it's set different
than "normal" which is how it's set using the command <tt>"stty
sane"</tt>.  Don't try to learn all the setting unless you want to
become a serial historian since many of the settings are only for slow
antique dumb terminals of the 1970's.  Most of the defaults should
work OK.

<tt/stty/ is documented in the man pages with a more detailed account
in the info pages.  Type <tt>"man stty"</tt> or <tt>"info stty"</tt>.

Many of the stty options start with an "o" (output) or an "i" (input).
For example: <tt/onlcr/.  Output is the flow of bytes out of the
computer while input is the flow of bytes into the computer.  The
"point of view" is the computer, not the serial port or the device
connected to the serial port.  For example, received input data comes
in on a cable and goes to the serial port chip.  This chip, after
converting the bits from the serial to parallel representation, then
sends it (via a program read) to the large serial port buffer in main
computer memory.  Thus the chip has both input and output but since
it's input data to the computer, its output is considered to be input.
The situation is similar for output flowing thru this chip.  The
"input" and "output" refer to the direction of flow with respect to the
computer and not the serial port hardware (the chip).

Whereas <tt/setserial/ only deals with actual serial ports, stty is
used both for serial ports and for virtual terminals such as the standard
Linux text interface on a PC monitor.  For the PC monitor, many of the
stty settings are meaningless.  Changing the baud rate, etc. doesn't
appear to actually do anything.

Here are some of the items stty configures: speed (bits/sec), parity,
bits/byte, # of stop bits, strip 8th bit?, modem control signals, flow
control, break signal, end-of-line markers, change case, padding, beep
if buffer overrun?, echo what you type to the screen, allow background
tasks to write to terminal?, define special (control) characters (such
as what key to press for interrupt).  See the <tt/stty/ man or info
page for more details.  Also see the man page: <tt/termios/ which
covers the same options set by stty but (as of mid 1999) covers
features which the stty man page fails to mention.


With some implementations of getty (getty_ps package), the commands
that one would normally give to stty are typed into a getty
configuration file: /etc/gettydefs.  Even without this configuration
file, the getty command line may be sufficient to set things up so
that you don't need stty.

One may write C programs which change the stty configuration, etc.
Looking at some of the documentation for this may help one better
understand the use of the stty command (and its many possible
arguments).  Serial-Programming-HOWTO may be useful but it's outdated.
The manual page: termios contains a description of the C-language
structure (of type termios) which stores the stty configuration in
computer memory.  Many of the flag names in this C-structure are
almost the same (and do the same thing) as the arguments to the stty
command.

<sect2> Flow control options
<p> To set hardware flow control use "crtscts".  For software flow
control there are 3 settings: ixon, ixoff, and ixany.

ixany: Mainly for terminals.  Hitting any key will restart the flow
after a flow-control stop.  If you stop scrolling with the "stop
scroll" key (or the like) then hitting any key will resume scrolling.
It's seldom needed since hitting the "scroll lock" key again will do
the same thing.

ixon: Enables the port to listen for Xoff and to stop transmitting
when it gets an Xoff.  Likewise, it will resume transmitting if it gets
an Xon.

ixoff: enables the port to send the Xoff signal out the transmit line
when its buffers in main memory are nearly full.  It protects the
device where the port is located from being overrun.

For a slow dumb terminal (or other slow device) connected to a fast
PC, it's unlikely that the PC's port will be overrun.  So you seldom
actually need to enable ixoff.  But it's often enabled "just in case".

<sect2> Using stty at a "foreign" terminal
<p>How do you use stty to view or set a terminal other than the
terminal you are currently using?  It's usually impossible to do it if
the foreign terminal is in use and has a shell running on it.  In
other cases for dealing with say ttyS2 while typing at another
terminal (such as tty1) use <tt> stty -F /dev/ttyS2 ...</tt> (or
--file instead of F).  If ... is -a it displays all the stty settings
(-a means all).

But if the foreign terminal (ttyS2 in this example) has a shell
running on it, then what you see will likely be deceptive and trying
to set it will not work.  This problem exists for virtual terminals
also such as dealing with tty3 from tty1, etc.  See <ref
id="two_term_interfaces" name="Two interfaces at a terminal"> to
understand it.

<sect2> Two interfaces at a terminal <label id="two_term_interfaces">
<p> When using a shell (such as bash) with command-line-editing
enabled there are two different terminal interfaces (what you see when
you type stty -a).  When you type in modern shells at the command line
you have a temporary "raw" interface (or raw mode) where each
character is read by the command-line-editor as you type it.  Once you
hit the &lt;return&gt; key, the command-line-editor is exited and the
terminal interface is changed to the nominal "cooked" interface
(cooked mode) for the terminal.  This cooked mode lasts until the next
prompt is sent to the terminal (which is only a small fraction of a
second).  Note that one never gets to type any command in this cooked
mode but what was typed in raw mode on the command line gets read by
the shell while in cooked mode.

When a prompt is sent to the terminal, the terminal goes from "cooked"
to "raw" mode (just like it does when you start an editor such as vim.
The prompt signals starting the command-line editor.  The settings for
the "raw" mode are based only on the basic stty settings taken from the
"cooked" mode.  Raw mode keeps these setting but changes several other
settings in order to change the mode to "raw".  It is not at all based
on the settings used in the previous "raw" mode.  Thus if one uses
stty to change settings for the raw mode, such settings will be
permanently lost as soon as one hits the &lt;return&gt; key at the
terminal that has supposedly been "set".

Now when one types stty to look at the terminal interface, one may
either get a view of the cooked mode or the raw mode.  You need to
figure out which one you're looking at.  It you use stty from a
foreign terminal (other than the terminal you are currently typing
at) then you will see the raw mode settings.  Any changes made will
only be made to the raw mode and will be lost when someone presses
&lt;return&gt; at the foreign terminal you tried to "set".  But if you
type a stty command to view/change the configuration of the terminal
you are using, and then hit &lt;return&gt; it's a different story.
The &lt;return&gt; puts the terminal in cooked mode.  Your changes are
saved and will still be there when the terminal goes back into raw
mode (unless of course it's a setting not allowed in raw mode).

This situation can create problems.  For example, suppose you corrupt
your terminal interface.  To restore it you go to another terminal and
"stty -F dev/ttyS1 sane" (or the like).  It will not work!  Of course
you can try to type "stty sane ..." at the terminal that is corrupted
but you can't see what you typed.  All the above not only applies to
dumb terminals but to virtual terminals used on a PC Monitor as well
as to the terminal windows in X.  In other words, it applies to almost
everyone who uses Linux.

Luckily, when you start up Linux, any file that runs stty at boot-time
will likely deal with a terminal (or serial port with no terminal)
that has no shell running on it so there's no problem for this special
case.

<sect2> Where to put the stty command ? <label id="stty_where">
<p> Should you need to have <tt/stty/ set up the serial interface each
time the computer starts up then you need to put the <tt/stty/ command
in a file that will be executed each time the computer is started up
(Linux boots).  It should be run before the serial port is used
(including running getty on the port).  There are many possible places
to put it.  If it gets put in more than one place and you only know
about (or remember) one of those places, then a conflict is likely.
So make sure to document what you do.

One place to put it would be in the same file that runs setserial when
the system is booted.  The location is distribution and version
dependent.  It would seem best to put it after the setserial command
so that the low level stuff is done first.  If you have directories in
the /etc tree where every file in them is executed at boot-time
(System V Init) then you could create a file named "stty" for this
purpose.

<sect2> Obsolete redirection method
<p>Prior to about 2000 you needed to use the redirection operator "<"
if you wanted to use stty on a foreign terminal.  For example to use
stty on ttyS2 sitting at tty1 you would type: stty .... < /dev/ttyS2.
After 2000 (provided your version of setserial is >= 1.17 and stty >=
2.0) a better method was created using the -F option: stty -F
/dev/ttyS2.  This will work when the old redirection method fails.  

The old redirection example above makes ttyS2 the standard input to
stty.  This gives the stty program a link to the "file" ttyS2 so that
it may "read" it.  But instead of reading the bytes sent to ttyS2 as
one might expect, it uses the link to find the configuration settings
of the port so that it may read or change them.  In olden days, some
people tried to use ``stty ... &gt /dev/ttyS2'' to set the terminal.
This didn't work.  Instead, it takes the message normal displayed
by the stty command for the terminal you are on (say tty1) and sends
this message to ttyS2.  But it doesn't change any settings for ttyS2.

Here's a problem with the old redirection operator (which doesn't
happen if you use the newer -F option instead).  Sometimes when trying
to use stty, the command hangs and nothing happens (you don't get a
prompt for a next command even after hitting &lt;return&gt;).  This is
likely due to the port being stuck because it's waiting for one of the
modem control lines to be asserted.  For example, unless you've set
"clocal" to ignore modem control lines, then if no CD signal is
asserted the port will not open and stty will not work for it (unless
you use the newer -F option).  A similar situation seems to exist for
hardware flow control.  If the cable for the port doesn't even have a
conductor for the pin that needs to be asserted then there is no easy
way to stop the hang.

One way to try to get out of the above hang is to use the newer -F
option and set "clocal" and/or "crtscts" as needed.  If you don't have
the -F option then you may try to run some program (such as minicom) on
the port that will force it to operate even if the control lines say
not to.  Then hopefully this program might set the port so it doesn't
need the control signal in the future in order to open: clocal or
-crtscts.  To use "minicom" to do this you likely will have to
reconfigure minicom and then exit it and restart it.  Instead of all
this bother, it may be simpler to just reboot the PC or via using a
virtual terminal kill the process using "top" (or "ps" to get the
process number and then "kill" to kill that process.

The obsolete redirection method (which still works in later versions)
is to type ``stty ... &lt /dev/ttyS2''.   If the new method using -F
works but the obsolete one hangs, it implies that the port is hung due
to a modem control line not being asserted.  Thus the obsolete
redirection method might still useful for troubleshooting.
<!-- stty.D end -->


<sect1> What is isapnp ?
<p> <tt/isapnp/ is a program to configure Plug-and-Play (PnP) devices
on the ISA bus including internal modems.  It comes in a package
called "isapnptools" and includes another program, "pnpdump" which
finds all your ISA PnP devices and shows you options for configuring
them in a format which may be added to the PnP configuration file:
/etc/isapnp.conf.  The isapnp command may be put into a startup file
so that it runs each time you start the computer and thus will
configure ISA PnP devices.  It is able to do this even if your BIOS
doesn't support PnP.  See Plug-and-Play-HOWTO.

<sect1> Connecting two PCs together via serial ports
<p>This is where you run a serial cable (crossover type = null-modem
type) between the serial ports of two PCs.  Then how do you use this
line?  One way is for one PC to run login on the serial line and for
the other PC to run say minicom or picocom to emulate a terminal.  See
Text-Terminal-HOWTO.  There is no network protocol used in this case
and no error detection.

The other method is to run a network protocol on the line.  For
example, to use PPP in combination with TCP/IP see Serial Laplink
HOWTO.  Although this HOWTO doesn't mention the old program "slattach"
(serial line attach) it can put the serial line into a networking mode
using the protocol you select.  Protocols for slattach include PPP or
SLIP (an older protocol widely used prior to PPP).

The Debian package, net-tools, contains slattach.  SLIP is provided as
a kernel module or can be built into the kernel (2.2, 2.4, or 2.6).

<sect1>Connect the serial port to a fast network: ser2net
<p>ser2net is a Linux program which will connect a network to the
serial port.  For example, someone connects to your PC via an ethernet
port or fast modem using say telnet.  Then (without ser2net) they
could remotely login to your PC and then run programs on your PC that
utilize a serial port on the PC.  However, it might be better if they
didn't need to login and use your software, but instead could
immediately connect to the serial port.  ser2net running on your PC
can make this happen.

It could be like a bridge between the ethernet cable and the serial
cable.  The ethernet cable would have TCP/IP protocol on it but the
serial port would just have the raw data taken out of the TCP/IP
packets.  Optionally, you could use TCP/IP packets on the serial line
too.  Since an ethernet port has high bandwidth, it could communicate
with several serial ports at the same time and also have data flowing
elsewhere as well.

To set up ser2net, you must specify which network ports (on the
ethernet) will connect to which serial ports.  Then when network
packets arrive at your PC over ethernet which are addressed to a
network port you've tied to a serial port, the data in those packets
flows to the serial port.  And conversely.  Of course the network
doesn't have to be ethernet.  It could be a cable modem or DSL line,
etc.

<sect> Speed (Flow Rate) <label id="speed_">

<p> By "speed" we really mean the "data flow rate" but almost everybody
incorrectly calls it speed.  The speed is measured in bits/sec (or
baud).  Speed is set using the "stty" command or by a program which
uses the serial port.  See <ref id="stty_" name="Stty">

<sect1> Very High Speeds <label id="high_speed">
<!-- high_speed.D begin  In Serial and Modem HOWTOs but some of the speed
section is unique to each HOWTO.
Feb. 2003, 
-->
<sect2> Speeds over 115.2kbps
<p> The top speed of 115.2k has been standard since the mid 1990's.
But by the year 2000, most new serial ports supported higher speeds of
230.4k and 460.8k.  Some also support 921.6k.  Unfortunately Linux
seldom uses these speeds due to lack of drivers.  Thus such ports
behave just like 115.2k ports unless the higher speeds are enabled by
special software.  To get these speeds you need to compile the kernel
with special patches or use modules until support is built into the
kernel's serial driver.

