A Port Monitor at Work: ttymon

Whenever you attempt to log in via a directly connected modem or alphanumeric terminal, ttymon goes to work, as follows.

As shown in the figure below, the init program is the first process to be started at boot time. Consulting its administrative file (/etc/inittab), init starts other processes as they are needed. Listed among those processes is the SAC.

SAC, in turn, automatically starts up the port monitors designated in its administrative file (/etc/saf/_sactab). The figure below shows only a single ttymon port monitor.

After ttymon has been started, it monitors the serial port lines for service requests.

Figure 14-1 How ttymon Helps Process a Login Request

When someone attempts to log in via an alphanumeric terminal or a modem, the serial port driver passes the activity to the operating system. The ttymon port monitor notes the serial port activity, and attempts to establish a communications link. ttymon determines what data transfer rate, line discipline, and handshaking protocol are required to communicate with the device.

Having established the proper parameters for communication with the modem or terminal, ttymon passes these parameters to the login program and transfers control to it.