This is configured as either COM1, COM2, COM3 or COM4. A special programmable interface chip, the 8250 serial communications controller chip, provides the DTE RS-232 interface signals. The 8250 has numerous registers used to implement the interface. The operating system provides a means for transmitting and receiving characters via the interface. The ROM BIOS extends this by adding the status signals of the various control lines.
Programmers thus tend to communicate with the serial card directly.
This necessitates writing the appropriate routines to control the serial interface. It is placed between the DTE and the DCE, so you can see the state of the various signal lines and perform interconnection if required. Using the breakout box is a matter of determining the signals being asserted and performing interconnection of signal lines if required. LED's are used to indicate the state of the signal line. In the following exercises, you will perform some of the functions of the modem.