DIGITAL OUTPUT DEVICE DIAGNOSTIC METHOD, DIGITAL OUTPUT DEVICE AND PLANT CONTROLLER

PROBLEM TO BE SOLVED: To simplify system software by a self-diagnostic function, to diagnose a final output stage by the use of the minimum number of external wires and to attain multiple output constitution. SOLUTION: In the digital output device 1, plural contact output parts are connected by parallel constitution and a delay circuit 3 applies time differences to the state changes of respective contacts. When respective contacts are normal, a pulse signal is obtained from a contact voltage balance state detecting circuit 2. When a signal rise detecting circuit 7 detects the rise of the pulse, a one-shot circuit 8 is triggered and a one-shot output 17 is latched to a diagnostic result latching flip flop 4 based upon a latch signal 18 outputted from a diagnostic timing generating circuit 5 to obtain a diagnostic result 15. The result 15 is entered into a computer through a bus interface 12, and at the occurrence of a failure, the result 15 is outputted and separated as necessity so as to turn off a switch 11. Consequently the reliability of the contact outputs can be improved by the minimum number of plant side external wires.