Subject: Linux Serial HOWTO
From: [email-masked] (Greg Hankins)
Newsgroups: comp.os.linux.announce,comp.os.linux.admin

Archive-name: linux/howto/serial
Last-modified: 6 Jan 94

Linux Serial-HOWTO version 1.0 Thu Jan 6 1994 
maintained by Greg Hankins <[email-masked]>.

** Note: This file supercedes the old Linux Serial-FAQ.

INDEX 
	0. 	Introduction
	0.1	New versions of this document
	0.2	Recommended reading
	0.3	Feedback	
	0.4 	Disclaimer
	1. 	Supported serial hardware	
	2. 	How are serial devices named?
	3. 	What is 'getty_ps 2.0.7b'?	
	3.1	Installation
	3.2	Customizing 'getty'
	4. 	How do I dial out with my modem?
	4.1	Hardware requirements
	4.2	Connecting to your modem
	5. 	How do I dial in and out with my modem?
	5.1	Note for UUCP users
	6. 	How do I set up a terminal connected to my PC?
	6.1	Hardware requirements
	6.2	Setting up 'getty'
	7. 	Can I use more than 2 serial ports?  
	8. 	Can Linux detect the serial devices automatically?	
	9. 	What numbers should my serial devices in /dev have?	
	9.1	Creating devices with the 'mknod' command
	10. 	How should I configure my modem?
	10.1	Dial out configuration
	10.2	Dial in and out configuration
	11. 	How can I hook up a printer to my serial port?
	12.	What are locks, and what are they for?
	13. 	What are UARTs?  How do they affect performance?  
	14. 	'kermit': a quick start
	15. 	Troubleshooting
	15.1	I keep getting "line XXX of inittab invalid"
	15.2	When I try to dial out, it says "/dev/cuaX: Device or 
		resource busy"
	15.3	I keep getting "respawning too fast, disableing for 5 minutes"
	15.4	I have a modem/terminal connected, but root can't login from 
		the serial device.
	15.5	I have my terminal connected to my PC, but after I type in a 
		login name, it just locks up.
	15.6	At high speeds, my modem looses characters, and my computer 
		gets really slow!
	16. 	Contributions

===============================================================================

0.	Introduction

	This is the Linux SERIAL-HOWTO.  This document is a rewrite of the 
	serial-FAQ.  The serial-FAQ is out of date, and should no longer be 
	distributed.

	This document describes the Linux serial drivers, and software
	used for serial communications. 

	I will attempt to provide answers to a variety of serial questions
	and common problems.

0.1	New versions of this document
	
	New versions of this document can be retrieved via anonymous ftp from 
	sunsite.unc.edu:/pub/Linux/docs.  It will also be posted to several 
	newsgroups, including comp.os.linux.announce.

0.2	Recommended reading

	- man pages for:
		init(1), getty(1), login(1)
		gettydefs(4)
		setserial(8)

	- your modem manual

	- "Managing UUCP and Usenet" by Tom O'Reilly and Grace Todino, 
	  (highly recommended by nearly everyone)

	- Other HOWTOs: UUCP-HOWTO, Printing-HOWTO

0.3 	Feedback

	Please send me any comments, suggestions, or additional material you 
	have.  I'm always eager to hear about what you think about the HOWTO.  
	I'm also always on the lookout for improvements!  But, please don't 
	just tell me "it sucks".  Tell me what exactly you don't understand, 
	or what could be clearer.

	If you find any wrong information, please contact me ASAP.

0.4	Disclaimer

	Your milage may vary.  The answers given may not work for all systems
	and all setup combinations.  

1. 	Supported serial hardware	

	Linux supports standard PC serial boards, internal modems, and a number
	of multiport serial boards, including the Usenet Serial Board II, the 
	Boca 4-port (BB-1004), 8-port (BB-1008), and 16-port (BB-2016) boards, 
	and AST FourPort boards and clones.  In general, Linux will support any
	serial board which uses a 8250, 16450, 16550, or 16550A (or compatible)
	UART, and assignes the UART registers to a contiguous block of 8 I/O 
	ports on the 386/486.

	Linux does not support any intelligent serial ports, nor is it likely
	that it will in the future.

