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10. Interesting Programs You Should Know About

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.

10.1 Serial Monitoring/Diagnostics Programs

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).

You may already have them. If not, download them from Serial Software. As of June 1998, I know of no diagnostic program in Linux for the serial port.

10.2 Changing Interrupt Priority

10.3 What is Setserial ?

This part is in 3 HOWTOs: Modem, Serial, and Text-Terminal. There are some minor differences, depending on which HOWTO it appears in.


If you have a Laptop (PCMCIA) don't use setserial until you read Laptops: PCMCIA. setserial is a program which allows you to tell the device driver software the I/O address of the serial port, which interrupt (IRQ) is set in the port's hardware, what type of UART you have, etc. Since there's a good chance that the serial ports will be automatically detected and set, many people never need to use setserial. In any case 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) tries to use setserial.

Setserial can also show how the driver is currently set. In addition, it can be made to probe the hardware I0 port addresses to try to determine the UART type and IRQ, but this has severe limitations. See Probing. Note that it can't set the IRQ or the port address in the hardware of PnP serial ports (but the plug-and-play features of the serial driver may do this). It can't read the PnP data stored in configuration registers in the hardware.

If you only have one or two built-in serial ports, they will usually get set up correctly without using setserial. Otherwise, if you add more serial ports (such as a modem card) you may need to deal with setserial. Besides the man page for setserial, check out info in /usr/doc/setserial.../ or /usr/share/doc/setserial. It should tell you how setserial is handled in your distribution of Linux.

Setserial is often run automatically at boot-time by a start-up shell-script for the purpose of assigning IRQs, etc. to the driver. Setserial will only work if the serial module is loaded (or if the equivalent was compiled into your kernel). If the serial module gets unloaded later on, the changes previously made by setserial will be forgotten by the kernel. But recent (2000) distributions may contain scripts that save and restore this. If not, then setserial must be run again to reestablish them. In addition to running via a start-up script, something akin to setserial also runs earlier when the serial module is loaded (or the like). Thus when you watch the start-up messages on the screen it may look like it ran twice, and in fact it has.

Setserial can set 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 needed at slow baud rates of 1200 or lower. It's also needed at higher speeds if there are a lot of "flow control" waits. See "closing_wait" in the setserial man page.

Setserial does not set either IRQ's nor I/O addresses in the serial port hardware itself. That is done either by jumpers or by plug-and-play. You must tell setserial the identical values that have been set in the hardware. Do not just invent some values that you think would be nice to use and then tell them to setserial. However, 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 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:

setserial -g /dev/ttyS*
you'll see some info about how that device driver is configured for your ports. Note that where it says "UART: unknown" it probably means that no uart exists. In other words you probably have no such serial port and the other info shown about the port is meaningless and should be ignored. If you really do have such a serial port, setserial doesn't recognize it and that needs to be fixed.

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 "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 without complaint. 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 "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 already 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.

While assignments made by setserial are lost when the PC is powered off, a configuration file may restore them (or a previous configuration) when the PC is started up again. In newer versions, what you change by setserial may get automatically saved to a configuration file. In older versions, the configuration file only changes if you edit it manually so the configuration always remains the same from boot to boot. See Configuration Scripts/Files


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. But 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 an another scan program?

With appropriate options, 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 Probing

The purpose of this is to see if there is a uart there, and if so, what its IRQ is. Use "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 pnpdump --dumpregs. To try to detect the physical hardware use for example :
setserial /dev/ttyS2 -v autoconfig
If the resulting message shows a uart type such as 16550A, then you're OK. If instead it shows "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 "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 the configuration file /etc/serial.conf or /var/lib/serial.conf which will be used next time you start Linux. At boot-time when the serial module loads (or the like), a probe for UARTs is made automatically and reported on the screen. But the IRQs shown may be wrong. The second report of the same is the result of a script which usually does no probing and thus provides no reliable information as to how the hardware is actually set. It only shows configuration data someone wrote into the script or data that got saved in /etc/serial.conf.

