PURPOSE:To detect the state of an upper computer on the lower station side by storing data indicating the state of a watchdog timer in an operation control unit on the basis of a sequence control task. CONSTITUTION:In normal operating state, the upper computer 1 outputs a processing signal (a) in every prescribed period. The signal (a) is inputted to the watchdog timer 21 to control the counting operation of the timer 21, so that the timer 21 is not overflowed. Since the signal (a) is not outputted when the computer 1 is failed, the timer 21 is overflowed. Therefore, the state of the computer 1 can be detected by monitoring the output of the timer 21. The output of the timer 21 is monitored by a sequence control task driven on the side of a field control station 2 in each constant period and latched in an internal switch 22 as a prescribed value by a signal (b) corresponding to the output. Therefore, the contents of the switch 22 are read out by the internal sequence on the station 2 side to detect the state of the computer 1.
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