Abstract: Herein disclosed is a duplex computer system to be used in process automations or factory automations. The system is constructed of two computer (or processor) systems connected with a common backplane bus and a plurality of I/O units to be controlled by those computer systems. Each of these systems is composed of a CPU having a self-diagnosis function for outputting a ready signal indicating whether or not its operations can be normally accomplished, a communication interface unit for interfacing with the backplane bus, and a duplex control unit that receives the ready signal output from the CPU in order to determine which of the computer systems is to be brought into a control status while the other is brought into a standby status.

Abstract: A dual computer system comprising a pair of processor units, and a dual control unit for controlling which of the two processor units are to be kept operating or on standby in case of failure of the operated unit; wherein the dual control unit controls which processor unit is to be operated through monitoring of the operating states of the two processor units, and comprising two independent interruption devices for indicating the switching of the two processor units through interruption. The system is effective in improving the continuity of the control at the time of switching.