Null modem, also called crossover, is a term associated with serial (RS-232) cables. A standard serial cable, also called an AT cable, has the wires inside the cable running straight through. Take a DB9 cable as an example. Pin 1 on one end of the cable would be connected to Pin 1 on the other end. Then Pin 2 to 2, 3 to 3, and so on. Null modem cables are serial cables that use an alternative pinout for different functionality.

A standard DB9 AT cable pinout (non-null modem)

Originally, all serial cables were AT cables and could not connect two devices (such as two computers) directly. They required a modem or similar equipment as a go-between. Null modem cables changed that old standard, allowing devices to be linked up directly with no middleman equipment. This allows older computers and other machines with serial ports to transfer data between each other directly, similar to more modern ethernet crossover cables.

Null modem cables work by switching around wire pairs when going from one end of the cable to the other. Returning to the DB9 example, a null modem cable would have Pin 2 on one side connected to Pin 3 on the other side. Then Pin 4 would be connected to Pin 6 and Pin 7 to Pin 8. Pins 1 and 9 are unused while Pin 5 still connects to itself, acting as the ground wire.

This type of null modem cable is called a Handshake cable. The way these cables are wired allows two connected devices to talk and listen to each other at the same time. Standard AT cables attempt to do the same thing cannot due to their straight-thru pinout. For example, if Pin 2 is connected to Pin 2, both pins are trying to “talk”. Whereas if Pin 2 is connected to Pin 3, one pin “talks” while the other “listens”.

While there are a few different types of handshake cables, the chart below shows the most common version and is the only type available as a stock item today. When someone says “null modem”, this is the version they are referring to. With serial cables now considered obsolete, the odds of running into one of the other types is practically non-existent unless you happen to be using very old, very specific equipment.

A standard pinout featuring the two most common null modem cables, DB9 and DB25

Null modem cables are still somewhat common today, having been used on many machines for a few decades. Along with full-fledged cables, there are also null modem adapters that can be used to change a straight-thru cable into null modem. When using an adapter, make sure to only put a null modem adapter on one end. The adapters change the pin configuration from straight-thru to null modem. So if null modem adapters are installed on both sides, the pins will line up (like a straight-thru cable) again.

One downside of null modem cables is their limited distance. AT cables can easily cover distances of up to 100-feet while null modem cables have a maximum distance of 25-feet. This 25-foot limit also applies when using a null modem adapter with a serial cable.