Serial

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

“Pinout” is a term describing how an electrical cable is wired. Some cables do not have pinouts because they only contain a single internal wire, like coax cables. But if a cable has multiple pins on the end of the cable, it will have a pinout.

Each type of multi-pin cable has a standard pinout or two, but these layouts are not set in stone. Some machines will require non-standard pinouts; this will require users to use a custom cable.

Pinouts also come into play when using a cable with two different ends. For example, going from DB9 (9 pins) to DB25 (25 pins) will mean the DB25 side has 16 unused, “dead” pins.

If you need to know what pinout a cable needs, ideally there will be a spec sheet handy showing it. The next best option is contacting the manufacturer of the equipment the cable will be used with to see if they have a spec sheet available. If you have a cable tester available, that can be used to see how the pins line up. As a last resort, a cable can also be cut open to verify the pinout.

In the guide below, we will be highlighting the standard pinout configuration for common types of multi-pin cables.

Ethernet Pinouts

Ethernet uses two main pinouts, straight and crossover. Straight cables are used to connect computers to other devices, like modems and routers. Crossover cables are used to connect two computers directly. The wires inside ethernet cables are color-coded to industry standards, making it easy to follow the standard pinout options.

Straight pinouts are divided into two different options, T-568A and T-568B. The “B” option is the standard today, although finding the “A” cables still in use in older buildings

Computer cables (data cables) are somewhat similar to audio and video cables. Instead of transmitting sound or images, they send data for your computer to use. This can be anything from sending over a Word document to streaming movies and TV shows. Technically Ethernet cables fall under this category, but Ethernet is such a broad topic it needed its own independent article. Common types of computer cables include:

• USB
• FireWire
• Serial
• DIN
• PS/2

USB

USB is the most common type of computer cable today, being found on computers, printers, hard drives, cell phones, and more. Along with transmitting data, USB is commonly used to recharge batteries on cell phones and other devices like video game controllers. There are several types of USB cables in various shapes and sizes. The two primary groups are USB 2.0 and USB 3.0.

USB 2.0 is capable of speeds up to 480 Mbps and comes in a few different varieties. When someone just says “USB”, they are generally referring to USB 2.0 Type A. These are the standard rectangle-shaped connections you can find on any computer. Next, there is Type B, which is also called a printer cable. And then there is the USB 2.0