It may be that two serial ports both have the same IO address set in the hardware. Of course this is not permitted 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 setserial to give the IRQ a fictitious value.

Boot-time Configuration

When the kernel loads the serial module (or if the "module equivalent" is built into the kernel) then only 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). You see this as a boot-time message just like as if setserial had been run.

To correct possible errors in IRQs (or for other reasons) there may be a file somewhere that runs setserial again. Unfortunately, if this file has some IRQs wrong, the kernel will still have incorrect info about the IRQs. This file should run early at boot-time before any process uses the serial port. In fact, your distribution may have set things up so that the setserial program runs automatically from a start-up script at boot-time. More info about how to handle this situation for your particular distribution might be found in file named "setserial..." or the like located in directory /usr/doc/ or /usr/share/doc/.

Before modifying a configuration file, you can test out a "proposed" setserial command by just typing it on the command line. In some cases the results of this use of setserial will automatically get saved in /etc/serial.conf when you shutdown. So if it worked OK (and solved your problem) then there's no need to modify any configuration file. See New configuration method using /etc/serial.conf.

Configuration Scripts/Files

Your objective is to modify (or create) a script file in the /etc tree that runs setserial at boot-time. Most distributions provide such a file (but it may not initially reside in the /etc tree). In addition, setserial 2.15 and higher often have an /etc/serial.conf file that is used by the above script so that you don't need to directly edit the script that runs setserial. In addition just using setserial on the command line (2.15+) may ultimately alter this configuration file.

So prior to version 2.15 all you do is edit a script. After 2.15 you may need to either do one of three things: 1. edit a script. 2. edit /etc/serial.conf or 3. run "setserial" on the command line which may result in /etc/serial.conf automatically being edited. Which one of these you need to do depends on both your particular distribution, and how you have set it up.

Edit a script (required prior to version 2.15)

Prior to setserial 2.15 (1999) there was no /etc/serial.conf file to configure setserial. Thus you need to find the file that runs "setserial" at boot time and edit it. If it doesn't exist, you need to create one (or place the commands in a file that runs early at boot-time). If such a file is currently being used it's likely 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. You might use "locate" to try to find such a file. For example, you could type: locate "*serial*".

The script /etc/rc.d/rc.serial was commonly used in the past. The Debian distribution used /etc/rc.boot/0setserial. Another file once used was /etc/rc.d/rc.local but it's not a good idea to use this since it may not be run early enough. It's been reported that other processes may try to open the serial port before rc.local runs resulting in serial communication failure. Today it's most likely in /etc/init.d/ but it isn't normally intended to be edited.

If such a file is supplied, it should contain a number of commented-out examples. By uncommenting some of these and/or modifying them, you should be able to set things up correctly. Make sure that you are using a valid path for 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 is a lot faster than doing repeated reboots to get it right.

For versions >= 2.15 (provided your distribution implemented the change, Redhat didn't) 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 New configuration method using /etc/serial.conf.

If you want setserial to automatically determine the uart and the IRQ for ttyS3 you would add something like:

/sbin/setserial  /dev/ttyS3 auto_irq skip_test autoconfig
Do this for every serial port you want to auto configure. Be sure to give a device name that really does exist on your machine. In some cases this will not work right due to the hardware. If you know what the uart and irq actually are, you may want to assign them explicitly with "setserial". For example:

/sbin/setserial /dev/ttyS3 irq 5 uart 16550A  skip_test

New configuration method using /etc/serial.conf

Prior to setserial version 2.15, the way to configure setserial was to manually edit the shell-script that ran setserial at boot-time. See Edit a script (after version 2.15: perhaps not). Starting with version 2.15 (1999) of setserial this shell-script is not edited but instead gets its data from a configuration file: /etc/serial.conf. Furthermore you may not even need to edit serial.conf because using the "setserial" command on the command line may automatically cause serial.conf to be edited appropriately.

