Simple Response Check in Ladder Logic
Birket Engineering Cookbook
The following rungs implement a simple 2-way response check for a device which is controlled by an output and provides a monitoring input. We use this check more than any other diagnostic test.
This method of implementing Stuck-On and Stuck-Off tests is simpler (by several instructions) than the pattern I have used before. Thanks to Ken K. for the improvement.
Some examples of devices which should use this check are:
|Relay or Contactor||Coil||Aux Contact|
|EStop Relay||Coil||Estop Bus|
|Coaster Brake||Valve||Brake Position Prox.|
|Air Cannon||Valve(s)||Tank Pressure Sensor|
Sample Code for a Motor Contactor with a coil and Aux contact
Note that the two OK rungs do not include a latch or acknowledge. Those functions are presumed to be handled elsewhere by the Latch & Ack section of the Diagnostics routine.
Construct Timer-On and Timer-Off Done bits from the output and set permissible response delays in each direction.
Require the Aux Contact to be On unless the output is Off or has only just turned On.
Require the Aux Contact to be Off unless the output is On or has only just turned Off.