PURPOSE:To improve the reliability, by providing a signal receiving/processing part to each triple device to check the soundness of the signal which is received from a central computer and setting the output to be fed to a majority circuit at an extreme value when the signal is faulty. CONSTITUTION:A central computer 1 is connected to transmission/reception processors 3a-3c via a bus 2 and couplers 22a-22c. The relay contacts 5a- 5c are connected to a processor 7 via an analog majority logic 6. The signal 11 detected through the processor 7 is fed to the processors 3a-3c respectively. These signals are processed by a command of an arithmetic processor 31 and then fed to the computer 1 via a transmission control unit 32. The signal sent from the computer 1 is fed to a memory 33 under the control of the processor 31 via the unit 32 and at the same time receives a D/A conversion 35 to be fed to the logic 6 through the contact 5a. The signal of the converter 35 receives an A/D conversion 34 again and is compared with the contents of the memory 33. If a discordance is detected through a self-diagnosing unit 37, a relay 4a is driven by a fault processing unit 38 to break the contact 5a.
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