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Simple Response Check in Ladder Logic

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.
  Motor 1     Motor 1
  Control Out     On Tmr
  MCC1_CTL     MCC1_TON
  O:1/0     T4:0 (0.50)
- ------[ ]------ + --------------- -----[TON]-----
    |    
    |   Motor 1
    |   Off Tmr
    |   MCC1_TOF
    |   T4:1 (0.50)
    + --------------- -----[TOF]-----
         
Require the Aux Contact to be On unless the output is Off
or has only just turned On.
  Motor 1     Motor 1
  Aux Input     Stuck-Off OK
  MCC1_AUX     OK_TST_124
  I:2/0     B20/124
+ ------[ ]------ + --------------- ------( )------
|   |    
| Motor 1 |    
| On Tmr |    
| MCC1_TON.DN |    
| T4:0.DN |    
+ ------[/]------ +    
         
Require the Aux Contact to be Off unless the output is On
or has only just turned Off.
  Motor 1     Motor 1
  Aux Input     Stuck-On OK
  MCC1_AUX     OK_TST_125
  I:2/0     B20/125
+ ------[/]------ + --------------- ------( )------
|   |    
| Motor 1 |    
| Off Tmr |    
| MCC1_TOF.DN |    
| T4:1.DN |    
+ ------[ ]------ +    

 

 
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