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9. Serial Port Devices /dev/ttyS2, etc.

For creating devices in the device directory see:

Creating Devices In the /dev directory

9.1 Devfs (The new Device File System)

This is a new type of device interface to Linux. It's optional starting with kernel 2.4. It's more efficient than the conventional interface and makes it easy to deal with a huge number of devices. The device names have all changed as well. But there's an option to continue using the old names. For a detailed description of it see: Also see the kernel documentation tree: filesystems/devfs.

The name changes (if used) are: 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.

9.2 Serial Port Device Names & Numbers

Devices in Linux have major and minor numbers (unless you use the new devfs). The serial port ttySx (x=0,1,2, etc.) has major number 4. You may see this (and the minor numbers too) by typing: "ls -l ttyS*" in the /dev directory.

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.

Dos/Windows use the COM name while the setserial program uses tty00, tty01, etc. Don't confuse these with dev/tty0, dev/tty1, etc. which are used for the console (your PC monitor) but are not serial ports. The table below is for the "standard" case (but yours could be different). The major/minor numbers don't exist with the devfs. For the PCI bus the IO addresses are different.

                            ISA IO                              devfs usb
 dos            major minor address  devfs       devfs usb      acm modem
COM1  /dev/ttyS0  4,  64;   3F8   /dev/tts/0  /dev/usb/tts/0 /dev/usb/acm/0
COM2  /dev/ttyS1  4,  65;   2F8   /dev/tts/1  /dev/usb/tts/1 /dev/usb/acm/1
COM3  /dev/ttyS2  4,  66;   3E8   /dev/tts/2  /dev/usb/tts/2 /dev/usb/acm/2
COM4  /dev/ttyS3  4,  67;   2E8   /dev/tts/3  /dev/usb/tts/3 /dev/usb/acm/3

9.3 USB (Universal Serial Bus) Ports

The serial ports on the USB are: /dev/ttyUSB0, /dev/ttyUSB1, etc. The devfs names for these are: /dev/usb/tts/0, /dev/usb/tts/1, etc. For many modems they are /dev/usb/acm/0, etc. (in devfs notation). For more info see the usb subdirectory in the kernel documentation directory for files: acm and usb-serial.

9.4 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 serial device in /dev which you specified during the installation Except if you have a bus mouse, then /dev/mouse will point to the bus mouse device).

Formerly (in the 1990s) the use of /dev/modem 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.

9.5 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 PnP configured yet). A report from setserial (at boot-time or run by you from the command line) should help you identify the non-modem port.

If you have two serial connectors it may be more difficult. 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 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 CMOS 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) it's 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 positive 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 and perhaps use a scrap of paper to prevent the mini clips from accidentally touching the metal of the connector. 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 (Alt-F2 for example) 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 if connected.

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 serial DB connector. Someone may build a PC from this and omit the connector. There may be a "serial" label on the motherboard but no ribbon cable connects to the pins next to this label. To use this port you must get a ribbon cable/connector. I've seen different wiring arrangements for such ribbon cables so beware.

9.6 Creating Devices In the /dev directory

If you don't have a device "file" that you need, you will have to create it with 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
You can use the MAKEDEV script, which lives in /dev. See the man page for it. This simplifies the making of devices. For example, if you needed to make the devices for ttyS0 you would type:
linux# cd /dev
linux# ./MAKEDEV ttyS0
This handles the devices creation and should set the correct permissions. For making multiport devices see Making multiport devices in the /dev directory.

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