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Abstract

Serial input/output multiplexing apparatus for bi-directional communication of input status signals and output control signals between remote devices and a programmable logic controller (P.L.C.) over a two-wire, asynchronous communication channel. A remote module includes an input interface circuit to receive input status signals from each of plural input devices, an output interface circuit for transmitting output control signals to each of plural output devices, and a controller for selectively coupling the input interface circuit or the output interface circuit to the two-wire asynchronous communication channel for transmitting serial status information to the communication channel or for supplying to the output interface circuit output control information serially received from the communication channel. A local module includes a local output interface circuit for applying the status information to the P.L.C., a local input interface circuit for receiving control information from the P.L.C. and a controller for selectively coupling the local output or input interface circuits to the communication channel for receiving status information or transmitting control information.

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