Choosing which serial port to use

Before adding a modem, ensure there is a port available on your system -- either directly on the COM1 or COM2 serial ports, or on a third-party multiport card. If you are installing a supported modem, use the Modem Manager to configure an SCO-supported serial card. Otherwise, you can use the Hardware/Kernel Manager or the mkdev serial command to add support for additional serial ports, or a vendor-supplied configuration utility for third-party intelligent serial port devices and drivers. See Chapter 21, ``Adding serial and parallel ports'' for more information.

For systems with two dumb (non-intelligent) single-port serial cards, /dev/tty1a and /dev/tty2a are the non-modem control devices associated with COM1 and COM2 respectively. /dev/tty1A and /dev/tty2A are the corresponding modem control devices for these ports.
The operating system gives these ports different device names because it uses different device-driver routines for each.

For systems with dumb multiport serial cards, /dev/tty1a through /dev/tty1h and /dev/tty2a through /dev/tty2p are non-modem control devices, and /dev/tty1A through /dev/tty1H and /dev/tty2A through /dev/tty2P are modem control devices.

Vendors of multiport smart serial cards implement their own drivers and device naming schemes; consult the documentation supplied with your smart serial card for details.

Make sure the serial port you have chosen for your modem is recognized at bootup (check /usr/adm/messages or use hwconfig(C)) and, if the modem is internal, make sure that the interrupt vector (IRQ) and base I/O address of the COM port do not conflict with any other device.

If you attempt to use both modem and non-modem control ports at the same time you will see the warning:

   cannot open: device busy