Source : Free On-Line Dictionary of Computing

null modem
     
         A cable, especially an {EIA-232}
        cable, for connecting serial ports on two computers directly,
        rather than via modems.  Since, according to the
        specification, both computers should transmit on pin three of
        their EIA-232 connectors and receive on pin two, a null modem
        cable needs to connect one computer's pin two to the other's
        pin three and vice versa.  It also needs to have male
        connectors at both ends (again, according to the
        specification).
     
        (1996-05-17)