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Output problems -- P3000

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  • Output problems -- P3000

    Hello All,

    I have a P3-550 with a 32 point Input and Output block, and a 16 point input sim. I have a program that when the sim input 1 is on -- it turns on Four outputs in parallel OUT 25, 26, 27 , 28 (line 1). These four outputs "Home" an axis when activated together. The problem is...the same Four Outputs are used as a 15 bit binary code to move to different positions ( 25 is the LSB).

    The problem is...if I have OUT 26 as an output on any other line (like line 2), then it won't work when called in line 1!!!! Can anyone explain this to me, or help me devise a workaround?

    Thanks, Brett

  • #2
    There should only be one discrete output to a physical output.

    If OUT 26 is a discrete output and you want to turn it on in more than one place:
    Use the logic to turn on boolean bits.
    Logic on line 1 that needs to turn on OUT 26, turns on boolean bit C101 ( or whatever tag name is given )
    Logic on line 2 that needs to turn on OUT 26, turns on boolean bit C102
    Logic on line 3 that needs to turn on OUT 26, turns on boolean bit C103

    Logic on line 4 has contacts C101, C102, and C103 in parallel.
    When any one of the contacts is made, then OUT 26 discreet output is made.
    In this manner the discrete output is in only one place, but can be turned on in multiple places.


    • #3
      Universal across all PLCs (as far as I know) you are only supposed to use an out coil in one place, otherwise the last line with the out always wins and the rest do nothing. Proper practice when having multiple lines control a single out is to do as RogerR stated and use internal bits and those bits turn the output coil on and off.
      You can however use a set and reset of the same output in multiple places in the ladder. Set an output on turns it on and stays on until somewhere else in the ladder code has a reset to turn the output off. You cannot use set and rst with an out coil of the same output either.


      • #4
        The problem you have is related to plc programming.
        when you trigger an output in more than one line, the last triggering is the one that is actually visible at the output port.
        let me explain the plc program scanning cycle:
        1. The input ports are read; an image table is prepared to be used by the program. At this moment, if an input port changes state, the change will not be updated in the input image of this scan cycle.
        2. The program starts executing. The output image is updated with each update of any output, but is not reflected back to the output ports.
        3. The scan cycle ends. At this moment, the output image is reflected in the hardware and the plc outputs switch accordingly. Then the reading of inputs is reflected in the input image which will be used in the next scan cycle.

        the way you trigger and re-trigger an ouput in multiple rungs will not be updated in real time. Only one update will be made, at the end of scan cycle. Hope this helps.


        • #5
          Don't sweat it, this is a very common trap for newer PLC programmers.