9. How Do I Set Up My Serial Ports For Higher Speeds? What Speed Should I Use With My Modem?

This section should help you figure out what speed to use when using your modem with a communications program, or with a getty program.

• If you have something slower than a 9600 bps (V.32) modem, set your speed to the highest speed your modem supports. For example 300 bps (V.21/Bell 103), 1200 bps (V.22/Bell 212A), or 2400 bps (V.22bis).
• If you have a 9600 bps (V.32) modem, with V.42bis data compression, use 38400 bps as your speed. V.42bis compression has a theoretical rate of 4:1, thus ``4 * 9600 = 38400''.
• If you have a 14400 bps (V.32bis) modem, with V.42bis data compression, use setserial with the spd_hi flag to configure your serial port to use 57600 bps (4 * 14400 = 57600).

  Use the spd_vhi flag if you have a 28800 or 33600 bps (V.FC or V.34) modem (4 * 28800 = 115200).

  Then, use 38400 bps as the speed in your communication program, or /etc/inittab. This is now the high speed you have set. Make sure you have 16550A UARTs.

  If your libc version is at least 5.x, there will be speeds named 57600 and 115200. libc lives in /lib, so look there for what version you have. You can use these directly (without using setserial), if your applications have been compiled to take advantage of this. There are many distributions out there, so the best thing to do would be to try using these higher speeds if you have a recent version of a Linux distribution.

Test your setserial setting on the command line first, and then when you have them working, put them into /etc/rc.d/rc.serial or /etc/rc.d/rc.local so that they are done at startup. In my /etc/rc.d/rc.local, I set ttyS3 to 115200 bps by doing:

/sbin/setserial /dev/ttyS3 spd_vhi

Make sure that you are using a valid path for setserial, and a valid device name. You can check the settings of a serial port by running:

setserial -a /dev/ttyS3