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.

Key Takeaways

• A null modem cable is used for direct serial device-to-device communication.
• Null modem cables are commonly associated with RS-232 interfaces.
• The main difference from a straight-through cable is the crossing of transmit and receive signals.
• DB9 and DB25 are common connector formats for legacy serial applications.
• Not all null modem cables use the same pinout.
• Basic cables may use only TX, RX, and signal ground.
• Other applications may require additional hardware flow-control connections.
• Always verify connector type, gender, pinout, and flow-control

“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

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

Key Takeaways

• Computer data cables transfer information between computers, peripherals, networks, storage devices, displays, and other equipment.
• Common technologies include USB, Ethernet, serial, HDMI, DisplayPort, Thunderbolt, DVI, and VGA.
• The correct cable depends on the connector, standard, data rate, distance, and application.
• USB-C describes a connector type and does not automatically indicate a specific data speed or capability.
• Ethernet cables are designed primarily for wired networking.
• HDMI and DisplayPort are commonly used for digital