What is a terminal strip
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.
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.
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 setserial to tell the driver how these jumpers were set. Today, when plug-and-play methods detect how the jumperless serial port is set, setserial is not really needed anymore unless you're having problems or using old hardware. Furthermore, if the configuration file used by setserial is wrong, then there's trouble. In this case, if you use setserial to try to find out how the port is configured, it may just repeat the incorrect information in the configuration file.
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 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 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 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 setserial to run at boot-time, you may have to take some action. 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 setserial .
While 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 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 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 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 setserial. check out info in /usr/doc/setserial. / or /usr/share/doc/setserial. This should tell you how setserial is handled for your distribution of Linux. While 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.
Serial module unload
If a serial module gets unloaded, the changes previously made by 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.
Slow baud rates of 1200 or less
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 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.
Giving the setserial command
Remember, that 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 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 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: 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: "UART: unknown" 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 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 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.
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 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:
- 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)
- Save what 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 setserial command on the command line and then shuts down the system. ("autosave-once" option)
- At every shutdown, save whatever setserial detects to the configuration file. ("autosave" option)
- Manually edit the configuration file to set the configuration. Don't ever do any automatic saves to it. ("manual" option)
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 setserial. See Edit a script (prior to version 2.15) .
You probe for a port with setserial only when you suspect that it has been enabled (by PnP methods, the BIOS, jumpers, etc.). Otherwise 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, 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 such probing 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. or lspci -vv. But if you want to detect hardware with setserial 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 a distribution-specific configuration file such as /etc/serial.conf or /etc/sysconfig/serial or /var/lib/setserial/autoserial.conf 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 setserial to give the IRQ a fictitious value.
While setserial may run via an initialization script, something akin to 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 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 setserial had been run.
To correct possible errors in IRQs (or for other reasons) there may be a script file somewhere that runs 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" setserial command by just typing it on the command line. In some cases the results of this use of 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 Configuration method using /etc/serial.conf, etc. .
Edit a script (required prior to version 2.15)
This is how it was done prior to 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 /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 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 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 Configuration method using /etc/serial.conf, etc. .
An example line in such a script was:
or, if you wanted setserial to automatically determine the uart and the IRQ for ttyS3 you would have used something like this:
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.
Configuration method using /etc/serial.conf, etc.
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 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 /etc/serial.conf (or /var/lib/setserial/autoserial.conf ).
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 serial.conf. 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 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 /etc/init.d/
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 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
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.
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.
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 "stty sane". 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.
stty is documented in the man pages with a more detailed account in the info pages. Type "man stty" or "info stty" .
Many of the stty options start with an "o" (output) or an "i" (input). For example: 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 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 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 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.
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 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".
Using stty at a "foreign" terminal
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 stty -F /dev/ttyS2. (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 Two interfaces at a terminal to understand it.
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 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 <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 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 <return> 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 <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.
Obsolete redirection method
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. > /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 <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 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. < /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.
11.5 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.
11.6 Connecting two PCs together via serial ports
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).
11.7 Connect the serial port to a fast network: ser2net
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.Source: www.tldp.org