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Chapter 4 CPU Function
4 - 2
4.2 Operation Processing
4.2.1 Operation Processing Method
1) Cycle 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 option card 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.
I/O reset
▶
Execution of self
▶
-diagnosis
Data clear
▶
Allocating I/O address and type
▶
Input conditions are read and stored into the input image area
before it starts processing.
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.
Stage for return processing after the CPU part has finish
ed 1 scan.
The END processing following processing is executed.
▶
Self-diagnosis
▶
Change present values of timer and counter, etc.
▶
Processing data communications between computer
link module and communications module.
▶
Checking the switch for mode setting.
2) Interrupt operation method
If a situation occurs which is requested to be urgently processed during execution of a PLC progra
m, this operation method processes immediately the operation, which corresponds to interrupt progr
am. The signal, which informs the CPU of those urgent conditions is called interrupt signal. The C
PU has two kinds of interrupt operation methods, which are internal and external interrupt signal me
thods.
Operation Start
Initialization
Input Image Area Refresh
Program operation processing
Program starts
Program ends
Output image area refresh
END processing