Unfortunately serial port manufacturers never got together on a
standard way to support high speeds, so the serial driver needs to
support a variety of hardware.  Once high speed is enabled, a standard
way to choose it is to set baud_base to the highest speed with
setserial (unless the serial driver does this for you).  The software
will then use a divisor of 1 to set the highest speed.  All this will 
hopefully be supported by the Linux kernel sometime in 2003.

A driver for the w83627hf chip (used on many motherboards such as 
the Tyan S2460) is at <url url="https://www.muru.com/linux/w83627hf/">

A non-standard way that some manufacturers have implemented high speed
is to use a very large number for the divisor to get the high speed.
This number isn't really a divisor at all since it doesn't divide
anything.  It's just serves as a code number to tell the hardware what
speed to use.  In such cases you need to compile the kernel with
special patches. 

One patch to support this second type of high-speed hardware is called
shsmod (Super High Speed Mode).  There are both Windows and Linux
versions of this patch.  See <url
url="http://www.devdrv.com/shsmod/">.  There is also a module for the
VIA VT82C686 chip <url url="http://www.kati.fi/viahss/">.  Using it
may result in buffer overflow.  

For internal modems, only a minority of them advertise that they
support speeds of over 115.2k for their built-in serial ports.
Does shsmod support these ??

<sect2> How speed is set in hardware: the divisor and baud_base <label
 id="divisor_">
<p> Speed is set by having the serial port's clock change frequency.
But this change happens not by actually changing the frequency of the
oscillator driving the clock but by "dividing" the clock's frequency.
For example, to divide by two, just ignore every other clock tick.
This cuts the speed in half.  Dividing by 3 makes the clock run at 1/3
frequency, etc.  So to slow the clock down (meaning set speed), we
just send the clock a divisor.  It's sent by the serial driver to a
register in the port.  Thus speed is set by a divisor.

If the clock runs at a top speed of 115,000 bps (common), then here
are the divisors for various speeds (assuming a maximum speed of
115,200): 1 (115.2k), 2 (57.6k), 3 (38.4k), 6 (19.2k), 12 (9.6k), 24
(4.8k), 48 (2.4k), 96 (1.2k), etc.  The serial driver sets the speed
in the hardware by sending the hardware only a "divisor" (a positive
integer).  This "divisor" divides the "maximum speed" of the hardware
resulting in a slower speed (except a divisor of 1 obviously tells the
hardware to run at maximum speed).

There are exceptions to the above since for certain serial port
hardware, speeds above 115.2k are set by using a very high divisor.
Keep that exception in mind as you read the rest of this section.
Normally, if you specify a speed of 115.2k (in your communication
program or by stty) then the serial driver sets the port hardware to
divisor 1 which sets the highest speed.

Besides using a very high divisor to set high speed, the conventional
way to do it is as follows: If you happen to have hardware with a
maximum speed of say 230.4k (and the 230.4k speed has been enabled in
the hardware), then specifying 115.2k will result in divisor 1.  For
some hardware this will actually give you 230.4k.  This is double the
speed that you set.  In fact, for any speed you set, the actual speed
will be double.  If you had hardware that could run at 460.8k then the
actual speed would be quadruple what you set.  All the above assumes
that you don't use "setserial" to modify things.

<sect2> Setting the divisor, speed accounting
<p> To correct this accounting (but not always fix the problem) you
may use "setserial" to change the baud_base to the actual maximal
speed of your port such as 230.4k.  Then if you set the speed (by your
application or by stty) to 230.4k, a divisor of 1 will be used and
you'll get the same speed as you set.

If you have very old software which will not allow you to tell it such
a high speed (but your hardware has it enabled) then you might want to
look into using the "spd_cust" parameter.  This allows you to tell the
application that the speed is 38,400 but the actual speed for this
case is determined by the value of "divisor" which has also been set
in setserial.  I think it best to try to avoid using this kludge.

There are some brands of UARTs that uses a very high divisor to set
high speeds.  There isn't any satisfactory way to use "setserial" (say
set "divisor 32770") to get such a speed since then setserial would
then think that the speed is very low and disable the FIFO in the
UART.

<sect2> Crystal frequency is higher than baud_base
<p> Note that the baud_base setting is usually much lower than the
frequency of the crystal oscillator since the crystal frequency of say
1.8432 MHz is divided by 16 in the hardware to get the actual top
speed of 115.2k.  The reason the crystal frequency needs to be higher
is so that this high crystal speed can generate clock ticks to take a
number of samples of each  bit to determine if it's a 1 or a 0.

Actually, the 1.8432 MHz "crystal frequency" may be obtained from a
18.432 MHz crystal oscillator by dividing by 10 before being fed to
the UART.  Other schemes are also possible as long as the UART
performs properly.

<!-- high_speed.D end  -->


<sect1>Higher Serial Throughput <label id="higher_thruput">
<p>
If you are seeing slow throughput and serial port overruns on a
system with (E)IDE disk drives, you can get <tt>hdparm</tt>.  This
is a utility that can modify (E)IDE parameters, including unmasking
other IRQs during a disk IRQ.  This will improve responsiveness
and will help eliminate overruns.  Be sure to read the man page very
carefully, since some drive/controller combinations don't like this
and may corrupt the filesystem.
<p>
Also have a look at a utility called <tt>irqtune</tt> that will change
the IRQ priority of a device, for example the serial port that your
modem is on.  This may improve the serial throughput on your system.
The <tt/irqtune/ FAQ is at <url url="http://www.best.com/~cae/irqtune"
name="http://www.best.com/~cae/irqtune">

<sect> Locking Out Others
<sect1> Introduction
<p> When you are using a serial port, you may want to prevent others
from using it at the same time.  However there may be cases where you
do want others to use it, such as sending you an important message if
you are using a text-terminal.

There are various ways of preventing others (or other processes) from
using your serial port when you are using it (locking).  This should
all happen automatically but it's important to know about this if it
gives you trouble.  If a program is abnormally exited or the PC
is abruptly turned off (by pulling the plug, etc.) your serial port
might wind up locked.  Even if the lock remains, it's usually
automatically removed when you want to use the serial port again.
But in rare cases it isn't.  That's when you need to understand what
happened.

One way to implement locking is to design the kernel to handle it but
Linux thus far has shunned this solution (with an exception involving
the cua device which is now obsolete).  Two solutions used by Linux
is to:
<enum>
<item> create lock-files
<item> modify the permissions and/or owners of devices such as /dev/ttyS2
</enum>

<sect1>Lock-Files <label id="lockfiles_">
<p> If you use the new device-filesystem (devfs) then see the next
section.  A lock-file is simply a file created to mean that a
particular device is in use.  They are kept in <tt>/var/lock</tt>.
Formerly they were in <tt>/usr/spool/uucp</tt>.  Linux lock-files are
usually named <tt/LCK../<EM/name/, where <EM/name/ may be a device
name, a process id number, a device's major and minor numbers, or a
UUCP site name.  Most processes (an exception is getty) create these
locks so that they can have exclusive access to devices.  For instance
if you dial out on your modem, some lockfiles will appear to tell
other processes that someone else is using the modem.  In older
versions (in the 1990s) there was usually only one lockfile per
process.  Lock files contain the PID of the process that has locked
the device.  Note that if a process insists on using a device that is
locked, it may ignore the lockfile and use the device anyway.  This is
useful in sending a message to a text-terminal, etc.

When a program wants to use a serial port but finds it locked with
lock-files it should check to see if the lock-file's PID is still in
use.  If it's not it means that the lock is stale and it's OK to go
ahead and use the port anyway (after removing the stale lock-files).
Unfortunately, there may be some programs that don't do this and give
up by telling you that a device is already in use when it really isn't.

If the lockfile only uses device names, the following problem could
arise:  If the same device has two different names then two different
processes could each use a different name for the same device.  This
results in lockfiles with different names that actually are the same
device.  Formerly each physical serial port was known by two different
device names: ttyS0 and cua0.  To solve this lockfile alias problem, 3
methods have been used.  It may be overkill since any one of these
methods would have fixed the problem.

<enum>
<item> The lock checking software was made aware of ttyS vs. cua.
<item> The device cua was deprecated
<item> Additional locks were created which use unique device numbers
 instead of names.
</enum>

Using alternate names such as /dev/modem for /dev/ttyS2 may cause
problems if one program opens /dev/ttyS2 while another program opens
/dev/modem.  This problem was supposedly fixed around 2000, but is
still present in 2005.  For dumb terminals, lockfiles are not used
since this would not permit someone else to send a message to your
terminal using the write or talk program.


<sect1> Change Owners, Groups, and/or Permissions of Device Files
<p> In order to use a device, you (or the program you run if you have
"set user id") needs to have permission to read and write the device
"file" in the /dev directory.  So a logical way to prevent others from
using a device is to make yourself the temporary owner of the device
and set permissions so that no one else can use it.  A program may do
this for you.  A similar method can be used with the group of the
device file.

While lock files prevent other process from using the device, changing
device file owners/permissions restricts other users (or the group)
from using it.  One case is where the group is permitted to write to
the port, but not to read from it.  Writing to the port might just
mean a message sent to a text-terminal while reading means destructive
reading.  The original process that needs to read the data may find
data missing if another process has already read that data.  Thus a
read can do more harm that a write since a read causes loss of data
while a write only adds extra data.  That's a reason to allow writes
but not reads.  This is exactly the opposite of the case for ordinary
files where you allow others to read the file but not write (modify)
it.  Use of a port normally requires both read and write permissions.

A program that changes the device file attributes should undo these
changes when it exits.  But if the exit is abnormal, then a device
file may be left in such a condition that it gives the error
"permission denied" when one attempts to use it again.

<sect>Serial Communications Programs And Utilities<label id="comms">
<sect1> List of Software
<p>
Here is a list of some communication software you can choose from,
available via FTP, if they didn't come with your Linux distribution.

<itemize>
<item><tt/ecu/ - a communications program
<item><url url="http://www.columbia.edu/kermit/" name="C-Kermit"> -
 portable, scriptable, serial and TCP/IP communications including file
 transfer, character-set translation, and zmodem support
<item><tt>gkermit</tt> Tiny GPLed kermit run only from the command line.
 Can't connect to another computer
<item><tt/gtkterm/ - a simple gtk terminal, X-based
<item><tt/minicom/ - telix-like communications program
<item><tt/picocom/ - like a small minicom but no automatic phone dialing
<item><tt/pppd/ - establishes a ppp connection on the serial line
<item><tt/seyon/ - X based communication program
<item><tt/xc/ - xcomm communication package

<item><tt/term/ and <tt/SLiRP/ offer TCP/IP functionality using a
 shell account.

<item><tt/screen/ is another multi-session program.  This one behaves
 like the virtual consoles.

<item><tt/callback/ is where you dial out to a remote modem and then
 that modem hangs up and calls you back (to save on phone bills).

<item><tt/mgetty+fax/ handles FAX stuff, and provides an alternate
 <tt/ps_getty/.

<item><tt/ZyXEL/ is a control program for ZyXEL U-1496 modems.  It
 handles dialin, dialout, dial back security, FAXing, and voice
 mailbox functions.


<item>SLIP and PPP software (if not in your Linux distribution) can be
found at <tt> <htmlurl
url="ftp://metalab.unc.edu/pub/Linux/system/network/serial"
name="ftp://metalab.unc.edu/pub/Linux/system/network/serial"></tt>.
</itemize>

<sect1>kermit and zmodem
<p> For use of kermit with modems see the Modem-HOWTO.  One can run
zmodem within the kermit program.  To do this (for ttyS3), add the
following to your <tt/.kermrc/ file:
<tscreen><verb>
define rz !rz < /dev/ttyS3 > /dev/ttyS3
define sz !sz \%0 > /dev/ttyS3 < /dev/ttyS3
</verb></tscreen>
Be sure to put in the correct port your modem is on.  Then, to use it,
just type <tt/rz/ or <tt>sz &lt;filename&gt;</tt> at the <tt/kermit/
prompt.

<sect>Serial Tips And Miscellany

<sect1> Serial Modules <label id="ser_module">
<p> Often the serial driver is provided as a module(s) such as
generic_serial.ko.  Drivers for USB serial ports and multiport cards
are often provided as modules.  Linux should automatically load any
needed module, so in most cases you have nothing to do.  

But sometimes you need to configure Linux to load certain modules or
gives parameters to the module or to the kernel.

Such parameters may be supplied to certain modules on the command line
for the kernel or in /etc/modules, /etc/modules.conf or
/etc/modprobe.conf.  Since kernel 2.2 you don't edit the modprobe.conf
file but use the program update-modules to change it.  The info that
is used to update modules.conf is put in /etc/modutils/.  

The Debian/GNU Linux has a file named /etc/modutils/setserial which
runs the serial script in /etc/init.d/ every time the serial module is
loaded or unloaded.  When the serial module is unloaded this script
will save the state of the module in /var/run/setserial.conf.  Then if
the module loads again this saved state is restored.  When the serial
module first loads at boot-time, there's nothing in
/var/run/setserial.conf so the state is obtained from
/etc/serial.conf.  So there are two files that save the state.  Other
distributions may do something similar.

Serial modules are found in subdirectories of
<tt>/lib/modules/.../kernel/drivers/</tt>.  For multiport cards, look
in the <tt/serial/ subdirectory and/or <tt/char/.  For USB serial,
look in the <tt>usb/serial</tt> subdirectory.  The module,
parport_serial is for PCI cards that contain both serial and parallel
ports.  

As a last resort, one may modify the serial driver by editing the
source code.  Much of the serial driver is found in the file serial.c.
For info regarding writing of programs for the serial port see
Serial-Programming-HOWTO.  It was revised in 1999 by Vern Hoxie but
that revision is not at LDP.

