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Thread: PLC Operations when Timer less than cycle Time

  1. #1

    Default PLC Operations when Timer less than cycle Time

    Question:
    How does the CR0303 and CR0403 handle a timer that is less than cycle time?
    I assume Interrupts are not used?

    Also, how does the PLC handle the cycle passes?

    Ex. Cycle time is 300msec, timer is set to turn on output (Timer.Q) at 150msec. One pushbutton input (PB) triggering input. One light output (LED).

    Note: I assume both PLC are reading inputs ONLY at the start of a cycle and changing outputs ONLY at the end of a cycle.

    Pass 1 - Input is read and Timer.IN goes high, Timer.ET begins counting to 150msec.
    Pass 2 - Timer is noticed expired, Input (Timer.ET) goes high. At end of cycle LED turns on.

    Now their are variable times the system could have responded in.
    If everything was perfect and the PB was pushed at the start of a cycle the input could come on in 2x cycles (600msec).

    If everything was not perfect and the PB was pushed AFTER inputs were read you are at 3x cycles (900msec).

    Thanks!

  2. #2
    Join Date
    May 2009
    Location
    Minneapolis, MN
    Posts
    299

    Default

    Yes there could be an error in the accumulated time based on high PLC cycle times. You could use the PLC.

    You can also look at the TIMER_READ_US function block to get the time directly from the processor.

  3. #3

    Default

    Erik,

    This is actually a question of HOW the PLC (CR0303 and CR0403) handles this operation and prevents problems.

    Depending on HOW this is handled a timer less than cycle time may be worthless. Is it?

    So how is the CPU & OS handling timers less than cycle time?

  4. #4
    Join Date
    May 2009
    Location
    Minneapolis, MN
    Posts
    299

    Default

    If you have a timer less than the PLC cycle then the timer would be potentially the PT of the TON PLUS the PLC cycle.

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