MITSUBISHI ELECTRIC 700 Series - Manual (Page 73)

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Operation Processing Method of PLC Function
3.5 Operation Processing Method of PLC Function
The operation processing method is the repeated operation of a stored program.
(1) Stored program system
1) In a stored program system, a sequence program to be operated is stored in
the internal memory beforehand.
2) When sequence program operation is executed, the sequence program stored
in the built-in PLC function is read to the CPU instruction by instruction to
execute the operation, and the corresponding devices are controlled according
to the results.
(2) Repeated operation system
In a repeated operation system, a sequence of operations is repeated.
The built-in PLC function repeats the following processings.
1) The built-in PLC function executes the sequence program stored in the
internal memory from step 0 in due order.
2) When the END instruction is executed, internal processings, such as timer/
counter present value updating and self-diagnostic checks, are performed, and
the execution returns to step 0 of the sequence program again.
Fig 3.5 Operation Processing Method of Built-in PLC Function
REMARKS
A processing from step 0 to next step 0 or from END to next END is called one scan.
Therefore, one scan is the sum of the processing time of a user-created program (step 0 to
END) and the internal processing time of the built-in PLC function.
Step 0
Step 1
Step 2
END
Built-in sequence function repeats
this operation.
Timer/counter present
value updating
Self-diagnostic
checks, etc.