FAULT DIAGNOSTIC DEVICE OF DUPLEXED SYSTEM

PURPOSE:To prevent the control output of the other system from sneaking by connecting a first fault diagnostic circuit and a second fault diagnostic circuit to output circuits, respectively before and behind each backflow stopping circuit, and giving a detection output of each fault diagnostic circuit to a terminal logical part of the own system. CONSTITUTION:Terminal logical parts 11, 12 and external output units 21, 22 are formed as a duplexed system against a control object, and output circuits of external output units 21, 22 of each system are connected to control objects 31, 32 - 3n by wired OR. Also, in the external output units 21, 22 of each system, backflow stopping circuits 51, 52 for preventing sneaking from the output circuit of the other system are inserted into the output circuits connected to the output side of a driving part driven to control by the terminal logical parts 11, 12 of the own system, respectively, first fault diagnostic circuits 41, 42 and second fault diagnostic circuits 61, 62 are connected to the output circuits before and behind each backflow stopping circuit 51, 52, respectively, and a detection output of each fault diagnostic circuit is given to the terminal logical parts 11, 12 of the own system. In such a way, each system can execute the fault diagnosis without being influenced by sneaking of a control signal of the other system.