SOME HINTS ON REDUCING NOISE IN RS-232

 Under many circumstances, RS-232 lines can go for quite
 long distances without noise problems.  If noise is a problem,
 several things may help:

 1.  Use shielded cable.

 2.  If you don't have shielded cable, use twisted pair for the
     following lines:

        TD/GND  RD/GND  DSR/DTR RTS/CTS RI/CD

 3.  Try putting a resistor (3.3K to 6K) between RD and ground
     on one end.