Emergency Null Modem Cable
At some point in time, many engineers from numerous backgrounds have at some time or other 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 RJ45 ends are :-
3 – TXD (red)
5 – RXD (green)
6 – GRND (yellow)
When doing a quick search to confirm the pins, the colours people report don’t always seem consistent. I would 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 GRND yellow cables together, 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. I had no tape or wire crimping tools so that was the best I could do at the time.
Obviously in an ideal world, solder and insulation is highly recommended, but as a quick get out of jail card it worked just fine.
The finished article, if you call it finished.
I didn’t think about it at the time, but in hindsight I could have stripped all the other colours and twisted the copper together to help provide some resilience.
It was good enough to get the few bits back and forth at 9600, to get an ancient bit of kit back up and serviceable on the network. I’ve since soldered and insulated it all, but have never used it since. It is still in my draw ready for use though.