Abstract: In a plant control system, a transmission channel includes loop transmission lines arranged in duplex. Equivalently connected to this transmission channel are a plurality of one-loop controller stations, a backup station backing up a disabled one of the plural controller stations and a display station for displaying the status of the controller stations and the backup station. Each of the controller stations, backup station and display station includes a built-in microcomputer. Each of these stations includes also a transmission interface circuit and a self-diagnostic circuit. The transmission interface circuit in each station selects one of the transmission lines for data transmission between its own station and the others. The self-diagnostic circuit in each station detects the presence of failure of normal operation of its own station and disconnects the disabled station from the transmission line, so that the other stations may not be adversely affected by the disabled station.

Abstract: Disclosed is a signal transmitting and receiving apparatus for transmitting and receiving parallel-by-word data signals, converting the parallel-by-word data signal to serial-by-word data signal, and transmitting the serial-by-word data signal to control units connected in a multi-drop connection configuration or receiving the signals transmitted from the control units.An external control mode and an internal control mode in receiving the parallel-by-word data signal, a simulation mode, and the prevention of competing status between a write timing of the parallel-by-word data to a buffer memory and a read timing of the data for converting it to serial-by-word data are described.