	The Usenet Serial Board II is available from:
	South Coast Computing Services, Inc.
	phone +1 713 661 3301
	email [email-masked]

2. 	How are serial devices named?

	There are 4 serial devices corresponding to COM1 - COM4, /dev/cua0 - 
	/dev/cua3 and /dev/ttyS0 - /dev/ttyS3. The /dev/ttyS* devices are for 
	dialin and /dev/cua* devices for dialout. 

	On some installations, two extra devices will be created, /dev/modem 
	for your modem and /dev/mouse for your mouse.  Both of these are 
	symbolic links to the appropriate /dev/cua* device which you specified 
	during the installation (unless you have a bus mouse, then /dev/mouse 
	will point to the bus mouse device).

	There has been some discussion on the merits of /dev/mouse and 
	/dev/modem.  I prefer using the "real" device names, and not the 
	symbolic links.  It just seems less confusing to me.  Use them if you 
	like, but make sure they point to the right device.

	On the other hand, if you use the links, and they point to the correct 
	devices, using /dev/modem may be simpler for some people.  It's really 
	up to you what you use.

3. 	What is 'getty_ps 2.0.7b'?  

	This is just another version of 'getty', which is a program that handles
	some of the login process when you log in to a UNIX box.  This version 
	is better than the stock 'getty' that comes with most recent Linux 
	distributions.  It was written by Paul Sutcliffe, Jr.  
	<[email-masked]>.  2.0.7b is the latest version, and supercedes 
	any older versions.  If you do not have this version of 'getty', I 
	highly recommend that you get it.  It can be found on the standard 
	Linux ftp sites.

3.1 	Installation

	Get the package from an ftp sites, and install according to the 
	instructions.

	Users of SLS 1.0.1/1.0.2 should be very carefull because there might 
	be two versions of 'getty_ps' lurking on your disks!  The default one 
	is 2.0.4 and the non-default one is 2.0.7b.  Make sure you use the 
	right one.

	Be sure that the new 'getty_ps' works correctly *before* you remove all 
	the other stuff!  I recommend changing one of your virtual console 
	lines from:
	c2:23456:respawn:/etc/getty tty2
	to:
	c2:23456:respawn:/etc/getty_ps tty2 VC console

	Put this line in your /etc/gettydefs:
	VC# B9600 SANE CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: #VC

	and restart 'init':
	yourbox# init q    	(for poegil init, use "kill -HUP 1")

	If you can log in with 'getty_ps', it's working.  Move 'getty_ps' to 
	'getty', or link it.  Be sure to update your /etc/inittab to correctly 
	invoke the new 'getty'.  'getty_ps' can handle the stock 'getty's 
	syntax.

	Once you have installed it, and are certain it is working, you can 
	remove any other versions of 'getty' you have.  Be sure to check in 
	/bin, /etc, /usr/bin, /usr/etc, /usr/man/man1, and /usr/man/cat1 for 
	any thing called "getty" - there are old versions lurking everywhere...
	Also, you can safely remove the old config file /etc/gettytab - you 
	only need /etc/gettydefs.

3.2	Customizing 'getty'

	There are lots of parameters you can tweak for each port you have.  
	These are implemented in seperate config files for each port (if you 
	want).  I'm not convinced you need them at all.  Mine works fine without
	them.  In general you should only need them to implement setups for 
	unusual situations.  You might consider using the ALTLOCK option, to 
	be sure that all locks are checked properly.  Here are a couple of 
	/etc/default sample files - note the /etc/default/getty will be used 
	by *all* instances of 'getty', and that /etc/default/getty.tty* will 
	only be used by that one port.

	/etc/default/getty:
	SYSTEM=<systemname>
	VERSION=/proc/version
	LOGIN=/bin/login
	ISSUE=/	etc/issue
	CLEAR=NO
	HANGUP=YES

	/etc/default/getty.ttySX:
	SYSTEM=<systemname>
	VERSION=/proc/version
	LOGIN=/bin/login
	ISSUE=/etc/issue
	CLEAR=NO
	HANGUP=YES
	INIT="" ATZ\r OK
	TIMEOUT=60
	WAITFOR=RING
	CONNECT="" ATA\r CONNECT \s\A
	ALTLINE=cuaX

