Linking two PCs with a null modem cable

Linking two computers without network cards

The best way to link two computers is to use an RJ45 cable to connect the machines' network cards. However, when one or both of the computers has no network card, there is still a fairly easy way to connect them, by using communication ports (found on every PC).

To connect the two computers, you can use a cable called a null modem cable.

What's a null modem cable?

A null modem cable is a 6-conductor shielded cable (meaning 6 wires surrounded by a ground strap), with a serial port connector at both ends. It acts as an inverter cable which matches up the data-sending pins and the data-receiving serial ports of both computers. Technically speaking, it used to link two DTEs without going through two DCEs. The resulting connection, however, cannot be longer than 250 metres.

To create a null modem cable, then, all that is required is to correctly solder the "correct" wires on both ends of the cable. A PC normally has two kinds of ports:

  • A 25-pin parallel port called DB25
  • A 9-pin serial port called DB9

So with these ports free on both computers, there are three possible ways to link them by cable:

  • A DB9-DB9 cable
  • A DB25-DB9 cable
  • A DB25-DB25 cable

DB9-DB9 null modem cable

DB9 Number 1 DB9 Number 2
Name # Name #
RD 2 TD 3
TD 3 RD 2
DTR 4 DSR+CD 6+1
SG 5 SG 5
DSR+CD 6+1 DTR 4
RS 7 CS 8
CS 8 RS 7

DB9-DB25 null modem cable

DB9 DB25
Name # Name #
RD 2 TD 2
TD 3 RD 3
DTR 4 DSR+CD 6+8
SG 5 SG 7
DSR+CD 6+1 DTR 20
RS 7 CS 5
CS 8 RS 4

DB25-DB25 null modem cable

DB25 DB25
Name # Name #
RD 3 TD 2
TD 2 RD 3
DTR 20 DSR+CD 6+8
SG 7 SG 7
DSR+CD 6+8 DTR 20
RS 4 CS 5
CS 5 RS 4
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