<sect1>Kernel Configuration <label id="kernel_conf">


<sect1>Number of Serial Ports Supported <label id="nr_ports">
<p>If you have more than 4 (or possibly 2) serial ports, then you must
insure that the kernel knows this.  It can be done by configuring the
kernel when compiling or by a parameter given to the kernel when it
starts (boot-prompt or kernel command line).

The kernel configuration parameters:
CONFIG_SERIAL_8250_RUNTIME_UARTS=4 and CONFIG_SERIAL_8250_NR_UARTS=4
set the maximum number of ordinary serial ports (UARTs) equal to 4.
If you have more than 4 ordinary serial ports, then you need to change
the 4 to whatever.  But you may override this via the kernel command
line for example: nr_uarts=16 (if serial support built into the
kernel) or 8250.nr_uarts=16 (if serial support is via a module).  The
boot loader such as lilo or grub can be told to do this.

<sect1> Serial Console (console on the serial port)
<p> See the kernel documentation in: Documentation/serial-console.txt.
Kernel 2.4+ has better documentation.  See also "Serial Console" in
Text-Terminal-HOWTO.

<sect1> Line Drivers
<p> For a text terminal, the RS-232 speeds are fast enough but the
usable cable length is often too short.  Balanced technology could
fix this.  The common method of obtaining balanced communication with
a text terminal is to install 2 line drivers in the serial line to
convert unbalanced to balanced (and conversely).  They are a
specialty item and are expensive if purchased new.

<sect1> Stopping the Data Flow when Printing, etc.
<p> Normally flow control and/or application programs stop the flow of
bytes when its needed.  But sometimes they don't.  The problem is that
output to the serial port first passes thru the large serial buffer
in the PC's main memory.  So if you want to abort printing, whatever is
in this buffer should be removed.  When you tell an application program
to stop printing, it may not empty this buffer so printing continues
until it's empty.  In addition, your printer has it's own buffer which
needs to be cleared.  So telling the PC to stop printing may not work
due to these two buffers that continue to supply bytes for the printer.
It's a problem with printer software not knowing about the serial port
and that modem control lines need to be dropped to stop the printer.

One way to insure that printing stops is to just turn off the printer.
With newer serial drivers, this works OK.  The buffers are cleared and
printing doesn't resume.  With older serial drivers, the PC's serial
buffer didn't clear and it would sometimes continue to print when the
printer was turned back on.  To avoid this, you must wait a time
specified by setserial's closing_wait before turning the printer back
on again.  You may also need to remove the print job from the print
queue so it won't try to resume.

<sect1>Known IO Address Conflicts
<sect2>Avoiding IO Address Conflicts with Certain Video Boards <label
 id="video_8514">
<p> The IO address of the IBM 8514 video board (and others) is
allegedly 0x?2e8 where ? is 2, 4, 8, or 9.  This may conflict (but
shouldn't if the serial port is well designed) with the IO address of
<tt/ttyS3/ at 0x02e8 if the serial port ignores the leading 0 hex
digit when it decodes the address (many do).  That is bad news if you
try to use <tt/ttyS3/ at this IO address.  Another story is that Linux
will not detect your internal modem on <tt/ttyS3/ but that you can use
<tt>setserial</tt> to put <tt/ttyS3/ at this address and the modem
will work fine.
<sect2>IO address conflict with ide2 hard drive
<p>The address of ttyS2 is 3e8-3ef while hard drive ide2 uses 3ee
which is in this range.  So when booting Linux you may see a report
of this conflict.  Most people don't use ide2 (the 3rd hard drive
cable) and may ignore this conflict message.  You may have 2 hard
drives on ide0 and two more on ide1 so most people don't need ide2.

<sect1> Known Defective Hardware
<sect2> Problem with AMD Elan SC400 CPU (PC-on-a-chip)
<p> This has a race condition between an interrupt and a status register
of the UART.  An interrupt is issued when the UART transmitter
finishes the transmission of a byte and the UART transmit buffer
becomes empty (waiting for the next byte).  But a status register of
the UART doesn't get updated fast enough to reflect this.  As a
result, the interrupt service routine rapidly checks and determines
(erroneously) that nothing has happened.  Thus no byte is sent to the
port to be transmitted and the UART transmitter waits in vain for a
byte that never arrives.  If the interrupt service routine had waited
just a bit longer before checking the status register, then it would
have been updated to reflect the true state and all would be OK.

There is a proposal to fix this by patching the serial driver.  But
Should linux be patched to accommodate defective hardware, especially
if this patch may impair performance of good hardware?

<sect>Troubleshooting <label id="trouble_shoot">
<p> See Modem-HOWTO for troubleshooting related to modems or getty for
modems.  For a Text-Terminal much of the info here will be of value as
well as the troubleshooting info in Text-Terminal-HOWTO.

<sect1> Serial Electrical Test Equipment <label id="ser_elect_test">
<!-- begin ser_elect_test.D.  in Serial, Text-Terminal -->
<sect2> Breakout Gadgets, etc.
<p> While a multimeter (used as a voltmeter) may be all that you need
for just a few serial ports, simple special test equipment has been
made for testing serial port lines.  Some are called "breakout ... "
where breakout means to break out conductors from a cable.  These
gadgets have a couple of connectors which connect to serial port
connectors (either at the ends of serial cables or at the back of a
PC).  Some have test points for connecting a voltmeter.  Others have
LED lamps which light when certain modem control lines are asserted
(turned on).  The color of the light may indicate the polarity of the
signal (positive or negative voltage).  Still others have jumpers so
that you can connect any wire to any wire.  Some have switches.

Radio Shack sells (in 2002) a "RS-232 Troubleshooter" (formerly called
"RS-232 Line Tester") Cat. #276-1401.  It checks TD, RD, CD, RTS, CTS,
DTR, and DSR.  A green light means on (+12 v) while red means off (-12
v).  They also sell a "RS-232 Serial Jumper Box" Cat.
#276-1403.  This permits connecting the pins anyway you choose.  Both
these items are under the heading of "Peripheral hookup helpers".
Unfortunately, they are not listed in the index to the printed
catalog.  They are on the same page as the D type connecters so look
in the index under "Connectors, Computer, D-Sub".  A store chain named
"Active Components" may have them.

<sect2> Measuring voltages
<p> Any voltmeter or multimeter, even the cheapest that sells for
about $10, should work fine.  Trying to use other methods for checking
voltage is tricky.  Don't use a LED unless it has a series resistor to
reduce the voltage across the LED.  A 470 ohm resistor is used for a
20 ma LED (but not all LED's are 20 ma).  The LED will only light for
a certain polarity so you may test for + or - voltages.  Does anyone
make such a gadget for automotive circuit testing??  Logic probes may
be damaged if you try to use them since the TTL voltages for which
they are designed are only 5 volts.  Trying to use a 12 V incandescent
light bulb is not a good idea.  It won't show polarity and due to
limited output current of the UART it probably will not even light up.

To measure voltage on a female connector you may plug in a bent paper
clip into the desired opening.  The paper clip's diameter should be no
larger than the pins so that it doesn't damage the contact.  Clip
an alligator clip (or the like) to the paper clip to connect up.  Take
care not to touch two pins at the same time with any metal object.

<sect2> Taste voltage
<p> As a last resort, if you have no test equipment and are willing to
risk getting shocked (or even electrocuted) you can always taste the
voltage.  Before touching one of the test leads with your tongue, test
them to make sure that there is no high voltage on them.  Touch both
leads (at the same time) to one hand to see if they shock you.  Then
if no shock, wet the skin contact points by licking and repeat.  If
this test gives you a shock, you certainly don't want to use your
tongue.

For the test for 12 V, Lick a finger and hold one test lead in it.
Put the other test lead on your tongue.  If the lead on your tongue is
positive, there will be a noticeable taste.  You might try this with
flashlight batteries first so you will know what taste to expect.
<!-- end ser_elect_test.D in Serial, Text-Terminal -->


<sect1> Serial Monitoring/Diagnostics
<p> A few Linux programs will monitor the modem control lines and
indicate if they are positive (1) or negative (0).  See section <ref
id="serial_mon" name="Serial Monitoring/Diagnostics">

<!-- currently in <sect>Troubleshooting -->
<!-- troubleshooting.D begin  (in Modem/Serial HOWTOs)
Change Log:
May  '05<>Linux Creates an Interrupt Conflict (your PC has an ISA slot)
Feb. '05: Revised "Serial Port Can't be Found", /proc tree
  error link
Feb. '04: Cannot open /dev/ttyS?
 example n.g.
Dec. '03: Scanport can also detect an enabled PnP port
June '03: Wvdial: busy message due to lockfile permissions
Feb. '03: Interrupts may be shared on PCI Bus
Jan. '03: LSR safety check error 
Dec. '02: IO error may mean IRQ conflict or IO address conflict.
July '02: typo: is doesn't => it doesn't, clarity re port not found
Dec. '00: /proc/tty/driver/serial shows info, I/O error+, pid 161 in
Nov. '00: which connector is ttyS1, etc. Input/output error, overrun
May  '00: address conflict
Apr. '00: 2 ports on same address
-->
<sect1>(The following subsections are in both the Serial and Modem HOWTOs)

<sect1> Serial Port Can't be Found
<label id="cant_find_port">
<p>There are 3 possibilities:
<enum>
<item>Your port is disabled since both the BIOS and Linux failed to
enable it.  It has no IO address.
<item>Your port is enabled and has an IO address but it has no ttyS
device number (like ttyS14) assigned to that address so the port
can't be used.  As a last resort, you may need to use "setserial" to
assign a ttyS number to it.
<item>Your port does have a ttyS number assigned to it (like ttyS14)
but you don't know which physical connector it is (on the back of your
PC).

See <ref id="identify_ttyS" name="Which Connector on the Back of
my PC is ttyS1, etc?"> 

</enum>

First check BIOS messages at boot-time (and possibly the BIOS menu for
the serial port).  Then for the PCI bus type lspci -v.  If this shows
something like "LPC Bridge" then your port is likely on the LPC bus
which is not well supported by Linux yet (but the BIOS might find it)
??  If it's an ISA bus PnP serial port, try "pnpdump --dumpregs"
and/or see Plug-and-Play-HOWTO.  If the port happens to be enabled
then the following two paragraphs may help find the IO port:

<sect2> Scanning/probing legacy ports
<p>This is mainly for legacy non-PCI ports and ISA ports that are not
Plug-and-Play.

Using "scanport" (Debian only ??) will scan all enabled bus ports and
may discover an unknown port that could be a serial port (but it
doesn't probe the port).  It could hang your PC.  If you suspect that
your port may be at a certain address, you may try manually probing
with setserial, but it's a slow tedious task if you have several
addresses to probe.  See <ref id="probing_ss" name="Probing">.

<sect1>Linux Creates an Interrupt Conflict (your PC has an ISA slot)
<p>If your PC has a BIOS that handles ISA (and likely PCI too) then
if you find a IRQ conflict, it might be due to a shortage of free
IRQs.  The BIOS often maintains a list of reserved IRQs, reserved for
legacy ISA cards.  If too many are reserved, the BIOS may not be able
to find a free IRQ and will erroneously assign an IRQ to the serial
port that creates a conflict.  So check to see if all the reserved
IRQs are really needed and if not, unreserve an IRQ that the serial
port can use.  For more details, see Plug-and-Play-HOWTO.

<sect1> Extremely Slow: Text appears on the screen slowly after long delays
 <label id="slow_">
<p> It's likely mis-set/conflicting interrupts.  Here are some of the
symptoms which will happen the first time you try to use a modem,
terminal, or serial printer.  In some cases you type something but
nothing appears on the screen until many seconds later.  Only the last
character typed may show up.  It may be just an invisible
&lt;return&gt character so all you notice is that the cursor jumps
down one line.  In other cases where a lot of data should appear on
the screen, only a batch of about 16 characters appear.  Then there is
a long wait of many seconds for the next batch of characters.  You
might also get "input overrun" error messages (or find them in logs).



 For more details on the symptoms and why this
happens see <ref id="irq_prob_details" name="Interrupt Problem
Details"> and/or <ref id="irq_conflict" name="Interrupt Conflicts">
and/or <ref id="irq_ng" name="Mis-set Interrupts">. 
If it involves Plug-and-Play devices, see also the Plug-and-Play-HOWTO.

As a quick check to see if it really is an interrupt problem, set the
IRQ to 0 with "setserial".  This will tell the driver to use
polling instead of interrupts.  If this seems to fix the "slow"
problem then you had an interrupt problem.  You should still try to
solve the problem since polling uses excessive computer resources.

Checking to find the interrupt conflict may not be easy since Linux
supposedly doesn't permit any interrupt conflicts and will send you a
<ref id="busy_err" name="/dev/ttyS?: Device or resource busy"> error
message if it thinks you are attempting to create a conflict.  But a
real conflict can be created if "setserial" has told the kernel
incorrect info.  The kernel has been lied to and thus doesn't think
there is any conflict.  Thus using "setserial" will not reveal the
conflict (nor will looking at /proc/interrupts which bases its info on
"setserial").  You still need to know what "setserial" thinks so that
you can pinpoint where it's wrong and change it when you determine
what's really set in the hardware.

What you need to do is to check how the hardware is set by checking
jumpers or using PnP software to check how the hardware is actually
set.  For PnP run either "pnpdump --dumpregs" (if ISA bus) or run
"lspci" (if PCI bus).  Compare this to how Linux (e.g. "setserial")
thinks the hardware is set.

