Emergency null modem cable

At some point in time, many IT engineers from numerous backgrounds have had an emergency requirement for a Null Modem Cable. I was in one such situation recently and had to cobble together one quick to get a box back up and running.

With just a pocket (pen) knife and a couple of Cisco DB9-8p8c (or RJ45 if you prefer) I was able to cobble one together. I needed DB9 at both ends so chopped of the RJ45 connectors.

The pins we are interested in (viewed from the transparent RJ45 ends) are :-

3 - TXD (red)
5 - RXD (green)
6 - GRND (yellow)

The first thing I noticed after doing a quick search to confirm interesting pins, is that the colours people report don't always agree. My advise would be to work from the pin number and confirm the colours.

Once identified it is simply a case of stripping the insulation on the required pins back, (red, green and yellow) to reveal the copper beneath, they could be twisted together again in the right order to make a Null Modem Cable with DB9 on both ends.

So I twisted the two yellow cables together to connect the ground, then the red and green cables were crossed and twisted together, Finally a paper clip was used to try and provide some strain relief to the cables.

In an ideal world, solder and insulation is highly recommended, but as a quick get out of jail card it worked fine.

The finished article, if you call it finished.

Rough and ready finished article. A Null modem cable.

Close up of the tough and ready finished article. A Null modem cable.

Finally, just to confirm, this did indeed enable me to restore service, all be it a 9600 8-1-none.