FAULT MONITORING TECHNIQUE FOR PROGRAMMABLE LOGIC CONTROLLERS

An isolated fault monitoring circuit for a AC port in an AC I/O module for a programmable logic controller is taught. Briefly stated, a bridge circuit is series connected to a fuse and an optocoupler. When the fuse which is in series with the AC input line is blown, the output of the optocoupler produces a voltage one-half of that during steady state operation thereby causing one of two comparators to go high which in turn causes an LED to light indicating a blown fuse. Should the AC input line be disconnected or otherwise not present, the optocoupler produces no output which thereby causes the second of the two optocouplers to have its output go high thereby causing a second of the two LEDs to light so indicating that condition. At any time during the fault condition only one of the LEDs will light, while no LEDs are lit during steady state operation.