HOW PLC SCAN CYCLE WORKS


HOW PLC
SCAN CYCLE WORKS?

Hi Friends …
Greetings , Today want to share with you how PLC actually
works. PLC’s work on the principal on SCAN CYCLE. Lets find out how PLC Work?
It works in Following Stages.



STAGE 1.Regenerate input signal

  •  Before the execution of the program,
    PLC reads the On/Off status of the external input signals into the input signal
    memory at a time.
  •   The On/Off status of the input
    signal during the execution of the program will not change the signal status in
    the input signal memory. The new On/Off status will be read in the next scan.
  •  There will be approximately a 10ms
    delay from the On→Off or Off→On changes to the status being recognized by the
    contact in the program. The delay time may be affected by the scan time in the program.


STAGE 2.Program processing


After the PLC reads the On/Off status of every input signal in the input signal
memory, it will start to execute every instruction in the program in order
starting from address 0. The execution result (On/Off of every output coil)
will be stored in order into the device memory.

STAGE 3. Regenerate output
  1.    When the program executes to END instruction,
    it will send the On/Off status of Y in the device memory to the output latched
    memory. The output latched memory is the coil of the output relay.
  2.    There will be a 10ms delay from OnàOff or Off àOn of the
    relay coil to the On/Off status of the contact.
  3.   There will be a 10 ~ 20us delay from OnàOff or Off àOn of the
    transistor module to the On/Off status of the contact.

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