<communications> A half-duplex communication channel can, at any given time, carry data in either one direction or the other, but not both. A full-duplex channel can carry data in both directions at once. A simplex channel can only ever carry data in one direction.
Early use of the term "duplex" arose in wireless, telegraph, and telephone communications, where "duplex" circuits between communicating terminals were capable of transmission in both directions, and "simplex" indicated transmission from a sender to a reciever, or broadcast to several potential receivers.
In communications between computers or computing processes, particularly those involving human keyboarding and/or reading, duplex came to mean the re-transmission of a keyboard character to the output display. Early input device such as the Teletype ASR-33 teleprinter, being descended from the electric typewriter, print all input characters as they are typed. In contrast, the video terminals that have replaced them require that, as well as the character be transmitted to the receiver, it should also be displayed on the screen ("echoed"). The echo can be effected locally, but this can confuse the operator because characters continue to appear on the screen, even when the communication circuit has failed.
Additionally, if the echo of the character was effected by the remote (receiving) process, the operator could see and correct not only typing errors, but also transmission errors. This crude but effective mechanism was called, somewhat misleadingly, "full duplex", and the echoing of characters by the sending process was called half duplex. The terms "remote echo" and "local echo" are now more common. Nearly all communications circuits used by computers are two-way, so the distinction between simplex and duplex is seldom made.
(12 Apr 1995)