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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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