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