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How to "lock" some system inputs in current state

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  • How to "lock" some system inputs in current state

    I'm relatively new learning PLC system design so I apologize if the answer to this should be obvious to me.

    I'm using a P3k at an outdoor remote facility. The system uses a combination of manual selector switches, HMI, and internal switches and sensors as inputs. In particular what I would like to do is use an input (such as a keyswitch) be able to lock some of the PLC inputs in their current state. These are typically selector switches that are easily accessible, about a dozen of them which are used as NO contacts. This way only authorized people with a key (and not vandals), can actually change the state of the inputs. What is the best way to implement this?

    Thanks in advance.

  • #2
    For each input you will transfer its state to a bit. On the top part of each rung use a normally open of the input through a contact representing the unlocked state to a storage bit. In parallel with the first two mentioned contacts you will use a normally open contact of the storage bit through a contact representing the locked state. This will hold whatever state the input had when the change is made to the locked state. Use the state of the storage bit for further processing. Note that the condition of the storage bit may not last through a power cycle.



    • #3
      When your lock input is in the unlock state, execute logic to copy the real world inputs to the proxy (internal) inputs. If it's in the lock state don't copy. Use the proxy bits on logic.


      • #4
        Thank you both for your input. Seems really simple now. Appreciate you helping me get on the right track.