One of my coworkers needed to make a null modem cable last week, and most of the sites he found made it far, far too difficult. All you really need is 3 wires. Here’s how to make your own null modem cable.
Most of the time, you only need three pins. In fact, I never needed more than three pins, no matter what I was connecting. Usually, a 3-wire null modem cable is more than sufficient.
Pinouts vary from device to device, and I’m going to assume you have pinouts for both devices you need, and the right connectors. Yes, RS-232 is a standard, but your connector could have 9 or 25 pins, or potentially even be on an RJ-11 or RJ-45 connector.
The 3-wire null modem cable
Here are the magical three pins for my 3-wire null modem cable. Transmit data to receive data in both directions (that’s two of the pins) and ground to ground. That’s it. You almost don’t even need a null modem wiring diagram, but if you’re visual, take a look at the wiring diagram on the right.
Let’s explain it another way. First you connect transmit from one side to receive on the other. Then you connect the remaining transmit pin to the remaining receive pin. Finally you connect the ground pin on one end to the ground pin on the other. If you have multiple ground pins available, you only have to use one on each side.
If you have a 9-pin serial port on each end, connect pin 2 to pin 3 on opposite sides, and pin 5 to pin 5. You can use dissimilar connectors, just make sure you connect equivalent to equivalent. Pins 2, 3, and 5 are all you need for a 3-wire null modem cable. Hence the name.
What kind of wire to use
The wire you use doesn’t matter much; you’re likely to have CAT5e on hand and that will be more than fine. By modern standards, null modems use a very slow connection. You could probably use coat hangers and get away with it. I’ve used phone wiring and even speaker wire in the past, back when my college classmates wanted to play Doom. I even got away with omitting ground once, but I don’t recommend that. It’s just one more wire, and we almost always have at least four to work with.
That’s really it, most of the time. There’s no reason to overcomplicate it. Especially when you’re connecting two devices sitting right next to each other. The 3-wire null modem cable has served me well for years, and it will do the same for you.