	/etc/default/uugetty.ttySX:
	ALTLOCK=cuaX
	TIMEOUT=60
	INIT="" ATZ\r OK
	DELAY=1
	CLEAR=NO

	/etc/default/uugetty.ttySX:
	SYSTEM=<systemname>
	VERSION=/proc/version
	LOGIN=/bin/login
	ISSUE=/etc/issue
	CLEAR=NO
	HANGUP=YES
	INIT="" ATZ1\r
	ALTLINE=cuaX
	ALTLOCK=cuaX
	INITLINE=cuaX
	TIMEOUT=60

4. 	How do I dial out with my modem?

4.1	Hardware requirements

	First, make sure you have the right hardware.  You probably want to 
	hook the modem to the DB25 pin port on your PC.  This requires a 
	straight through cable, with no pins crossed.  (The 9 pin port works 
	nicely for a mouse, since most serial mice already have a DB9 
	connector.)  

	For an internal modem, you will not need a modem cable.  Be sure to
	set the interrupt to something that is not being used.

4.2	Connecting to your modem

	For now, make sure that nothing is using that port.  Check to see if 
	there is a 'getty' watching it, or it is not locked by anything else.  
	If there is a 'getty' process, check your /etc/inittab file for entries
	using your serial port.  Comment the line out (with "#"), and restart 
	'init'.  Do not comment any lines that contain a tty* device, only 
	ones containing ttyS* or cua*.  tty* are your virtual consoles.

	Use 'kermit' to test the setup.  You should set your line to 
	/dev/cua*, and the speed to the highest speed your modem can handle.  
	Try dialing out, or talking to the modem.  If you are properly connected
	your modem should respond to "AT" commands.  If you say "AT" to your 
	modem, it should respond with "OK".  Once you are sure that the modem 
	is functional, you can use more complex programs, like 'Seyon', or 
	'xconn'.

5.	How do I dial in and out with my modem?

	Get your modem to dial out correctly.  

	Replace the stock 'getty' with 'getty_ps' as described in HOWTO section
	2.  Update /etc/gettydefs to include entries for modems:
	(note that the entries point to each other, these are not for fixed 
	speed)

	# Modem entries
	38400# B38400 CS8 # B38400 SANE -ISTRIP HUPCL #@S @L @B login: #19200

	19200# B19200 CS8 # B19200 SANE -ISTRIP HUPCL #@S @L @B login: #9600

	9600# B9600 CS8 # B9600 SANE -ISTRIP HUPCL #@S @L @B login: #2400

	2400# B2400 CS8 # B2400 SANE -ISTRIP HUPCL #@S @L @B login: #1200

	1200# B1200 CS8 # B1200 SANE -ISTRIP HUPCL #@S @L @B login: #300

	300# B300 CS8 # B300 SANE -ISTRIP HUPCL #@S @L @B login: #38400

	If you have a 9600 bps or faster modem with flow control, you can lock
	your serial port speed and let the modem handle the translation to 
	other bps rates. Then, instead of the step down series of lines listed 
	below, /etc/gettydefs only needs to contain one line for the modem: 

	38400# B38400 CS8 # B38400 SANE -ISTRIP HUPCL #@S login: #38400
 	or
	19200# B19200 CS8 # B19200 SANE -ISTRIP HUPCL #@S login: #19200

	If you have your modem set up to do RTS/CTS hardware flow control, you
	can add CRTSCTS to the entries.

	Next, make sure that you have a dialin and dialout device for the port
	and your modem is on.  If you have your modem on /dev/cua1, you will 
	need a device called /dev/ttyS1.  If you don't have the correct devices,
	see the question on how to create devices, and create the devices.

	Edit your /etc/inittab, so that 'getty' is run on your serial port:

	Add the following line:
	S1:456:respawn:/etc/getty ttyS1 38400
	(				^-- put your highest modem speed here)

	Restart 'init':
	yourbox# init q    	(for poegil init, use "kill -HUP 1")

	Now Linux will be watching your serial port for connections.

	Dial in from another site and log in to you Linux system.  Rejoice.

5.1	Note for UUCP users

	For sites with UUCP feeds, you probably want to use 'uu_getty', instead
	of plain 'getty'.  It's designed for use with UUCP.  Look at the README
	for it for exact differences.  Most importantly, it checks the lock 
	files that UUCP creates.