<sect1> Somewhat Slow: I expected it to be a few times faster
<p> An obvious reason is that the baud rate is set too slow.  It's
claimed that this once happened by trying to set the baud rate to a
speed higher than the hardware can support (such as 230400).

Another reason may be that whatever is on the serial port (such as a
modem, terminal, printer) doesn't work as fast as you thought it did.


Another possible reason is that you have an obsolete serial port: UART
8250, 16450 or early 16550 (or the serial driver thinks you do).  See

<ref id="uart_" name="What Are UARTS?"> 
Use "setserial -g /dev/ttyS*".
If it shows anything less than a 16550A, this may be your problem.
If you think that "setserial" has it wrong check it out.  See <ref
id="set_serial" name="What is Setserial"> for more info.  If you
really do have an obsolete serial port, lying about it to setserial
will only make things worse.

<sect1>The Startup Screen Shows Wrong IRQs for the Serial Ports
<label id="irqs_shown_wrong">
<p> For non-PnP ports, Linux does not do any IRQ detection on startup.
When the serial module loads it only does serial device detection.
Thus, disregard what it says about the IRQ, because it's just assuming
the standard IRQs.  This is done, because IRQ detection is unreliable,
and can be fooled.  But if and when setserial runs from a start-up
script, it changes the IRQ's and displays the new (and hopefully
correct) state on on the startup screen.  If the wrong IRQ is not
corrected by a later display on the screen, then you've got a problem.

So, even though I have my <tt/ttyS2/ set at IRQ 5, I still see
<tscreen><verb>
ttyS02 at 0x03e8 (irq = 4) is a 16550A
</verb></tscreen>
at first when Linux boots.  (Older kernels may show "ttyS02" as
"tty02" which is the same as ttyS2).  You may need to use
<tt/setserial/ to tell Linux the IRQ you are using.

<sect1> "Cannot open /dev/ttyS?: Device or resource busy
<p> See <ref id="busy_err" name="/dev/tty? Device or resource busy">

<sect1> "Cannot open /dev/ttyS?: Permission denied"
<p> Check the file permissions on this port with "ls -l /dev/ttyS?"_
If you own the ttyS? then you need read and write permissions: crw
with the c (Character device) in col. 1.  It you don't own it then it
will work for you if it shows rw- in cols. 8 & 9 which means that
everyone has read and write permission on it.  Use "chmod" to change
permissions.  There are more complicated (and secure) ways to get
access like belonging to a "group" that has group permission.  Some
programs change the permissions when they run but restore them when
the program exists normally.  But if someone pulls the plug on your
PC it's an abnormal exit and correct permissions may not be restored.

<sect1> "Cannot open /dev/ttyS?"
<p>Unless stty is set for clocal, the CD pin may need to be asserted
in order to open a serial port.  If the physical port is not connected
to anything, or if it's connected to something that is not powered on
(such an external modem) then there will be no voltage on CD
from that device.  Thus the "cannot open" message.  Either set clocal
or connect the serial port connector to something and power it on.

Even if a device is powered on and connected to a port, it may
sometimes prevent opening the port.  An example of this is where the
device has negated CD and the CD pin on your PC is negated (negative
voltage). 

<sect1> "Operation not supported by device" for ttyS?
<p> This means that an operation requested by setserial, stty, etc.
couldn't be done because the kernel doesn't support doing it.
Formerly this was often due to the "serial" module not being loaded.
But with the advent of PnP, it may likely mean that there is no modem
(or other serial device) at the address where the driver (and
setserial) thinks it is.  If there is no modem there, commands (for
operations) sent to that address obviously don't get done.  See <ref
id="io-irq_in_hdw" name="What is set in my serial port hardware?">

If the "serial" module wasn't loaded but "lsmod" shows you it's now
loaded it might be the case that it's loaded now but wasn't loaded
when you got the error message.  In many cases the module will
automatically loaded when needed (if it can be found).  To force
loading of the "serial" module it may be listed in the file:
/etc/modules.conf or /etc/modules.  The actual module should reside
in: /lib/modules/.../misc/serial.o.

<sect1> "Cannot create lockfile. Sorry" <label id="lockfile_">
<p> Sometimes when it can't create a lockfile you get the erroneous
message: "... Device or resource busy" instead of the one above.
When a port is "opened" by a program a lockfile is created in
/var/lock/.  Wrong permissions for the lock directory will not allow a
lockfile to be created there.  Use "ls -ld /var/lock" to see if the
permissions are OK.  Giving rwx permissions for the root owner and the
group should work, provided that the users that need to dialout belong
to that group.  Others should have r-x permission.  Even with this
scheme, there may be a security risk.  Use "chmod" to change
permissions and "chgrp" to change groups.  Of course, if there is no
"lock" directory no lockfile can be created there.  For more info on
lockfiles see <ref id="lockfiles_" name="What
Are Lock Files">

<sect1> "Device /dev/ttyS? is locked."
<p> This means that someone else (or some other process) is supposedly
using the serial port.  There are various ways to try to find out what
process is "using" it.  One way is to look at the contents of the
lockfile (/var/lock/LCK...).  It should be the process id.  If the
process id is say 100 type "ps 100" to find out what it is.  Then if
the process is no longer needed, it may be gracefully killed by "kill
100".  If it refuses to be killed use "kill -9 100" to force it to be
killed, but then the lockfile will not be removed and you'll need to
delete it manually.  Of course if there is no such process as 100 then
you may just remove the lockfile but in most cases the lockfile should
have been automatically removed if it contained a stale process id
(such as 100).

<sect1> "/dev/tty? Device or resource busy" <label id="busy_err">
<p> This means that the device you are trying to access (or use) is
supposedly busy (in use) or that a resource it needs (such as an IRQ)
is supposedly being used by another device and can't be shared.
This message is easy to understand if it only means that the device is
busy (in use).  But it sometimes means that a needed resource is already
in use (busy).  What makes it even more confusing is that in some cases
neither the device nor the resources that it needs are actually
"busy".  

In olden days, if a PC was shutdown by just turning off the power, a
bogus lockfile might remain and then later on one would get this bogus
message and not be able to use the serial port.  Software today is
supposed to automatically remove such bogus lockfiles, but as of 2006
there is still a problem with the "wvdial" dialer program related to
lockfiles.  If wvdial can't create a lockfile because it doesn't have
write permission in the /var/lock/ directory, you will see this
erroneous message.  See <ref id="lockfile_" name= "Cannot create
lockfile. Sorry"> 

The following example is where interrupts can't be shared (at least
one of the interrupts is on the ISA bus).  The ``resource busy'' part
often means (example for <tt/ttyS2/) ``You can't use <tt/ttyS2/ since
another device is using ttyS2's interrupt.'' The potential interrupt
conflict is inferred from what "setserial" thinks.  A more accurate
error message would be ``Can't use <tt/ttyS2/ since the setserial data
(and kernel data) indicates that another device is using <tt/ttyS2/'s
interrupt''.  If two devices use the same IRQ and you start up only
one of the devices, everything is OK because there is no conflict yet.
But when you next try to start the second device (without quitting the
first device) you get a "... busy" error message.  This is because the
kernel only keeps track of what IRQs are actually in use and actual
conflicts don't happen unless the devices are in use (open).   The
situation for I/O address (such as 0x3f8) conflict is similar.

This error is sometimes due to having two serial drivers: one a module
and the other compiled into the kernel.  Both drivers try to grab the
same resources and one driver finds them "busy".

There are two possible cases when you see this message:
<enum>
<item> There may be a real resource conflict that is being avoided.
<item> Setserial has it wrong and the only reason <tt/ttyS2/ can't be
 used is that setserial erroneously predicts a conflict.
</enum>

What you need to do is to find the interrupt setserial thinks
<tt/ttyS2/ is using.  Look at /proc/tty/driver/serial.  You should
also be able to find it with the "setserial" command for <tt/ttyS2/.   

Bug in old versions:  Prior to 2001 there was a bug which wouldn't let
you see it with "setserial".  Trying to see it would give the same
"... busy" error message.

To try to resolve this problem reboot or: exit or gracefully kill all
likely conflicting processes.   If you reboot: 1. Watch the boot-time
messages for the serial ports.  2. Hope that the file that runs
"setserial" at boot-time doesn't (by itself) create the same conflict
again.

If you think you know what IRQ say <tt/ttyS2/ is using then you may
look at /proc/interrupts to find what else (besides another serial
port) is currently using this IRQ.  You might also want to double
check that any suspicious IRQs shown here (and by "setserial") are
correct (the same as set in the hardware).  A way to test whether or
not it's a potential interrupt conflict is to set the IRQ to 0
(polling) using "setserial".  Then if the busy message goes away, it
was likely a potential interrupt conflict.  It's not a good idea to
leave it permanently set at 0 since it will put more load on the CPU.

<sect1>"Input/output error" from setserial, stty, pppd, etc.
<p> This means that communication with the serial port isn't working
right.  It could mean that there isn't any serial port at the IO
address that setserial thinks your port is at.  It could also be an
interrupt conflict (or an IO address conflict).  It also may mean that
the serial port is in use (busy or opened) and thus the attempt to
get/set parameters by setserial or stty failed.  It will also happen
if you make a typo in the serial port name such as typing "ttys"
instead of "ttyS".  

<sect1>"LSR safety check engaged"
<p>LSR is the name of a hardware register.  It usually means that 
there is no serial port at the address where the driver thinks your
serial port is located.  You need to find your serial port and
possibly configure it.  See <ref id="locate_port" name="Locating the
Serial Port: IO address IRQs"> and/or <ref id="set_serial" name="What
is Setserial">

<sect1>Overrun errors on serial port
<p> This is an overrun of the hardware FIFO buffer and you can't
increase its size.  Bug note (reported in 2002): Due to a bug in some
kernel 2.4 versions, the port number may be missing and you will only
see "ttyS" (no port number).  But if devfs notation such as "tts/2"
was being used, there was no bug.  See <ref id="higher_thruput" name="Higher Serial Thruput">.




<sect1> Port gets characters only sporadically
<p> There could be some other program running on the port.  Use "top"
(provided you've set it to display the port number) or type "ps
-alxw".  Look at the results to see if the port is being used by
another program.  Be on the lookout for the gpm mouse program which
often runs on a serial port.

<sect1> Troubleshooting Tools
<p> These are some of the programs you might want to use in
troubleshooting:
<itemize>
<item> "lsof /dev/ttyS*" will list serial ports which are open.
<item> "setserial" shows and sets the low-level hardware configuration
 of a port (what the driver thinks it is).  See <ref id="set_serial"
 name="What is Setserial">
<item> "stty" shows and sets the configuration of a port (except for
 that handled by "setserial").
  See the section <ref id="stty_" name="Stty"><item> "modemstat" or "statserial" or "watch head
/proc/tty/driver/serial" will show the current state of various modem
signal lines (such as DTR, CTS, etc.).  The one in /proc also shows
byte flow and errors.
<item> "irqtune" will give serial port interrupts higher
 priority to improve performance.
<item> "hdparm" for hard-disk tuning may help some more.
<item> "lspci" shows the actual IRQs, etc. of hardware on the PCI bus.
<item> "pnpdump --dumpregs" shows the actual IRQs, etc. of hardware for
 PnP devices on the ISA bus.
<item> Some "files" in the /proc tree (such as ioports, interrupts,
 and tty/driver/serial).
</itemize>

<!-- troubleshooting.D end -->


<sect1> Almost all characters are wrong; Many missing or many extras
<p>Perhaps a baud mismatch.  If one port sends at twice the speed that
the other port is set to receive, then every two characters sent will
be received as one character.  Each bit of this received character
will be a sample of two bits of what is sent and will be wrong.  Also,
only half the characters sent seem to get received.  For flow in the
reverse direction, it's just the opposite.  Twice as many characters
get received than were sent.  A worse mismatch will produce even worse
results.

A speed mismatch is not likely to happen with a modem since the modem
autodetects the speed.  One cause of a mismatch may be due to serial
port hardware that has been set to run at very fast speeds.  It may
actually operate at a speed say 8 times that of which you (or an
application) set it via software.  See <ref id="high_speed" name="Very
High Speeds">

<sect> Interrupt Problem Details <label id="irq_prob_details">
<p> While the section <ref id="trouble_shoot" name="Troubleshooting">
lists problems by symptom, this section explains what will happen if
interrupts are set incorrectly.  This section helps you understand what
caused the symptom, what other symptoms might be due to the same
problem, and what to do about it.

<sect1> Types of interrupt problems
<p> The "setserial" program will show you how serial driver thinks the
interrupts are set.  If the serial driver (and setserial) has it right
then everything regarding interrupts should be OK.  Of course a
/dev/ttyS must exist for the device and Plug-and-Play (or jumpers)
must have set an address and IRQ in the hardware.  Linux will not
knowingly permit an interrupt conflict and you will get a "Device or
resource busy" error message if you attempt to do something that would
create a conflict.

Since the kernel tries to avoid interrupt conflicts and gives you the
"resource busy" message if you try to create a conflict, how can
interrupt conflicts happen?  Easy.  "setserial" may have it wrong and
erroneously predicts no conflict when there will actually be a real
conflict based on what is set in the hardware.  When this happens
there will be no "... busy" message but a conflict will physically
happen.  Performance is likely to be extremely slow.  Both devices
will send identical interrupt signals on the same wire and the CPU
will erroneously think that the interrupts only come from one device.
This will be explained in detail in the following sections.

