Serial HOWTO

11. Serial Port Devices /dev/tts/2 = /dev/ttyS2, etc.

11.1 Serial Port Names: ttyS4, etc

Once upon a time the names of the serial ports were simple. Except for some multiport serial cards they were named /dev/ttyS0, /dev/ttyS1, etc. Then around the year 2000 came the USB bus with names like /dev/ttyUSB0 and /dev/ttyACM1 (for the ACM modem on the USB bus).

11.2 The PCI Bus

Since DOS provided for 4 serial ports on the old ISA bus: COM1-COM4, ttyS0-ttyS3 most serial ports on the newer PCI bus used higher numbers such as ttyS4 or ttyS14 (prior to kernel 2.6.13). But since most PCs only came with one or two serial ports, ttyS0 and possibly ttyS1 (for the second port) the PCI bus now may use ttyS2 (kernel 2.6.15 on). All this permits one to have both ISA serial ports and PCI serial ports on the same PC with no name conflicts. 0-1 (or 0-3) are reserved for the old ISA bus (or the newer LPC bus) and 2-upward (or 4-upward or 14-upward) are used for PCI. It's not required to be this way but it often is. On-board serial ports on motherboards which have both PCI and ISA slots are likely to still be ISA ports. Even for all-PCI-slot motherboards, the serial ports are often not PCI. Instead, they are either ISA, on an internal ISA bus or on a LPC bus which is intended for slow legacy I/O devices: serial/parallel ports and floppy drives.

11.3 Serial Port Device Names & Numbers

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

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

For creating the old devices in the device directory see:

Creating Devices In the /dev directory

11.4 More on Serial Port Names

Dos/Windows use the COM name while the messages from the serial driver use ttyS00, ttyS01, etc. Older serial drivers (2001 ?) used just tty00, tty01, etc.

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

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

11.5 USB (Universal Serial Bus) Serial Ports

For more info see the usb subdirectory in the kernel documentation directory for files: usb-serial, acm, etc.

11.6 Link ttySN to /dev/modem

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

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

11.7 Devfs (The Improved but Obsolete Device File System)

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

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

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

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

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

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

11.8 Which Connector on the Back of my PC is ttyS1, etc?

Inspect the connectors

Inspecting the connectors may give some clues but is often not definitive. The serial connectors on the back side of a PC are usually DB connectors with male pins. 9-pin is the most common but some are 25-pin (especially older PCs like 486s). There may be one 9-pin (perhaps ttyS0 ??) and one 25-pin (perhaps ttyS1 ??). For two 9-pin ones the top one might be ttyS0.

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

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

Send bytes to the port

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

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

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

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

Connect a device to the connector

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

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

Missing connectors

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

11.9 Creating Devices In the /dev directory

If you don't use devfs (which automatically creates such devices) and don't have a device "file" that you need, you will have to create it. Use the mknod command or with the MAKEDEV shell script. Example, suppose you needed to create ttyS0:

linux# mknod -m 666 /dev/ttyS0 c 4 64

linux# MAKEDEV ttyS0

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

This handles the devices creation and should set the correct permissions. For making multiport devices see Making multiport devices in the /dev directory.