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:

Device Output Input
Relay or Contactor Coil Aux Contact
EStop Relay Coil Estop Bus
Shut-Off Valve Solenoid Proof-of-Closure
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.

SRCLL Figure 1

Require the Aux Contact to be On unless the output is Off or has only just turned On.

SRCLL Figure 2

Require the Aux Contact to be Off unless the output is On or has only just turned Off.

SRCLL Figure 3