Half-Duplex and Full-Duplex

A duplex communications system is one that controls whether a point-to-point connection allows data to be transmitted in both directions simultaneously (full-duplex), or only in one direction at a time (half-duplex).

With full-duplex transmission mode, the slave device (such as an outstation) can simultaneously send data updates to, and receive commands (such as controls) from, the master device.

With half-duplex transmission mode, the master device alone controls the communication link (and therefore the direction of data flow). The slave device can only send messages when requested to do so by the master device.