Linux doesn't complain when you assign two devices the same IRQ
provided that neither device is in use.  As each device starts up
(initializes), it asks Linux for permission to use its hardware
interrupt.  Linux keeps track of which interrupt is assigned to whom,
and if your interrupt is already in use, you'll see this "... busy"
error message.  Thus if two devices use the same IRQ and you start up
only one of the devices, everything is OK.  But when you next try to
start the second device (without quitting the first device) you get
"... busy" error message.

<sect1> Symptoms of Mis-set or Conflicting Interrupts
<p> The symptoms depend on whether or not you have a modern serial port
with FIFO buffers or an obsolete serial port without FIFO buffers.
It's important to understand the symptoms for the obsolete ones also
since sometimes modern ports seem to behave that way.

For the obsolete serial ports, only one character gets thru every
several seconds.  This is so slow that it seems almost like nothing is
working (especially if the character that gets thru is invisible (such
a space or newline).  For the modern ports with FIFO buffers you
will likely see bursts of up to 16 characters every several seconds.

If you have a modem on the port and dial a number, it seemingly may
not connect since the CONNECT message may not make it thru.  But after
a long wait it may finally connect and you may see part of a login
message (or the like).  The response from your side of the connection
may be so delayed that the other side gives up and disconnects you,
resulting in a NO CARRIER message.

If you use minicom, a common test to see if things are working is to
type the simplest "AT" command and see if the modem responds.  Typing
just at&lt;enter&gt; should normally (if interrupts are OK) result in
an immediate "OK" response from the modem.  With bad interrupts you
type at&lt;enter&gt; and may see nothing.  But then after 10 seconds
or so you see the cursor drop down one line.  What is going on is that
the FIFO is behaving like it can only hold one byte.  The "at" you
typed caused it to overrun and both letters were lost.  But the final
&lt;enter&gt; eventually got thru and you "see" this invisible
character by noticing that the cursor jumped down one line.  If you were
to type a single letter and then wait about 10 seconds, you should see
it echo back to the screen.  This is fine if your typing speed is less
that one word per minute :-)

<sect1> Mis-set Interrupts <label id="irq_ng">
<p> If you don't understand what an interrupt does see <ref
id="interrupt_" name="Interrupts">.  If a serial port has one IRQ set
in the hardware but a different one set in the device driver, the
device driver will not catch any interrupts sent by the serial port.
Since the serial port uses interrupts to call its driver to service
the port (fetching bytes from its 16-byte receive buffer or putting
another 16-bytes in its transmit buffer) one might expect that the
serial port would not work at all.

But it still may work anyway --sort of.  Why?  Well, besides the
interrupt method of servicing the port there's an undocumented slow
polling method that doesn't need interrupts.  The way it works is that
every so often the device driver checks the serial port to see if it
needs anything such as if it has some bytes that need fetching from
its receive buffer.  If interrupts don't work, the serial driver falls
back to this polling method.  But this polling method was not intended
to be used a substitute for interrupts.  It's so slow that it's not
practical to use and may cause buffer overruns.  Its purpose may have
been to get things going again if just one interrupt is lost or fails
to do the right thing.  It's also useful in showing you that
interrupts have failed.  Don't confuse this slow polling method with
the fast polling method that operates on ports that have their IRQs
set to 0.

For the 16-byte transmit buffer, 16 bytes will be transmitted and then
it will wait until the next polling takes place (several seconds
later) before the next 16 bytes are sent out.  Thus transmission is
very slow and in small chunks.  Receiving is slow too since bytes that
are received by the receive buffer are likely to remain there for
several seconds until it is polled.

This explains why it takes so long before you see what you typed.
When you type say AT to a modem, the AT goes out the serial port to
the modem.  The modem then echos the AT back thru the serial port to
the screen.  Thus the AT characters have to pass twice thru the serial
port.  Normally this happens so fast that AT seems to appear on the
screen at the same time you hit the keys on the keyboard.  With slow
polling delays at the serial port, you don't see what you typed
until perhaps 15 seconds later.  Even then, you don't often see all
you typed but only the first several characters.	

What about overruns of the 16-byte receive buffer?  This will happen
with an external modem since the modem just sends to the serial port
at high speed which is likely to overrun the 16-byte buffer.  But for
an internal modem, the serial port is on the same card and it's likely
to check that this 16-byte receive buffer has room for more bytes
before putting received bytes into it.  In this case there will be no
overrun of this receive buffer, but text will just appear on your
screen in 16-byte chunks spaced at intervals of several seconds.

Even with an external modem you might not get overruns.  If just a few
characters (under 16) are sent you don't get overruns since the buffer
likely has room for them.  But attempts to send a larger number of
bytes from your modem to your screen may result in overruns.  However,
more than 16 (with no gaps) can get thru without overruns if the
timing is right.  For example, suppose a burst of 32 bytes is sent
into the port from the external cable.  The polling might just happen
after the first 16 bytes came in so it would pick up these 16 bytes
OK.  Then there would be space for the next 16 bytes so that entire 32
bytes gets thru OK.  While this scenario is not very likely, similar
cases where 17 to 31 bytes make thru are more likely.  But it's even
more likely that only an occasional 16-byte chunk will get thru with
possible loss of data.

If you have an obsolete serial port with only a 1-byte buffer (or it's
been incorrectly set to work like a 1-byte buffer) then the situation
will be much worse than described above and only one character will
occasionally make it thru the port.  Every character received causes
an overrun (and is lost) except for the last character received.  This
character is likely to be just a line-feed since this is often the
last character to be transmitted in a burst of characters sent to your
screen.  Thus you may type AT&lt;return&gt to the modem but never see
AT on the screen.  All you see several seconds later is that the
cursor drops down one line (a line feed).  This has happened to me
with a 16-byte FIFO buffer that was behaving like a 1-byte buffer.

When a communication program starts up, it expects interrupts to be
working.  It's not geared to using this slow polling-like mode of
operation.  Thus all sorts of mistakes may be made such as setting up
the serial port and/or modem incorrectly.  It may fail to realize when
a connection has been made.  If a script is being used for login, it
may fail (caused by timeout) due to the polling delays.

<sect1> Interrupt Conflicts <label id="irq_conflict">
<p> When two devices have the same IRQ number it's called sharing
interrupts.  Under some conditions this sharing works out OK.
Starting with kernel version 2.2, ISA serial ports may, if the
hardware is designed for this, share interrupts with other serial
ports.  Devices on the PCI bus may share the same IRQ interrupt with
other devices on the PCI bus (provided the software supports this).
In other cases where there is potential for conflict, there should be
no problem if no two devices with the same IRQ are ever "in use" at
the same time.   More precisely, "in use" really means "open" (in
programmer jargon).  In cases other than the exceptions mentioned
above (unless special software and hardware permit sharing), sharing
is not allowed and conflicts arise if sharing is attempted.

Even if two processes with conflicting IRQs run at the same time, one
of the devices will likely have its interrupts caught by its device
driver and may work OK.  The other device will not have its interrupts
caught by the correct driver and will likely behave just like a
process with mis-set interrupts.  See <ref id="irq_ng" name="Mis-set
Interrupts"> for more details.

<sect1> Resolving Interrupt Problems
<p> If you are getting a very slow response as described above, then
one test is to change the IRQ to 0 (uses fast polling instead of
interrupts) and see if the problem goes away.  Note that the polling
due to IRQ=0 is orders of magnitude faster than the slow "polling" due
to bad interrupts.  If IRQ=0 seems to fix the problem, then there was
likely something wrong with the interrupts.  Using IRQ=0 is very
resource intensive and is only a temporary fix.  You should try to
find the cause of the interrupt problem and not permanently use IRQ=0.

Check /proc/interrupts to see if the IRQ is currently in use by another
process. If it's in use by another serial port you could try "top"
(type f and then enable the TTY display) or "ps -e" to find out which
serial ports are in use.  If you suspect that setserial has a wrong
IRQ then see <ref id="what_is_io_irq" name="What is the current IO
address and IRQ of my Serial Port ?">

<sect>What Are UARTs?  How Do They Affect Performance? <label id="uart_">
<sect1> Introduction to UARTS
<p> UARTs (<BF/U/niversal <BF/A/synchronous <BF/R/eceiver
<BF/T/ransmitter) are serial chips on your PC motherboard (or on an
internal modem card).   The UART function may also be done on a chip
that does other things as well.  On older computers like many 486's,
the chips were on the disk IO controller card.  Still older computer
have dedicated serial boards.

When PCs all had parallel bus architecture, the UART's purpose was to
convert bytes from the PC's parallel bus to a serial bit-stream.  The
cable going out of the serial port is serial and has only one wire for
each direction of flow.  The serial port sends out a stream of bits,
one bit at a time.  Conversely, the bit stream that enters the serial
port via the external cable was converted to parallel bytes that the
computer can understand.  UARTs deal with data in byte sized pieces,
which is conveniently also the size of ASCII characters.

Say you have a terminal hooked up to a serial port on your PC.  When
you type a character, the terminal gives that character to its
transmitter (also a UART).  The transmitter sends that byte out onto
the serial line, one bit at a time, at a specific rate.  On the PC
end, the receiving UART takes all the bits and reconstruct the byte
(parallel on older PCs) and puts it in a buffer.  For newer PCs that
might have a PCI-e serial port, the UART doesn't need to convert
parallel-to-serial since the PCI-e "bus" is already a serial line.
But the PCI-e line carries an encoded signal which must be decoded and
then greatly slowed down to the speed of the RS-232 serial line.

Along with converting between serial and parallel, the UART does some
other things as a byproduct (side effect) of its primary task.  The
voltage used to represent bits is also converted (changed).  Extra
bits (called start and stop bits) are added to each byte before it is
transmitted.  See the Serial-HOWTO section <ref id="volt_shape"
name="Voltage Waveshapes"> for details.  Also, while the flow rate
(in bytes/sec) on the parallel bus inside the computer is very high,
the flow rate out the UART on the serial port side of it is much
lower.  The UART has a fixed set of rates (speeds) which it can use at
its serial port interface.

<sect1> Two Types of UARTs
<p> There are two basic types of UARTs: dumb UARTS and FIFO UARTS.
Dumb UARTs are the 8250, 16450, early 16550, and early 16650.  They
are obsolete but if you understand how they work it's easy to
understand how the modern ones work with FIFO UARTS ( late 16550,
16550A, and higher numbers).  Note that the driver for all of them is
still labeled a "8250" driver in Linux where you may see it in compile
options if you compile your own kernel, etc.

There is some confusion regarding 16550.  Early models had a bug and
worked properly only as 16450's (no FIFO).  Later models with the bug
fixed were named 16550A but many manufacturers did not accept the name
change and continued calling it a 16550.  Most all 16550's in use
today are like 16550A's.  Linux will report it as being a 16550A even
though your hardware manual (or a label note) says it's a 16550.  A
similar situation exists for the 16650 (only it's worse since the
manufacturer allegedly didn't admit anything was wrong).  Linux will
report a late 16650 as being a 16650V2.  If it reports it as 16650 it
is bad news and only is used as if it had a one-byte buffer.

<sect1> FIFOs <label id="fifo_">
<p> To understand the differences between dumb and FIFO (First In,
First Out queue discipline) first let's examine what happens when a
UART has sent or received a byte.  The UART itself can't do anything
with the data passing thru it, it just receives and sends it.  For the
obsolete dumb UARTS, the CPU gets an interrupt from the serial device
every time a byte has been sent or received.  The CPU then moves the
received byte out of the UART's buffer and into memory somewhere, or
gives the UART another byte to send.  The obsolete 8250 and 16450
UARTs only have a 1 byte buffer.  That means, that every time 1 byte
is sent or received, the CPU is interrupted.  At low transfer rates,
this is OK.  But, at high transfer rates, the CPU gets so busy dealing
with the UART, that is doesn't have time to adequately tend to other
tasks.  In some cases, the CPU does not get around to servicing the
interrupt in time, and the byte is overwritten, because they are
coming in so fast.  This is called an "overrun" or "overflow".

FIFO UARTs help solve this problem.  The 16550A (or 16550) FIFO chip
comes with 16 byte FIFO buffers.  This means that it can receive up to
14 bytes (or send 16 bytes) before it has to interrupt the CPU.  Not
only can it wait for more bytes, but the CPU then can transfer all (14
to 16) bytes at a time.  This is a significant advantage over the
obsolete UARTs, which only had 1 byte buffers.  The CPU receives less
interrupts, and is free to do other things.  Data is rarely lost.
Note that the interrupt threshold of FIFO buffers (trigger level) may
be set at less than 14.  1, 4 and 8 are other possible choices.  As of
late 2000 there was no way the Linux user could set these directly
(setserial can't do it).  While many PC's only have a 16550 with
16-byte buffers, better UARTS have even larger buffers.

Note that the interrupt is issued slightly before the buffer gets full
(at say a "trigger level" of 14 bytes for a 16-byte buffer).  This
allows room for a couple more bytes to be received before the
interrupt service routine is able to actually fetch all these bytes.
The trigger level may be set to various permitted values by kernel
software.  A trigger level of 1 will be almost like an obsolete UART
(except that it still has room for 15 more bytes after it issues the
interrupt).

Now consider the case where you're on the Internet.  It's just sent
you a short webpage of text.  All of this came in thru the serial
port.  If you had a 16-byte buffer on the serial port which held back
characters until it had 14 of them, some of the last several
characters on the screen might be missing as the FIFO buffer waited to
get the 14th character.  But the 14th character doesn't arrive since
you've been sent the entire page (over the phone line) and there are
no more characters to send to you.  It could be that these last
characters are part of the HTML formatting, etc. and are not
characters to display on the screen but you don't want to lose format
either.

