Crossover Cable and Null Modem Wiring

Setting up communication networks or interfacing with devices is a common task at DMC. Usually we try to prepare ahead of time so that we have all the hardware we need, but in a pinch, we do occasionally have to make our own cables or have a better understand of how things work so we can debug communications or monitor specific I/O lines.

Crossover Versus Patch Cables

I'll start by giving some background information. Typical, old-school communication systems work by each device having a dedicated send line and a dedicated receive line. There is often a 'master' device and a 'slave' device. The pin outs for the send versus the receive line are different for the master versus the slave so that a straight through, or 'patch' cable can be used for the two devices to communicate.

This becomes an issue when you want two of the same type of device to communicate with each other. Two computers, for example, which, for these purposes can both be considered slaves, trying to communicate directly without a hub or similar device, will have issues if a standard patch or straight-through cable is used. The Send Lines will both be connected to each other, so neither is listening to the others' message.

This issue can be connected with the cabling. If you use a crossover cable or a null-modem adapter, the send and receive lines are swapped.

Ethernet Cables

Below is a diagram explaining the typical pin out for an Ethernet cable. Note which cables are swapped to make a crossover cable. One thing to note - making crossover cables is not typically necessary any more. Most modern network switches and computer Ethernet ports automatically determine what device you are connected to and internally switch the input and output pins accordingly.

Null Modem Adapters

For a standard RS-232, DB-9 connection, a null modem is needed, to swap the send and receive signals pins 2 and 3 must be swapped. If no complicated hand-shaking is needed, pin 5 should be connected straight through.

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