NULL MODEM

PC with 25-pin connector
PC I   PC II
Pin 2   Pin 2 
Pin 3   Pin 3 
Pin 5   Pin 7
Pin 7   Pin 5
Pin 8   Pin 4
Pin 6   Pin 20
Pin 4   Pin 6

PC with nine-pin connector
PC I   PC II
Pin 2   Pin 3
Pin 3   Pin 2 
Pin 5   Pin 5
Pin 7   Pin 8
Pin 8   Pin 7
Pin 6   Pin 4
Pin 4   Pin 6