There is a "timeout" to prevent the above problem.  The "timeout"
works like this for the receive UART buffer: If characters arrive one
after another, then an interrupt is issued only when say the 14th
character reaches the buffer.  But if a character arrives and the next
character doesn't arrive soon thereafter, then an interrupt is issued
anyway.  This results in fetching all of the characters in the FIFO
buffer, even if only a few (or only one) are present.  There is also
"timeout" for the transmit buffer as well.

<sect1> Why FIFO Buffers are Small

<p>You may wonder why the FIFO buffers are not larger.  After all,
memory is cheap and it wouldn't cost much more to use buffers in the
kilo-byte range.  The reason is flow control.  Flow control stops the
flow of data (bytes) on serial line when necessary.  If a stop signal
is sent to serial port, then the stop request is handled by software
(even if the flow control is "hardware").  The serial port hardware
knows nothing about flow control.

If the serial port buffer contains 64 bytes ready to send when it
receives a flow control signal to stop sending, it will send out the
64 bytes anyway in violation of the stop request.  There is no
stopping it since it doesn't know about flow control.  If the buffer
was large, then many more bytes would be sent in violation of flow
control's request to stop.

<sect1> UART Model Numbers
<p> Here's a list of some UARTs.  <em/TL/ is <em/T/rigger <em/L/evel
<itemize>
<item> 8250, 16450, early 16550: Obsolete with 1-byte buffers
<item> 16550, 16550A, 16C552: 16-byte buffers, TL=1,4,8,14;
 115.2 kbps standard, many support 230.4 or 460.8 kbps
<item> 16650:  32-byte buffers. 460.8 kbps
<item> 16750:  64-byte buffer for send, 56-byte for receive. 921.6 kbps
<item> 16850, 16C850: 128-byte buffers.  460.8 kbps or 1.5 mbps
<item> 16950
<item> Hayes ESP: 1k-byte buffers.
</itemize>

For V.90 56k modems, it may be a several percent faster with a 16650
(especially if you are downloading large uncompressed files).  The
main advantage of the 16650 is its larger buffer size as the extra
speed isn't needed unless the modem compression ratio is high.  Some
56k internal modems may come with a 16650 ??

Non-UART, and intelligent multiport boards use DSP chips to
do additional buffering and control, thus relieving the CPU
even more.  For example, the Cyclades Cyclom, and Stallion
EasyIO boards use a Cirrus Logic CD1400 RISC UART, and many
boards use 80186 CPUs or even special RISC CPUs, to handle the
serial IO.

Many 486 PCs (old) and all Pentiums (or the like) should have at least
16550As (usually called just 16550's) with FIFOs.  Some better
motherboards today (2000) even have 16650s.  For replacing obsolete
UARTs with newer ones in pre 1990 hardware see the Appendix: Obsolete
...

<sect> Pinout and Signals <label id="pinout_">
<sect1> Pinout of 9-pin and 25-pin serial connectors
<p> The pin numbers are often engraved in the plastic of the
connector but you may need a magnifying glass to read them.
Note DCD is sometimes labeled CD.  The numbering of the pins on a
female connector is read from right to left, starting with 1 in the
upper right corner (instead of 1 in the upper left corner for the male
connector as shown below).  --> direction is out of PC.

<tscreen><verb>
  ___________                    ________________________________________
  \1 2 3 4 5/  Looking at pins   \1  2  3  4  5  6  7  8  9  10 11 12 13/
   \6 7 8 9/  on male connector   \14 15 16 17 18 19 20 21 22 23 24 25/
    ------                         -----------------------------------
Pin #	Pin #	Acronym	 Full-Name   Direction	What-it-May-Do/Mean
9-pin	25-pin
 3	 2	TxD	Transmit Data	  -->	Transmits bytes out of PC
 2	 3	RxD	Receive Data	  <--	Receives bytes into PC
 7	 4	RTS	Request To Send	  -->	RTS/CTS flow control
 8	 5	CTS	Clear To Send	  <--	RTS/CTS flow control
 6	 6	DSR	Data Set Ready	  <--	I'm ready to communicate
 4	20	DTR	Data Terminal Ready-->  I'm ready to communicate
 1	 8	DCD	Data Carrier Detect<--	Modem connected to another
 9	22	RI	Ring Indicator	  <--	Telephone line ringing
 5	 7	SG	Signal Ground

  9-Pin DB9 Connector                    25-Pin DB-25 Connector
1     DCD   Carrier Detect             1           Chassis Ground             
2     RxD   Receive Data               2     TxD   Transmit Data
3     TxD   Transmit Data              3     RxD   Receive Data
4     DTR   Data Terminal Ready        4     RTS   Request To Send
5     SG    Signal Ground              5     CTS   Clear To Send
6     DSR   Data Set Ready             6     DSR   Data Set Ready
7     RTS   Request To Send            7     SG    Signal Ground
8     CTS   Clear To Send              8     DCD   Carrier Detect
9     RI    Ring Indicator            20     DTR   Data Terminal Ready
                                      22     RI    Ring Indicator
</verb></tscreen>

<sect1> Signals May Have No Fixed Meaning
<p> Only 3 of the 9 pins have a fixed assignment: transmit, receive
and signal ground.  This is fixed by the hardware and you can't change
it.  But the other signal lines are controlled by software and may do
(and mean) almost anything at all.  However they can only be in one of
two states: asserted (+12 volts) or negated (-12 volts).  Asserted is
"on" and negated is "off".  For example, Linux software may command
that DTR be negated and the hardware only carries out this command and
puts -12 volts on the DTR pin.  A modem (or other device) that
receives this DTR signal may do various things.  If a modem has been
configured a certain way it will hang up the telephone line when DTR
is negated.  In other cases it may ignore this signal or do something
else when DTR is negated (turned off).

It's like this for all the 6 signal lines.  The hardware only sends
and receives the signals, but what action (if any) they perform is up
to the Linux software and the configuration/design of devices that you
connect to the serial port.  However, most pins have certain functions
which they normally perform but this may vary with the operating
system and the device driver configuration.  Under Linux, one may
modify the source code to make these signal lines behave differently
(some people have).

<sect1> Cabling Between Serial Ports <label id="cabling_">
<p> A cable from a serial port always connects to another serial port.
An external modem or other device that connects to the serial port has
a serial port built into it.  For modems, the cable is always straight
thru: pin 2 goes to pin 2, etc.  The modem is said to be DCE (Data
Communications Equipment) and the computer is said to be DTE (Data
Terminal Equipment).  Thus for connecting DTE-to-DCE you use
straight-thru cable.  For connecting DTE-to-DTE you must use a
null-modem cable (also called a crossover cable).  There are many ways
to wire such cable (see examples in Text-Terminal-HOWTO subsection:
"Direct Cable Connection")

There are good reasons why it works this way.  One reason is that the
signals are unidirectional.  If pin 2 sends a signal out of it (but is
unable to receive any signal) then obviously you can't connect it to
pin 2 of the same type of device.  If you did, they would both send
out signals on the same wire to each other but neither would be able
to receive any signal.  There are two ways to deal with this
situation.  One way is to have a two different types of equipment
where pin 2 of the first type sends the signal to pin 2 of the second
type (which receives the signal).  That's the way it's done when you
connect a PC (DTE) to a modem (DCE).  There's a second way to do this
without having two different types of equipment: Connect pin sending
pin 2 to a receiving pin 3 on same type of equipment.  That's the way
it's done when you connect 2 PCs together or a PC to a terminal
(DTE-to-DTE).  The cable used for this is called a null-modem cable
since it connects two PCs without use of a modem.  A null-modem cable
may also be called a cross-over cable since the wires between pins 2
and 3 cross over each other (if you draw them on a sheet of paper).
The above example is for a 25 pin connector but for a 9-pin connector
the pin numbers 2 and 3 are just the opposite.

The serial pin designations were originally intended for connecting a
dumb terminal to a modem.  The terminal was DTE (Data Terminal
Equipment) and the modem was DCE (Data Communication Equipment).
Today the PC is usually used as DTE instead of a terminal (but real
terminals may still be used this way).  The names of the pins are the
same on both DTE and DCE.  The words: "receive" and "transmit" are
from the "point of view" of the PC (DTE).  The transmit pin from the
PC transmits to the "transmit" pin of the modem (but actually the
modem is receiving the data from this pin so from the point of view of
the modem it would be a receive pin).

The serial port was originally intended to be used for connecting DTE
to DCE which makes cabling simple: just use a straight-thru cable.
Thus when one connects a modem one seldom needs to worry about which
pin is which.  But people wanted to connect DTE to DTE (for example a
computer to a terminal) and various ways were found to do this by
fabricating various types of special null-modem cables.  In this case
what pin connects to what pin becomes significant.

<sect1> RTS/CTS and DTR/DSR Flow Control <label id="rts_cts">
<p> This is "hardware" flow control.  Flow control was previously
explained in the <ref id="flow_control" name="Flow Control">
subsection but the pins and voltage signals were not.  Linux only
supports RTS/CTS flow control at present (but a special driver may
exist for a specific application which supports DTR/DSR flow control).
Only RTS/CTS flow control will be discussed since DTR/DSR flow control
works the same way.  To get RTS/CTS flow control one needs to either
select hardware flow control in an application program or use the
command:<newline>
stty -F /dev/ttyS2 crtscts (or the like).  This enables RTS/CTS
hardware flow control in the Linux device driver.

Then when a DTE (such as a PC) wants to stop the flow into it, it
negates RTS.  Negated "Request To Send" (-12 volts) means "request NOT
to send to me" (stop sending).  When the PC is ready for more bytes
it asserts RTS (+12 volts) and the flow of bytes to it resumes.  Flow
control signals are always sent in a direction opposite to the flow of
bytes that is being controlled.  DCE equipment (modems) works the same
way but sends the stop signal out the CTS pin.  Thus it's RTS/CTS flow
control using 2 lines.

On what pins is this stop signal received?  That depends on whether we
have a DCE-DTE connection or a DTE-DTE connection.  For DCE-DTE it's a
straight-thru connection so obviously the signal is received on a pin
with the same name as the pin it's sent out from.  It's RTS-->RTS (PC
to modem) and CTS<--CTS (modem to PC).  For DTE-to-DTE the connection
is also easy to figure out.  The RTS pin always sends and the CTS pin
always receives.  Assume that we connect two PCs (PC1 and PC2)
together via their serial ports.  Then it's RTS(PC1)-->CTS(PC2) and
CTS(PC1)<--RTS(PC2).  In other words RTS and CTS cross over.  Such a
cable (with other signals crossed over as well) is called a "null
modem" cable.  See <ref id="cabling_" name="Cabling Between Serial
Ports">

What is sometimes confusing is that there is the original use of RTS
where it means about the opposite of the previous explanation above.
This original meaning is: I Request To Send to you.  This request was
intended to be sent from a terminal (or computer) to a modem which, if
it decided to grant the request, would send back an asserted CTS from
its CTS pin to the CTS pin of the computer: You are Cleared To Send to
me.  Note that in contrast to the modern RTS/CTS bi-directional flow
control, this only protects the flow in one direction: from the
computer (or terminal) to the modem.  This original use appears to be
little used today on modern equipment (including modems).

<sect2> The DTR and DSR Pins
<p> Just like RTS and CTS, these pins are paired.  For DTE-to-DTE
connections they are likely to cross over.  There are two ways to use
these pins.  One way is to use them as a substitute for RTS/CTS flow
control.  The DTR pin is just like the RTS pin while the DSR pin
behaves like the CTS pin.  Although Linux doesn't support DTR/DSR flow
control, it can be obtained by connecting the RTS/CTS pins at the PC
to the DSR/DTR pins at the device that uses DTR/DSR flow control.  DTR
flow control is the same as DTR/DSR flow control but it's only one-way
and only uses the DTR pin at the device.  Many text terminals and some
printers use DTR/DSR (or just DTR) flow control.  In the future, Linux
may support DTR/DSR flow control.  The software has already been
written but it's not clear when (or if) it will incorporated into the
serial driver.

The normal use of DTR and DSR (not for flow control) is as follows: A
device asserting DTR says that its powered on and ready to operate.
For a modem, the meaning of a DTR signal from the PC depends on how
the modem is configured.  Negating DTR is sometimes called "hanging
up" but it doesn't always do this.  One way to "hang up" (negate DTR)
is to set the baud rate to 0 using the command "stty 0".  Trying to do
this from a "foreign" terminal may not work due to the two-interface
problem.  See <ref id="two_term_interfaces" name="Two interfaces at a
terminal">.  For internal modem-serial_ports it worked OK with a port
using minicom but didn't work if the port was using wvdial.  Why?

<sect1> Preventing a Port From Opening
<p> If "stty -clocal" (or getty is used with the "local" flag negated)
then a serial port can't open until DCD gets an assert (+12 volts)
signal.

<sect> Voltage Waveshapes <label id="volt_shape">

<sect1> Voltage for a Bit
<p> At the RS-232 serial port, voltages are bipolar (positive or
negative with respect to ground) and should be about 12 volts in
magnitude (some are 5 or 10 volts).   For the transmit and receive
pins +12 volts is a 0-bit (sometimes called "space") and -12 volts is
a 1-bit (sometimes called "mark").  This is known as inverted logic
since normally a 0-bit is both false and negative while a one is
normally both true and positive.  Although the receive and transmit
pins are inverted logic, other pins (modem control lines) are normal
logic with a positive voltage being true (or "on" or "asserted") and a
negative voltage being false (or "off" or "negated").  Zero voltage
has no meaning (except it usually means that the unit is powered off).

