Simplex communication is a form of radio-wave or electronic communications system that allows for the transmission of only one signal at a time or the reception of one signal at a time. Early forms of short-wave radios and telephones used by the military, as well as citizen's band (CB) radios employed by over-the-road semi-tractor trailer truckers in the US and elsewhere, utilized simplex communication principles. While the technology is outdated in modern-day radio transmitters as of 2011, as well as telephones that allow full-duplex, or two-way simultaneous communication, it is still employed in other modern-day forms of technology.
Communication circuits are often limited by a transducer's ability to process radio waves into analog or digital signals that contain audio information for the listener or speaker. Where early radios could only process one communications channel at a time, portable two-way radio and cell phone technology as of 2011 has the capability to handle multiple channels simultaneously. A more correct term for simplex communication that is used to distinguish it from dedicated simplex systems is that of half-duplex communication. A half-duplex system is one that can both transmit and receive signals, but not at the same time, such as a CB radio. A true simplex device, however, is something like a computer keyboard that transmits data to a computer's microprocessor, but has no capability of receiving data back from the computer itself.
The simplex circuit is still used in many advanced technologies simply because there is no need for a duplex level of circuitry. Examples of this include self-guided missiles that receive launch instructions from a ship or aircraft such as the first generation of US Tomahawk cruise missile deployed by the US and UK as of 2011. While the first generation of the missile could be reprogrammed in flight, it initially had no capability of responding to this form of simplex communication. The fourth generation of the missile, the Tomahawk Block IV, can also receive simplex targeting updates on maps stored in its on-board memory while in flight. Where the fourth generation of the missile differs, however, is that it can reset its own inertial guidance systems through two-way satellite communications while en-route to a target, giving it a form of autonomous machine level duplex communications capability.
Other common forms of simplex communication that only have a need for a one-way transmission of a signal include television and radio program broadcasts or the streaming and distribution of audio and video presentations on the Internet. Early computer printers also had no ability to communicate back to the computer once they received a print job, and were also a form of simplex communication. Many common-day electronic circuits can be looked at as a form of simplex technology where the receiver doesn't have the need or time to respond electronically to an input signal, and this includes systems people rely on every day, such as traffic signals at roadway intersections.