6. 	How do I set up a terminal connected to my PC.

6.1	Hardware requirements

	Make sure you have the right kind of cable!  A null modem cable bought 
	at a computer store should do it.  At a minimum, you should have:

	              	2 - 3     
	              	3 - 2     	
		      	7 - 7     		
		       20 - 20    

	If that doesn't work, try the following (a full null modem cable):

			2 - 3     			Pin names:
			3 - 2    			1 Frame Ground 
			4 - 5     			2 TxD
			5 - 4    			3 RxD 
			6 - 20   			4 RTS 
			7 - 7    			5 CTS 
			8 - 20   			6 DSR 
		       20 - 6				7 Signal Ground
                       20 - 8				8 DCD
						       20 DTR

	You will need pins 4 and 5 if you want to do any kind of hardware flow
	control.

	Connect your terminal to your computer.  If you can, tell you terminal
	to ignore modem control signals.  Try using 9600 bps, 8 data bits, 1 
	stop bit, no parity bits.

6.2	Setting up 'getty'

	Replace the stock 'getty' with 'getty_ps' as described in section 2.
	Add an entry for 'getty' to use for your terminal in /etc/gettydefs.  
	Mine looks like this:

	# 9600 baud Dumb Terminal entry
	DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: 
	#DT9600

	(the above should all be on one line)	

	You might add HUPCL so the login is killed whenever you switch of
	the terminal (provided you use a proper null-modem cable).

	Edit your /etc/inittab file to run getty on the serial port.  Mine 
	looks like this:

	S1:456:respawn:/etc/getty ttyS1 DT9600 vt100

	restart 'init'
	yourbox# init q    	(for poegil init, use "kill -HUP 1")

	At this point, you should see a login prompt on your terminal.  Rejoice.
	Party.  Log in on your terminal and PC.  'talk' to yourself from 
	accross the room.

7. 	Can I use more than 2 serial ports?  Can I use a modem, a terminal, 
	and my mouse?

	That depends.  The number of serial ports you can use is limited by the
	number of interrupts (IRQ) we have to use.  Each serial devices must 
	be assigned it's own interrupt.  On a PC, each serial port counts as a 
	device.  However, there are specially designed multiport serial cards 
	which have multiple serial ports on one device.  (For those of you who 
	are hardware minded, all it means that is that there is a OR gate 
	combining all of the IRQ outputs from each serial port.)

	Your PC will normally come with COM1 and COM3 at IRQ 4, and COM2 and 
	COM4 at IRQ 3.  To use more than 2 serial devices, you will have to 
	give up an interrupt to use.  A good choice is to reassign an interrupt
	from your parallel port.  Your PC normally comes with IRQ 5 and IRQ 7 
	set up as interrupts for your parallel ports, but few people use 2 
	parallel ports.  You can reassign one of the interrupts to a serial 
	device, and still happily use your parallel port.  You will need the 
	'setserial' program to do this.  In addition, you have to play with the
	jumpers on your boards.	 

	You will need to set things up so that there is one, and only one 
	interrupt for each serial device.  Here is how I set mine up (in 
	/etc/rc.local - you should do it upon startup somewhere):

	/etc/setserial /dev/cua0 irq 3		# mouse
	/etc/setserial /dev/cua1 irq 4		# terminal
	/etc/setserial /dev/cua2 irq 5		# modem 
	/etc/setserial /dev/cua3 irq 9		# modem
	# my printer is hooked up on my parallel port IRQ 7 - no change 

	The problem is that all of the bus lines on the ISA bus are 
	electrically connected together, in parallel, and all of the bus lines 
	are driven by TTL gates.  If there are two devices simultaneously 
	trying to drive an IRQ line, they end up "fighting" over that line.  
	You get undefined behavior when one of the TTL gates is trying to drive
	the IRQ line low, and the other TTL gate is trying to drive the IRQ 
	line high.  Usually one of the gates will consistently win, with the 
	net result that the computer will only see interrupts from one of the 
	serial devices --- but this is not guaranteed; there may be very 
	erratic behavior as a result.