A range of voltages is allowed.  The specs say the magnitude of a
transmitted signal should be between 5 and 15 volts but must never
exceed 25 V.  Any voltage received under 3 V is undefined (but some
devices will accept a lower voltage as valid).  One sometimes sees
erroneous claims that the voltage is commonly 5 volts (or even 3
volts) but it's usually 11-12 volts.  If you are using a EIA-422
(RS-422) port on a Mac computer as an RS-232 (requires a special
cable) or EIA-423 (RS-423) then the voltage will actually be only 5 V.
The discussion here assumes 12 V.

Note that normal computer logic normally is just a few volts (5 volts
was once the standard) so that if you try to use test equipment
designed for testing 3-5 volt computer logic (TTL) on the 12 volts of a
serial port, it may damage the test equipment.

<sect1> Voltage Sequence for a Byte <label id="byte_seq">
<p> The transmit pin (TxD) is held at -12 V (mark) at idle when nothing
is being sent.  To start a byte it jumps to +12 V (space) for the
start bit and remains at +12 V for the duration (period) of the start
bit.  Next comes the low-order bit of the data byte.  If it's a 0-bit
nothing changes and the line remains at +12 V for another bit-period.
If it's a 1-bit the voltage jumps from +12 to -12 V.  After that comes
the next bit (-12 V if a 1 or +12 V if a 0), etc., etc.  After the
last data bit, a parity bit may be sent and then a -12 V (mark) stop
bit.  Then the line remains at -12 V (idle) until the next start bit.
Note that there is no return to 0 volts and thus there is no simple
way (except by a synchronizing signal) to tell where one bit ends and
the next one begins for the case where 2 consecutive bits are the same
polarity (both zero or both one).

A 2nd stop bit would also be -12 V, just the same as the first stop
bit.  Since there is no signal to mark the boundaries between these
bits, the only effect of the 2nd stop bit is that the line must remain
at -12 V idle twice as long.  The receiver has no way of detecting the
difference between a 2nd stop bit and a longer idle time between
bytes.  Thus communications works OK if one end uses one stop bit and
the other end uses 2 stop bits, but using only one stop bit is
obviously faster.  In rare cases 1 1/2 stop bits are used.  This means
that the line is kept at -12 V for 1 1/2 time periods (like a stop bit
50% wider than normal).

<sect1> Parity Explained <label id="parity_def">
<p> Characters are normally transmitted with either 7 or 8 bits of
data.  An additional parity bit may (or may not) be appended to this
resulting in a byte length of 7, 8 or 9 bits.  Some terminal emulators
and older terminals do not allow 9 bits.  Some prohibit 9 bits if 2
stop bits are used (since this would make the total number of bits too
large: 12 bits total after adding the start bit).

The parity may be set to odd, even or none (mark and space parity may
be options on some terminals or other serial devices).  With odd
parity, the parity bit is selected so that the number of 1-bits in a
byte, including the parity bit, is odd.  If a such a byte gets
corrupted by a bit being flipped, the result is an illegal byte of
even parity.  This error will be detected and if it's an incoming byte
to the terminal an error-character symbol will appear on the screen.
Even parity works in a similar manner with all legal bytes (including
the parity bit) having an even number of 1-bits.  During set-up, the
number of bits per character usually means only the number of data
bits per byte (7 for true ASCII and 8 for various ISO character sets).

A "mark" is a 1-bit (or logic 1) and a "space" is a 0-bit (or logic
0).  For mark parity, the parity bit is always a one-bit.  For space
parity it's always a zero-bit.  Mark or space parity (also known as
"sticky parity") only wastes bandwidth and should be avoided if
feasible.  The <tt/stty/ command can't set sticky parity but it's
supported by serial hardware and can be dealt with by programming in
C.  "No parity" means that no parity bit is added.   For terminals
that don't permit 9 bit bytes, "no parity" must be selected when using
8 bit character sets since there is no room for a parity bit.

<sect1> Forming a Byte (Framing)
<p> In serial transmission of bytes via RS-232 ports, the low-order
bit is always sent first (the bit-order).  Serial ports on PC's use
asynchronous communication where there is a start bit and a stop bit
to mark the beginning and end of a byte.  This is called framing and
the framed byte is sometimes called a frame.  As a result a total of
9, 10, or 11 bits are sent per byte with 10 being the most common.
8-N-1 means 8 data bits, No parity, 1 stop bit.  This adds up to 10
bits total when one counts the start bit.  One stop bit is almost
universally used.  At 110 bits/sec (and sometimes at 300 bits/sec) 2
stop bits were once used but today the 2nd stop bit is used only in
very unusual situations (or by mistake since it still works OK that
way but wastes bandwidth).

Don't confuse this type of framing with the framing used for a packet
of bytes on a network.  The serial port just frames every byte.  For a
network, many bytes are framed into a packet (sometimes called a
frame).  For a network frame, instead of a start bit, there is a
sequence of bytes called a header.  On a network that uses serial
ports (with modems), a report of a frame error usually refers to a
multi-byte frame and not the serial port frame of a single byte.

<sect1> How "Asynchronous" is Synchronized

<p> The RS-232 serial port as implemented on PC is asynchronous which
in effect means that there is no "clock" signal sent with "ticks" to
mark when each bit is sent..  There are only two states of the
transmit (or receive) wire: mark (-12 V) or space (+12 V).  There is
no state of 0 V.  Thus a sequence of 1-bits is transmitted by just a
steady -12 V with no markers of any kind between bits.  For the
receiver to detect individual bits it must always have a clock signal
which is in synchronization with the transmitter clock.  Such a clock
would generate a "tick" in synchronization with each transmitted (or
received) bit.

For asynchronous transmission, synchronization is achieved by framing
each byte with a start bit and a stop bit (done by hardware).  The
receiver listens on the negative line for a positive start bit and
when it detects one it starts its clock ticking.  It uses this clock
tick to time the reading of the next 7, 8 or 9 bits.  (It actually is
a little more complex than this since several samples of a bit are
normally taken and this requires additional timing ticks.)  Then the
stop bit is read, the clock stops and the receiver waits for the next
start bit.  Thus async is actually synchronized during the reception
of a single byte but there is no synchronization between one byte and
the next byte.

<sect> Other Serial Devices (not async RS-232) <label id="non_rs232">

<sect1> Successors to RS-232 <label id="non_232">
<p> A number of EIA (or RS) standards have been established for higher
speeds and longer distances using twisted-pair (balanced) technology.
Balanced transmission make possible higher speeds, and can be a
thousand times faster than unbalanced RS-232.  For a given speed, the
distance (maximum cable length) may be many times longer with twisted
pair.  But PC's, prior to about 2004??, kept being made with the
quasi-obsolete RS-232 since it works OK with modems and mice since the
cable length is short.

High speed serial ports (over 460.8 kbps) will often support both
RS-232 and EIA-485/EIA-422 (RS-485/RS-422) modes .  (Note that for
non-RS-232 I've used the "EIA" designation instead of the more
commonly used "RS" but they both mean the same thing.)  At such high
speeds RS-232 is not of much use (except for a very short cable).

<sect1> EIA-422-A (balanced) and EIA-423-A (unbalanced)
<p> EIA-423 is just like the unbalanced RS-232 except that the
voltage is only 5 volts.  Since this falls within RS-232 specs it
can be connected to a RS-232 port.  Its specs call for somewhat
higher speeds than the RS-232 (but this may be of little help on a
long run where it's the unbalance that causes interference).  Since
EIA-423 is not much of an improvement over RS-232, it is seldom used
except on old Mac computers.

EIA-422 is twisted pair (known as "balanced" or "differential) and is
(per specs) exactly 100 times as fast as EIA-423 (which in turn is
somewhat faster than RS-232).  Apple's Mac computer used it prior to
mid-1998 with its RS-232/EIA-422 port.  The Mac used a small round
"mini-DIN-8" connector and named these serial ports as "modem port",
"printer port", and/or "GeoPort".

Mac also provided conventional RS-232 but at only at 5 volts (which
is still legal RS-232).  To make it work like at RS-232 one must use
a special cable which (signal) grounds RxD+ (one side of a balanced
pair) and use RxD- as the receive pin.  While TxD- is used as the
transmit pin, for some reason TxD+ should not be grounded.  See <url
url="http://www.modemshop.com/csm-comm-faq.html" name="Macintosh
Communications FAQ">.  However, due to the fact that Macs (and
upgrades for them) cost more than PC's, they are not widely as host
computers for Linux.

<sect1> EIA-485
<p> This is like EIA-422 (balanced = differential).  It is
half-duplex.  It's not just point-to-point but is like ethernet or the
USB since all devices (nodes) on it share the same "bus".  It may be
used for a multidrop LAN (up to 32 nodes or more).  Unfortunately,
Linux currently doesn't support this and you can only use it under
Linux only for point-to-point where it behaves like RS-232.  So read
further only if you are curious about how its features would work if
only Linux supported them. 

Since many nodes share the same twisted pair, there's a need to use
the electrical tri-state mode.  Thus, besides the 0 and 1 binary
states, there is also an open circuit state to permit other nodes to
use the twisted pair line.  Instead of a transmitter keeping a 1-state
voltage on the line during line idle, the line is open circuited and
all nodes just listen (receive mode).

The most common architecture is master/slave.  The master polls the
slaves to see if they have anything to send.  A slave can only
transmit just after it's been polled.  But EIA-485 is just an
electrical specification and doesn't specify any protocol for the
master/slave interaction.  In fact, it doesn't even specify that there
must be a master and slaves.  So various protocols have been used.
Based on a discussion of 485 on the linux-serial mailing list in March
2003, it seems likely that none of these master/slave protocols are
currently supported by Linux.

There is an alternative implementation where two pair of wires are used
for sending data.  One pair is only for the Master to send to the Slaves.
Since no one transmits on this line except the master, there is no
need for it to be tri-state.  Thus the Master may just be RS-232 but
the slaves must still be EIA-485.  See
<url url="http://www.hw.cz/english/docs/rs485/rs485.html"> for more
details.

<sect1> EIA-530
<p> EIA-530-A (balanced but can also be used unbalanced) at 2Mbits/s
(balanced) was intended to be a replacement for RS-232 but few have
been installed.  It uses the same 25-pin connector as RS-232.

<sect1> EIA-612/613
<p> The High Speed Serial Interface ( HSSI = EIA-612/613) uses a
50-pin connector and goes up to about 50 Mbits/s but the distance is
limited to only several meters.  For Linux there are PCI cards
supporting HSSI.  The companies that sell the cards often provide (or
point you to) a Linux driver.  A howto or the like is needed for this
topic.

<sect1> The Universal Serial Bus (USB)
<p> The Universal Serial Bus (USB) is being built into PCI chips.
Newer PC's have them.  It was originally 12 Mbps but is now 480 Mbps
over a twisted pair with a 4-pin connector (2 wires are power supply).
It also is limited to short distances of at most 5 meters (depends on
configuration).  Linux supports the bus, although not all devices that
can plug into the bus are supported.

It is synchronous and transmits in special packets like a network.
Just like a network, it can have several devices physically attached
to it, including serial ports.  Each device on it gets a time-slice of
exclusive use for a short time.  A device can also be guaranteed the
use of the bus at fixed intervals.  One device can monopolize it if no
other device wants to use it.  It's not simple to describe in detail.

For serial ports on the USB bus, there are numerous configuration
options to use when compiling the kernel.  They all start with:
CONFIG_USB_SERIAL.  Each one is usually for a certain brand/model of
serial port, although generic is also an option.  See the
Configuration Help file in the kernel documentation.

For documentation, see the USB directory in /usr/share/doc/kernel ...
and look at the file: usb-serial.txt.  The modules that support usb
serial devices are found in the modules tree:
kernel/drivers/usb/serial.  It would be nice to have a HOWTO on the
USB.  See also <url url="http://www.linux-usb.org"> and/or <url
url="http://www.qbik.ch/usb/">.

<sect1> Firewire
<p> Firewire (IEEE 1394) is something like the USB only faster (800
Mbps is planned).  The protocol on the bus is claimed to be more
efficient than USB's.  It uses two twisted pair for data plus two
power conductors (6 conductors in all).  A variants uses only 4
conductors.  You may compile firewire support into the Linux kernel.
Like USB, it's also limited to short distances.

<sect1> MIDI
<p>Sound cards often have a 15-pin game port connector used for MIDI.
They are for connecting a musical keyboard to a PC so that you can
create musical recordings.  You could also connect a MIDI sound
system.  The MIDI standard uses 31250 baud (1M/32) which is not
available on an ordinary serial port.  Some MIDI devices are designed
so that they can be connected directly to an ordinary serial port.

Besides the 15-pin connector, a 5-pin DIN connector is also a MIDI
standard but the flow of sound is only one way thru it so for
bidirectional sound you need 2 of them.  Breakout cables often have a
15-pin connector on one end and 2 or more 5-pin connectors on the
other end.  The /dev/midi00 is for MIDI.

<sect1> Synchronization & Synchronous <label id="sync">
<p> Beside the asynchronous RS-232 (and others) there are a number of
synchronous serial port standards.  In fact RS-232 includes
synchronous specifications but they aren't normally implemented for
serial ports on PC's.  But first we'll explain what a synchronous
means.

