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Hello everyone,


I am come back to the forum since too much time. I come back to Unitronics' PLC programming and I have got an issue with a routine.


I am currently working with a generator routine which looks like:



We have a panel with 9 bottons to select a state for each generator (Day, Night, Standby). Therefore we have 6 combinations:

  1. Day,Night,Standby
  2. Night,Day,Standby
  3. Standby,Day,Night
  4. Standby,Night,Day
  5. Day,Standby,Night
  6. Night,Standby,Day

And if it is not any of these ones,it will work with this state like standard one: ''Day,Night,Standby"


Now the problems come. Time functions:




Standby should work when there is a failure in one of the other ones.


I made a simple visual checker with 3 pictures simulating the state of the generators, but I do not know why routines Gen1,Gen2 and Gen3 are always "ON".


I have attached the file in this link.


Thanks for your help.



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  • MVP 2023

When MB20 turns off, you stop calling "StandardRoutine". When you stop calling Standard Routine, you stop calling "Gen1" routine. When you stop calling Gen1 routine, the PLC cannot turn off MB17, thus it stays on.


Even if MB20 stays on, when the time expires in StandardRoutine, ladder rung 1, then Gen1 subroutine is no longer being called and MB17 cannot be turned off.


Conditional subroutine calls can be very problematic. Design your logic to minimize (or eliminate) the need for conditional subroutine calls.

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Hi Sirs,


I rethink all the ladder logic, I would like to ask you if you notice at first look anything wrong, with ladder logic's LOGIC


I have the next pieces working. I am going to start working with a normal routine(Basic), generator 1 Day, generator 2 Night, generator 3 Standby and a transformer which is the main one.


I am simulating this program with bits instead with inputs and outputs.


I have the next componets:


Generator 1 start is MB5

Generator 2 start is MB6

Generator 3 start is MB7


Phase failure rele transformer is MB0

Phase failure rele Gen 1 is MB10

Phase failure rele Gen 2 is MB11

Phase failure rele Gen 3 is MB12


Breaker Switch Transformer is MB1

Breaker Switch Gen1 is MB2

Breaker Switch Gen2 is MB3

Breaker Switch Gen3 is MB4


Gen routine BIT MB8


PFR House  MB9








This is my program idea, I will performance and I will simulate. Could anyone say me if there is some confict condition with ladder logic in this program?



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