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Modbus TCP issue with Advanced HMI software

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  • Modbus TCP issue with Advanced HMI software

    Why is it necessary to to turn on overide in order for Advanced HMI program to access PLC instruction? For example: You have to set ovr flag on X001 before Advanced HMI button will change the state of X001 in the PLC.

  • #2
    Short answer: Because Physical IO always wins.

    Slightly longer answer: It has to do with the way that physical IO and communications are handled in the PLC. Comm updates typically happen first, Physical IO happens second, so the Physical IO always overwrites what the comms has set for Physical Addresses.
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    • #3
      Why are you using physical I/O addresses for HMI target registers in the first place??


      • #4
        best practice is to have the HMI(s) address internal bits. If you need to change a physical I/O you can have the internal bit do what you need it to in a PLC. Rarely would you force a physical input on though, instead if you are using the HMI to do some debugging you could have the rung in the PLC go true is the physical input or the internal bit goes true, to simulate the physical input like a microswitch instread of forcing it on logically.