All IBM PC and compatible computers are equipped with serial ports for communicating with external devices. A serial port sends and receives data one bit at a time over one wire. While it takes eight times as long to transfer each byte of data this way in comparison to a parallel port, only a few wires are required. Usually, the serial port is "Full-duplex", meaning it can send and receive data simultaneously. This is possible when there are separate lines; one for transmitting and one for receiving. Some devices support only one way communication and only have two wires - one for transmitting and one for the signal ground.

Two terms that are important to be familiar with are DTE & DCE. DTE stands for Data Terminal Equipment, and DCE stands for Data Communications Equipment. These terms are used to indicate the pin-out for the connectors on a device and the direction of the signals on the pins. Your computer is a DTE device, while most other devices are usually DCE devices. DTE & DCE DETERMINATION GUIDE DTE DEVICES Terminals (Gender: Male) DCE DEVICES Modems, CSU/DSU, Multiplexers (Gender: Female) SELECTABLE DTE OR DCE DEVICES Routers, Hubs, Switches, Access Servers (Gender: Either) Selectable devices usually have a jumper, switch, or software command used to select either DTE or DCE.

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