Mitsubishi Electronics 700 Series User Manual

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

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.