10. Setting serial device addressesHow 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, 1200, or 2400 bps.
• If you have a 9600 bps (V.32) modem, with V.42bis data compression. use 38400 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 (V.FC or V.34) modem (4 * 28800 = 115200).

  Then, use 38400 as the speed in your comm program, or inittab. This is now the high speed you have set. There is no speed named 57600 or 115200 (although support was added in 1.1.65 and will be used soon). Make sure you have 16550A UARTs :-).

Put your modifications 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/cua3 spd_vhi