	Standard IRQ assignments:
              IRQ  0    Timer channel 0
	      IRQ  1    Keyboard
              IRQ  2    Cascade for controller 2
              IRQ  3    Serial port 2
              IRQ  4    Serial port 1
              IRQ  5    Parallel port 2 (Reserved in PS/2)
              IRQ  6    Floppy diskette
              IRQ  7    Parallel port 1
              IRQ  8    Real-time clock
              IRQ  9    Redirected to IRQ2
              IRQ 10    Reserved
              IRQ 11    Reserved
              IRQ 12    Reserved (Auxillary device in PS/2)
              IRQ 13    Math coprocessor
              IRQ 14    Hard disk controller
              IRQ 15    Reserved

	There is really no Right Thing to do when choosing interrupts.  Just 
	make sure it isn't being used.  5 is a good choice.  If 5 is also
	taken, I suggest 2 (which is the same as 9).

8. 	Can Linux detect the serial devices automatically?

	Yes.  To get Linux to detect and set up the serial devices automatically
	on startup, add the line:
	#define CONFIG_AUTO_IRQ
	to /usr/src/linux/chr_drv/serial.c (look for where the other #defines 
	are).
	You'll have to recompile your kernel.

9. 	What numbers should my serial devices in /dev have?

	The recommended convention is:

	/dev/ttyS0 major 4, minor 64	/dev/cua0 major 5, minor 64
	/dev/ttyS1 major 4, minor 65	/dev/cua1 major 5, minor 65
	/dev/ttyS2 major 4, minor 66	/dev/cua2 major 5, minor 66
	/dev/ttyS3 major 4, minor 67	/dev/cua3 major 5, minor 67

9.1 	Creating devices with the 'mknod' command

	If you don't have a device, you will have to create it with the 'mknod' 
	command.

	Example, suppose you needed to create devices for cua0 (COM1):

	yourbox# mknod /dev/cua0 c 5 64
	yourbox# mknod /dev/ttyS0 c 4 64

	Note that all distributions should come with the correct devices
	already made.

10. 	How should I configure my modem?

	Modems have several registers you can change to your liking.
	You'll have to get your modem connected before you try to set registers.
	Consult your modem manual for more detail on registers.

10.1	Dial out configuration

	For dial out use only, configure it however you want.

	I like to see result codes, so I set Q0.  I also like to see what I'm 
	typing, so I set E1.

10.2	Dial in and out configuration

	For dial in and dial out use, you have to set it up a certain way.

	Here is what you have to set: (preceed each register with the AT 
				       command - ie ATE0, etc...)

	E0	Command echo OFF	
	Q1	NO result codes are reported		

	If you don't shut up the modem completely, it will talk to 'getty'.
	'getty' will think someone is trying to log in, and will spawn 'login'.
	This will cause all sorts of confusion.  Not setting these correctly 
	will probably cause the dreaded "respawning to fast" message.

	&C1	DCD is on after connect *only*

	If DCD is always on, there will always be a connection, and 'getty' 
	will try to log people in.  Not setting this can cause "device busy" 
	errors.

	Other things you should set:

	&S0	DSR is always on
	&D3	DTR on/off resets modem	
	enable your data compression (setting depend on modem manufacturer, 
	consult your modem manual)
	autobaud		     (same applies here)

11. 	How can I hook up a printer to my serial port?

	The answer to this question can be found in the Printing-HOWTO.  It is 
	available on both tsx-11.mit.edu, and sunsite.unc.edu.  There
	may be more than one of these around.  Look for the one maintained by
	Brian McCauley <[email-masked]>.

12. 	What are locks, and what are they for?

	Locks are simply a file saying that a particular device is in use.  
	They are kept in /usr/spool/uucp.  Locks are named LCK..<name>, where 
	<name> is either a device name, or a UUCP site name.  Certain processes
	create these locks so that they can have exclusive access to devices, 
	for instance if you dial out on your modem, a lock will appear telling 
	other processes that someone is using the modem already.  Locks mainly 
	contain the PID of the process that has locked the device.  Most 
	programs look at the lock, and try to determine if that lock is still 
	valid by checking the process table for the process that has locked
	the device.  If the lock is found to be valid, the program should exit.  
	If not, some programs remove the stale lock, and use the device, 
	creating their own lock in the process.