This was intended so that you don't need to edit any file in order to set up (or change) what setserial does each time that Linux is booted. But there are serious pitfalls because it's not really "setserial" that edits serial.conf. Confusion is compounded because different distributions handle this differently. In addition, you may modify it so that it works differently.

What often happens is this: When you shut down your PC the script that runs "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.conf file. 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.

If 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 at least one distribution, the removal of "###AUTOSAVE###" from the first line is automatically done after the first time you shutdown just after installation. The serial.conf file should contain some comments to explain this.

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 setserial to set it to "uart none". 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.

BUG: As of July 1999 there is a bug/problem since with ###AUTOSAVE### only the setserial parameters displayed by "setserial -Gg /dev/ttyS*" get saved but the other parameters don't get saved. Use the -a flag to "setserial" to see all parameters. This will only affect a small minority of users since the defaults for the parameters not saved are usually OK for most situations. It's been reported as a bug and may be fixed by now.

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 /etc/init.d/{set}serial stop. 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".


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

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 an internal modem and retain ttyS0 and ttyS1, then you should attempt to find an unused IRQ and set it both on 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 may be one you can use for a modem. To set the IRQ in hardware you may need to use isapnp, a PnP BIOS, or patch Linux to make it PnP. To help you determine which spare IRQ's you might have, type "man setserial" and search for say: "IRQ 11".

Laptops: PCMCIA

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.

10.4 Stty


stty does much of the configuration of the serial port but since application programs (and the getty program) often handle it, 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. Don't try to learn all the setting unless you want to become a serial guru. Most of the defaults should work OK and some of the settings are needed only for certain obsolete dumb terminals made in the 1970's.

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

Whereas 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 at 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 stty man or info page for more details. Also see the man page: 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 is useful. 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.

Flow control options

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 restarts 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 the the PC's port will be overrun. So you seldom actually need to enable ixoff. But it's often enabled "just in case".

Using stty at a "foreign" terminal

Using stty to configure the terminal that you are currently using is easy. Doing it for a different (foreign) terminal or serial port may be impossible. For example, let's say you are at the PC monitor (tty1) and want to use stty to deal with the serial port ttyS2. Prior to about 2000 you needed to use the redirection operator "<". After 2000 (provided your version of setserial is >= 1.17 and stty >= 2.0) there is a better method using the -F option. This will work when the old redirection method fails. Even with the latest versions be warned that if there is a terminal on ttyS2 and a shell is running on that terminal, then what you see will likely be deceptive and trying to set it will not work. See Two interfaces at a terminal to understand it.

The new method is ``stty -F /dev/ttyS2 ...'' (or --file instead of F). If ... is -a it displays all the stty settings. The old redirection method (which still works in later versions) is to type ``stty ... </dev/ttyS2''. If the new method works but the old one hangs, it implies that the port is hung due to a modem control line not being asserted. Thus the old method is still useful for troubleshooting. See the following subsection for details.

Old redirection method

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 <return>). 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.

The old redirection method 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. Some people tried to use ``stty ... > /dev/ttyS2'' to set the terminal. This will not do it. 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.

Two interfaces at a terminal

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 <return> 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 anything to this cooked mode but what was typed in raw mode gets executed 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 since you are starting the command-line editor). The settings for the "raw" mode are based only on the basic 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 <return> 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 another (foreign) terminal 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 <return> at the terminal you tried to "set". But if you type a stty command at your terminal (without the -F option or redirection) and then hit <return> it's a different story. The <return> 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.

Where to put the stty command ?

Should you need to have stty set up the serial interface each time the computer starts up then you need to put the 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.

10.5 What is isapnp ?

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.

10.6 What is slattach?

It's "serial line attach". It puts the serial line into a networking mode. You can thus network two computers together via a serial line using, for example, the slip protocol. But for the ppp protocol, you need to start pppd on the serial line.

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