Half-duplex is a type of communication in which data can flow back and forth between two devices, but not simultaneously. Each device in a half-duplex system can send and receive data, but only one device can transmit at a time.
An example of a half-duplex device is a CB (citizens band) radio. The CB protocol, which is used by truckers, police officers, and other mobile personnel, allows users to communicate back and forth on a specific radio frequency. However, since the CB protocol only supports half-duplex communication, only person can speak at a time. This is why people communicating over two-way radios often say "over" at the end of each statement. It is a simple way of telling the recipient he or she can respond if necessary.
Most communication protocols are designed to be full-duplex, rather than half duplex. Full-duplex communication allows computers and other devices to communicate back and forth at the same. While some computer networks can be set to half-duplex mode to limit bandwidth, full-duplex communication is much more common.
NOTE: Half-duplex is sometimes abbreviated "HDX."