13.  	What are UARTs?  How do they affect performance?

	UARTs (Universal Asyncronous Receiver Transmitter) are chips inside
	your communication devices (terminal, PC).  There is one on each end.  
	Their purpose is to convert characters to bits, send it down the line, 
	and then rebuild characters again on the other end.  The UARTs are 
	asyncronous devices, bacause the time interval between transmission of 
	characters is not fixed.

	Say you have a terminal hooked up to your PC.  When you type a 
	character, the terminal presents it to it's UART.  The UART shifts 
	that character (a byte) out onto the serial line one bit at at time, 
	at a specific rate.  Hence, we have the rates 110, 300, 1200, 2400, 
	... bits/sec (bps).  This is simple a measure of how fast the UART is 
	sending bits.  On the other end, the receiving UART takes all the bits 
	and rebuilds the character (byte).

	There are several different types of UARTs.  You have probably heard of
	dumb UARTs - the 8250 and 16450, and smart or FIFO UARTs - the 16550A.    
	To understand their differences, first let's examine what happens when
	a UART has received a character.

	The UART itself can't do anything with the character, it just sends and 
	receives them.  The CPU gets an interrupt ever time a character is done
	(receiving or sending).  The CPU then moves that character out of the 
	UART and into memory somewhere.  The 8250 and 16450 UARTs only have a 
	1 character buffer.  That means, that every time a character is done, 
	it interrupts the CPU.  At low rates, this is OK.  But, at high transfer
	rates, the CPU get so busy dealing with the UART, that is doesn't have 
	time to tend to other tasks.  In some cases, the CPU does not get 
	around to servicing the interrupt in time, and the character is 
	overwritten.  

	That's where the 16550A UARTs come in.  These chips come with 16 
	character FIFOs.  This means that it can receive or transmit up to 16 
	characters before it has to interrupt the CPU.  Not only can it wait, 
	but the CPU then can transfer all 16 characters at a time.  This is a 
	significant advantage over the other UARTs, which only have the 1 
	character buffer.  The CPU receives significantly less interrupts, and 
	is free to do other things.

	In general, the 8250 and 16450 UARTs should be fine for speeds up to 
	19200 bps.  After 19200, you might start seeing character losses, and a 
	reduction in computing speed.

	Keep in mind that these dumb UART types are not bad, or harmful, they 
	just aren't good for high speeds.  You should have no problem connecting
	a terminal, or a mouse to these UARTs.  But, for a high speed modem,
	the 16550A is definately a must.

	You can buy serial cards with the FIFO UARTs for a little more money,
	just ask your dealer what type of UART is on the card.  Or if you want 
	to upgrade your existing card, you can simply purchase 16550A chips and 
	replace your existing 16450 UARTs.  They are pin-to-pin compatible.
	Some cards come with socketed UARTs for this purpose, if not you can 
	solder.  Note, that you'll probably save yourself a lot of trouble by
	just getting a new card :-).

14. 	'kermit': a quick start.

	'kermit' is one of the many communications programs available to you.  
	It has been ported to nearly ever platform imaginable.
	If you are a beginning user, 'kermit' is a simple way to start using 
	your modem.
  
	Fire up 'kermit' by typing "kermit".  You'll need to set a few basic 
	things up before you can use your modem.  You can keep these commands 
	in $HOME/.kermrc so you don't have to type them every time.  Here is my
	.kermrc as an example.  You'll have to adjust things to fit your setup.

	set line /dev/cua3  	# serial device you want to use
	set modem v42-telebit	# which type of modem you are using
	set speed 38400		# speed	

	At this point, you can just type "c" to connect to your modem, and dial
	manually with "at" commands.  Or, from the kermit> prompt, you can use 
	the "dial" command.

	If you want to use the zmodem file transfer protocol, you can include:
	define rz !rz </dev/cua3 >/dev/cua3
	define sz !sz \%0 >/dev/cua3 </dev/cua3

	in your .kermrc (be sure to put in your correct devices).

	This has been a really quick intro, for more info on 'kermit', RTFM. 

15. 	Troubleshooting:

15.1 	I keep getting "line XXX of inittab invalid"

	Make sure you are using the correct syntax for your version of 'init'.
	The different 'init's that are out there use different syntax in the 
	/etc/inittab file.  Make sure you are using the correct syntax for your
	version of 'getty'. 
 
