Chapter 4. CPU module
4 - 2
4.2 Operation Processing
4.2.1 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
-
•
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.
4
I/O modules reset
4
Execution of self-diagnosis
4
Data clear
4
I/O module address allocation or type registration
•
Input module conditions are read and stored into the input image area before
operation processing of a program.
•
Program is sequentially executed from the first step to the last step
•
The contents stored in the output image area is output to output modules when
operation processing of a program is finished.
•
Stage for return processing after the CPU module has finished 1 scan. The
following processing are executed.
4
Self-diagnosis
4
Change of the present values of timer and counter, etc.
4
Processing data communications between computer link module and
communications module.
4
Checking the switch for mode setting.
Initialization
Input image area refresh
Program operation processing
Program start
~
Program end
Output image area refresh
END processing
Operation Start
