A 25-pin connector on the back panel of the computer serves as the general
purpose  serial interface . this connector can drive a wide range of
different peripherals, including an external modem or a serial printer.

For pin connections, see  appendix e .

 introduction to serial circuitry   contents of the receive data register 
 setting the baud rate              how output data is transmitted 
 setting the receive mode           specifying the register contents