15.2 	When I try to dial out, it says "/dev/cuaX: Device or resource busy"

	This is one of the most frequent errors people get.  It can happen for 
	a variety of reasons.  If you followed the instructions carefully, this
	should not happen, so double check to make sure you did everything 
	right.  

	This problem usually arrises when DCD is not set correctly.  DCD should
	only be set when there is an actual connection (ie someone is dialed 
	in), not when 'getty' is watching the port.  When 'getty' sees DCD get 
	set, it will lock the port and probably spawn 'login'.  Check to make 
	sure that your modem is configured to only set DCD when there is a 
	connection.  DTR and RTS should be set whenever something is using, or 
	watching the line, like 'getty', 'kermit', or some other comm program.

	Another common cause of "device busy" errors, is that you set up your 
	serial port with an interrupt already taken by something else.  As each
	device 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 taken, your device won't be able to initialize 
	properly.  The device really doesn't have much of any way to tell you 
	that this happened, except that when you try to use it, it will return 
	a device-busy error.

	A fix for this may be to remake the kernel, after doing "make config", 
	and answering "no" to all the devices that you don't actually have.  
	If they're compiled in, the software for these devices may be grabbing 
	your interrupt, even though the physical device does not exist.  Also, 
	you can search through the source and include files.  For example, 
	"grep IRQ /usr/src/linux/include/linux/*" yields a 
	"#define MOUSE_IRQ 5" line in busmouse.h.  Another good place to look 
	is in /usr/src/linux/net/tcp/Space.c.  

	You might experience problems when you use the WAITFOR or WAITCHAR 
	options in your /etc/defaults/getty.whatever file.  Try taking them out.

15.3 	I keep getting "respawning too fast, disableing for 5 minutes".

	Make sure your modem is configured correctly.  Look at registers E and 
	Q.  This usually occurs when your modem is chatting with 'getty'.

15.4 	I have a modem/terminal connected, but root can't login from the serial
   	device.

	This is done on purpose for security reasons.  Generally, you should 
	only have root logins enabled on the console.  It is considered 
	dangerous to allow root logins on anything other than the console.  

	But, you asked...  Edit /etc/login.defs, and find the line that says 
	CONSOLE.  Add the name of the serial device to it.  Say I have my 
	terminal on /dev/ttyS1.  I would change the line:
	CONSOLE tty1:tty2:tty3:tty4:tty5:tty6:tty8
	to
	CONSOLE tty1:tty2:tty3:tty4:tty5:tty6:tty8:ttyS1

15.5 	I have my terminal connected to my PC, but after I type in a login name,
	it just locks up.

	You probably don't have CLOCAL in your /etc/gettydefs entry for the 
	terminal.  You need CLOCAL.  Here is what it should look like:

	# 9600 baud Dumb Terminal entry
	DT9600# B9600 CS8 CLOCAL # B9600 SANE -ISTRIP CLOCAL #@S @L login: 
	#DT9600

	Next, "kill -HUP" the 'getty' process to make it read the entry again.

15.6 	At high speeds, my modem looses characters, and my computer gets really 
	slow!

	If you are trying to run your modem at > 19200 bps, and you don't have 
	16550A UARTs, you should upgrade them.  See the section about UARTs.

16. 	Contributions	
	
	There was no possible way to write this HOWTO alone.  Much of the 
	material is repeated verbatim from the original sources.

	Contributions, ideas, suggestions, and material:
	(in alphabetical order)

	Brandon S. Allbery <[email-masked]>
	Karl Buck <[email-masked]>
	Bryan Curnutt <[email-masked]>
	Arthur Donkers <[email-masked]>
	Karlheinz Hagen <[email-masked]>
	Michael Hamilton <[email-masked]>
	Wayne Hayes <[email-masked]>
	John Henders <[email-masked]>
	Ed Carp <[email-masked]>
	Cheng Chang Huang <[email-masked]>
	Camillus Jayewardena <[email-masked]>
	Jerry Kaidor <[email-masked]>
	Juha Laiho <[email-masked]>
	Bambang N. Prastowo <[email-masked]>
	Mark Schwabacher <[email-masked]>
	Andrew Tefft <[email-masked]>
	Theodore Ts'o <[email-masked]>

END OF SERIAL-HOWTO