<sect2> Defining Asynchronous vs Synchronous

<p> Asynchronous (async) means "not synchronous".  In practice, an
async signal is what the async serial port sends and receives which is
a stream of bytes with each byte framed by a start and stop bit.
Synchronous (sync) is most everything else.  But this doesn't explain
the basic concepts.

In theory, synchronous means that bytes are sent out at a constant
rate one after another in step with a clock signal tick.  There is
often a separate wire or channel for sending the clock signal.  The
clock signal might also be embedded in the transmitted bytes.
Asynchronous bytes may be sent out erratically with various time
intervals between bytes (like someone typing characters at a
keyboard).

When a file is being sent thru the async serial port, the flow of
bytes will likely be at the speed of the port (say 115.2k) which is a
constant rate.  This flow may frequently start and stop due to flow
control.  Is this sync or async?  Ignoring the flow control stops, it
might seem like sync since it's a steady flow.  But it's not because
there is no clock signal and the bytes could have been sent
erratically since they are framed by start/stop bits.

Another case is where data bytes (without any start-stop bits) are put
into packets with possible erratic spacing between one packet and the
next.  This is called sync since the bytes within each packet are
transmitted synchronously.

<sect2> Synchronous Communication
<p> Did you ever wonder what all the unused pins are for on a 25-pin
connector for the serial port?  Most of them are for use in
synchronous communication which is seldom implemented in chips for
PC's.  There are pins for sync timing signals as well as for a sync
reverse channel.  The RS-232 spec provides for both sync and async
but PC's use a UART (Universal Asynchronous Receiver/Transmitter) chip
such as a 16450, 16550A, or 16650 and can't deal with sync.  For sync
one needs a USRT chip or the equivalent where the "S" stands for
Synchronous.  A USART chip supports both synchronous and asynchronous.
Since sync is a niche market, a sync serial port is likely to be quite
expensive.

SCC stands for "Serial Communication Controller" or "Serial Controller
Chip".  It's likely old terminology and since it doesn't say "sync"
or "async" it might support both.

Besides the sync part of the RS-232, there are various other EIA
synchronous standards.  For RS-232, 3 pins of the connector are
reserved for clock (or timing) signals.  Sometimes it's a modem's task
to generate some timing signals making it impossible to use
synchronous communications without a synchronous modem (or without a
device called a "synchronous modem eliminator" which provides the
timing signals).

Although few serial ports are sync, synchronous communication
does often take place over telephone lines using modems which use
V.42 error correction.  This strips off the start/stop bits and puts
the data bytes in packets resulting in synchronous operation over the
phone line.

<sect> Other Sources of Information
<sect1> Books
<p>
<enum>
<item> Axleson, Jan: Serial Port Complete, Lakeview Research, Madison,
	WI, 1998.
<item> Black, Uyless D.: Physical Layer Interfaces & Protocols, IEEE
	Computer Society Press, Los Alamitos, CA, 1996.
<item> Campbell, Joe: The RS-232 Solution, 2nd ed., Sybex, 1982.
<item> Campbell, Joe: C Programmer's Guide to Serial Communications,
       2nd ed., Unknown Publisher, 1993.
<item>  <url url="http://www.ora.com/catalog/posix/" name="Levine, Donald:
	POSIX Programmer's Guide">, O'Reilly, 1991.
<item> Nelson, Mark: Serial Communications Developer's Guide, 2nd ed.,
       Hungry Minds, 2000.
<item> Putnam, Byron W.: RS-232 Simplified, Prentice Hall, 1987.
<item> Seyer, Martin D.: RS-232 Made Easy, 2nd ed., Prentice Hall,
	1991.
<item>  <url
  url="http://heg-school.aw.com/cseng/authors/stevens/advanced/advanced.nclk"
 name="Stevens, Richard W.: Advanced Programming in the UNIX Environment">,
 (ISBN 0-201-56317-7; Addison-Wesley)
<item> Tischert, Michael & Bruno Jennrich: PC Intern, Abacus 1996.
 Chapter 7: Serial Ports
</enum>

Notes re books:
<enum>
<item>"... Complete" has hardware details (including register) but the
 programming aspect is Window oriented.
<item>"Physical Layer ..." covers much more than just RS-232.
</enum>

<sect1> Serial Software
<p> If it's not available in your Linux distribution try:<newline>
<url url="http://www.ibiblio.org/pub/Linux/system/serial/"
name="Serial Software"> for Linux software for the serial ports
including getty and port monitors.<newline>
<url url="http://www.ibiblio.org/pub/Linux/system/serial/"
name="Serial Communications"> for communication programs.

<itemize>
<item> <tt/irqtune/ will give serial port interrupts higher
 priority to improve performance.  Using <tt/hdparm/ for hard-disk tuning
 may help some more.

<item> <tt/modemstat/ and <tt/statserial/ show the current state of
 various modem control lines.  See <ref id="serial_mon" name="Serial
Monitoring/Diagnostics">
</itemize>

<sect1> Related Linux Documents
<p>
<itemize>
<item>man pages for: <tt>setserial</tt> and <tt/stty/
<item> <url url="http://www.gnu.org/manual/glibc/html_chapter/libc_12.html"
name="Low-Level Terminal Interface"> part of "GNU C Library Reference
manual" (in libc (or glibc) docs package).  It covers the detailed
meaning of "stty" commands, etc.
<item>Modem-HOWTO: modems on the serial port
<item>PPP-HOWTO: help with PPP (using a modem on the serial port)
<item>Printing-HOWTO: for setting up a serial printer
<item>Serial-Programming-HOWTO: for some aspects of serial-port programming
<item>Text-Terminal-HOWTO: how they work and how to install and configure
<item>UPS-HOWTO: setting up UPS sensors connected to your serial port
<item>UUCP-HOWTO: for information on setting up UUCP
</itemize>

<sect1> Serial Mailing List
<p>
 The Linux serial mailing list.  To join, send email to <tt><htmlurl
 url="mailto:majordomo@vger.rutgers.edu"
 name="majordomo@vger.kernel.org"></tt>, with ``<tt>subscribe
 linux-serial</tt>'' in the message body.  If you send ``<tt/help/'' in
 the message body, you get a help message.  The server also serves
 many other Linux lists.  Send the ``<tt/lists/'' command for a list
 of mailing lists.

<sect1> Internet
<p>
<itemize>
<item> <url url="http://en.wikipedia.org/wiki/Serial_port" name
="Serial port - Wikipedia">
<item> <url url="http://serial.sourceforge.net/" name="Linux Serial
 Driver home page">  Includes info about PCI support.
<!-- 
<item> Serial Suite by Vern Hoxie was a collection of blurbs about the
 care and feeding of the Linux serial port plus some simple programs.
 Not available.
-->
<item> <label id="vern_"> Serial-Programming-HOWTO (not yet available
from the Linux Documentation Project).  It's now on my website:
<url
url="http://www.lafn.org/~dave/linux/Serial-Programming-HOWTO.txt">
See also: <url
url="http://www.lafn.org/~dave/linux/Serial-Programming-HOWTO-B.txt"
name="Serial-Programming-HOWTO by Peter Baumann">
<item> <url
url="http://www.lafn.org/~dave/linux/terminalIO.html" name="Terminal
IO by Vern Hoxie">
<url
url="http://www.lafn.org/~dave/linux/termios.txt" name="Termios man
page revision by Vern Hoxie">

<item> A white paper discussing serial communications and multiport
 serial boards was available from Cyclades at <tt><htmlurl
 url="http://www.cyclades.com" name="http://www.cyclades.com"></tt>.

<item> <url
url="http://www.freebsd.org/doc/en_US.ISO8859-1/articles/serial-uart/"
name="Serial and UART Tutorial (FreeBSD)">
</itemize>

<sect> Appendix A: Very Obsolete Hardware/Software

<sect1>Replacing pre 1990 UARTS
<p> Many 486 PCs (old) and all Pentiums (or the like) should have
modern 16550As (usually called just 16550's) with FIFOs.  If you have
something really old (pre 1990), the chip may unplug so that you may
be able to upgrade by finding a plug-in 16550A chip and replacing your
old existing 16450 UART.  If the functionality has been built into
another type of chip, you are out of luck.  If the UART is socketed,
then upgrading would be easy if you could find a replacement.  The new
and old are pin-to-pin compatible.  It may be more feasible to just
buy a new serial card on the Internet (few retail stores stock them
as of 2000) or find a used one.

<sect1>Two Ports with the Same IO address
<p>Modern kernels should not allow the opening of ports with the same
IO address.  But one may probe for ports even though they are not
open.  If two ports have the same IO address then old fashioned
probing by sending commands to the address  will erroneously indicate
only one port.  But modem device detection at boot-time should
discover both ports and report the conflict.  In olden days, all sorts
of errors were reported/observed for devices illegally attempting to
use the same IO address.  See <ref id="probing_ss" name="Probing">.

<sect1>Configuring by modifying source code
<p>In the past, to get a certain serial port supported, one might need
to modify the C source code, perhaps by adding a #define to it.
Today, the use of parameters for modules or the kernel, or the use of
configuration options should handle all cases (except possible for
antique hardware ??).

<sect1> Modems on Multiport Cards Obsolete for Sending at 56k
<p>For a modem to transmit at nearly 56k requires that it be a special
digital modem and have a digital connection to a digital phone line
(such as a T1 line).  Modems used with serial cards (the modems may
either be on the serial card or on another card) normally have no such
digital connection so they can't be used at the 56k speed, and thus
are obsolete unless one doesn't need to send at 56k.  In other words
they are obsolete for ISP servers but might be OK for small business
or home use.

A partial exception to the above are modem banks that connect to
multiport serial cards where the modem bank can access multiplexed
digital phone lines.   Thus one could use a multiport serial card with
a few 56k digital modems for sending at 56k.  For both analog and
digital modems there is one modem on each serial port so there needs
to be an external cable (modem bank to multiport) for each modem.
This can lead to a large number of cables.  So it's less clutter (and
cheaper) to use internal modems without a multiport card.  This makes
even this "exception" obsolete for high volume work.  It's somewhat
analogous to the lower cost of an internal modem for a desktop PC as
compared to the higher cost (and more cabling) for an external modem.
See Modem-HOWTO: Modem Pools, Digital Modems.

<sect1>Lock-Files if you used the depreciated devfs
<p> The abandoned device-filesystem (devfs) has the /dev directory
with subdirectories.  As of late 2001, there were problems with
lockfiles.  For example, the lockfile mechanism considered
dev/usb/tts/0 and /dev/tts/0 to be the same device with name "0".
Ditto for all other devices that had the same "leaf" name.  Also, if
some applications use the old name for a device and other applications
use the devfs name for the same  device, then the lockfiles will have
different names.  But the serial driver should know they are the same.

<!-- serial_port_names.D 
in Modem and Serial HOWTOs
Feb  2010: moved devfs part of this file to the Obsolete section of
Serial-HOWTO -->

<sect1>Devfs (The deprecated Device File System.  History)
<p>Kernel 2.4 introduced the now obsolete optional "device file system"
(devfs) with a whole new set of names for everything.  Some people
used it but the majority probably didn't.  But in 2003-4, it was
claimed that devfs had unsolvable problems and starting with kernel
2.6.12 it was replaced with "udev" (kernels prior to 2.6.12 also could
use udev but with some problems).  Although udev doesn't provide all
the functionality of devfs, it does handle hot plugging.  Also, the
use of udev isn't required to run Linux so some people don't use it.
But many distributions install it by default.

Devfs was a good idea and was claimed to be more efficient than udev.
But unfortunately, the author of devfs didn't maintain it for long and
it allegedly became not too well maintained.  So for better or worse
we now have udev instead although the debate of devfs vs. udev still
continues.  For a detailed description of devfs see: <url
url="http://www.atnf.csiro.au/~rgooch/linux/docs/devfs.html"> Also see
the kernel documentation tree: filesystems/devfs.

The names of devices for the devfs can be used in udev, but usually
are not and may not be simple to activate.  Here's the devfs names for
serial devices: ttyS1 becomes tts/1, ttyUSB1 becomes /usb/tts/1, and
ttyACM1 is /usb/acm/1.  Note that the number 1 above is just an
example.  It could be replaced by 0, 2, 3, 4, etc.  Some more examples
of udev names: ttyS2 becomes tts/2 (Serial port), tty3 becomes vc/3
(Virtual Console), ptyp1 becomes pty/m1 (PTY master), ttyp2 becomes
pty/s2 (PTY slave).  "tts" looks like a directory which contains
devices "files": 0, 1, 2, etc.  All of these new names were put in the
/dev directory although optionally one may put them elsewhere.

For devfs, device names in the /dev directory are created automatically
by the corresponding driver.  Thus, if serial support comes from a
module and that module isn't loaded yet, there will not be any serial
devices in the /dev directory.  This can be confusing: you physically
have serial ports but don't see them in the /dev directory.  However,
if a device name is requested (attempt to open it) by a communication
program and the serial module isn't loaded, the kernel is supposed to
try to find a driver for it and create a name for it in the /dev
directory.

This works OK if it finds a driver.  But suppose there is no driver
found for it.  For example, if you try to use "setserial" to configure
a port that the driver failed to detect, it claims there is no such
port.  How does one create a devfs port in this case?

For multiport devices for example, /dev/ttyF9 becomes /dev/ttf/9, or
in a later version /dev/tts/F9.  Substitute for F (or f) whatever
letter(s) your multiport board uses for this purpose.  A multiport
driver is supposed to create a devfs name similar to the above and put
it into the /dev directory

<p> END OF Serial-HOWTO </article>
