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Chapter 5 Power Supply / CPU
5-6
5.3 Operation Processing
5.3.1 Operation Processing Method
1) Cyclic operation
A PLC program is sequentially executed from the first step to the last step, which is called scan. This sequential
processing is called cyclic operation. Cyclic operation of the PLC continues as long as conditions do not change
for interrupt processing during program execution. This processing is classified into the following stages:
Stages Processing
-
Operation Start
Initialization
Input image area refresh
Program operation processing
Program starts
~
Program ends
Output image area refresh
END processing
•
Stage for the start of a scan processing. it is executed only one
time when the power is applied or reset is executed. It executes
the following processing..
▶ I/O reset
▶ Execution of self-diagnosis
▶ Data clear
▶
A
llocatin
g
I/O address and t
yp
e
Program is sequentially executed from the first step to the last step
Program operation processing
The contents stored in the output image area is output to output part
when operation processing of a program is finished.
Input conditions are read and stored into the input image area
before starts processing.
Stage for return processing after the CPU part has finished 1 scan.
The END processing following processing is executed.
▶ Self-diagnosis
▶ Change the present values of timer and counter, etc.
▶ Processing data communications between computer link module
and communications module.
▶ Checking the switch for mode setting.