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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
6 - 4
.
Scan time without constant scan setting
END
0
END
5ms
Scan time with constant scan setting (7ms)
Sequence program
END processing
END
0
END
Waiting time
5ms
2ms
7ms
Scan time for multiple programs with constant scan setting (10ms)
Waiting time
8ms
10ms
Figure 6.1 Constant scan operation
Sequence program
END processing
0
END 0
6ms
5ms
0
END
0
6ms
1ms
7ms
Sequence program A
Sequence program B
Sequence program C
END processing
2ms
9ms
10ms
.
END 0
END
0
5ms
2ms
7ms
1ms
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