Understanding modem commands and setup strings

SCO OpenServer Handbook
Chapter 24, Adding modems

To create a new dialer, you need to understand how modem commands are used. You can enable or disable desired features by sending commands in a setup string to the modem. For example, the following setup string is used in the dialHA24 dialer:

   ATQ0E0T&D2&C1S0=0X4S2=043

You can change these strings to suit a different communications protocol or modem-specific commands by consulting the documentation for your modem. Though the setup commands may seem confusing because they are concatenated, there are two basic types of Hayes commands:

basic modem commands (for example, Q0 and &D2)
modem S-registers (for example, S0=0)

Hayes-compatible command strings always begin with the AT (attention) command.