Mitsubishi Electric melsec q03ud(E)cpu - 6.2 Constant Scan

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6.2 Constant Scan
(1) Definition
Scan time of the CPU module is not constant because the processing time varies depending on the
execution status of instructions used in a sequence program.
This function allows sequence programs to be executed repeatedly, maintaining its scan time
constant.
(2) Application
I/O refresh is performed before every sequence program execution.
This function is used to maintain I/O refresh intervals constant even if the execution time of each
sequence program differs
.
Scan time without constant scan setting
Scan time with constant scan setting (7ms)
Scan time for multiple programs with constant scan setting (10ms)
Figure 6.1 Constant scan operation
END
5ms
0 END 0 END 0 END 0
6ms
5ms
Sequence program
END processing
END
5ms
0
END
0
2ms 6ms
END
0
1ms
5ms
END
0
2ms
7ms7ms 7ms
Sequence program
END processing
Waiting time
8ms 2ms
10ms
9ms
1ms
10ms
Sequence program A
Sequence program B
Sequence program C
END processing
Waiting time