Download MELSEC-L CPU Module User`s Manual (Function Explanation

3.2
Constant Scan
Scan time of the CPU module is not constant because the processing time varies depending on the execution status of
instructions used in a program. By using this function, scan time can be maintained constant.
(1) Application
I/O refresh is performed before program execution. By using this function, the constant I/O refresh intervals can
be maintained even the time required for program execution changes.
• Scan time without constant scan setting
Program execution
END 0
END
0
END processing
END 0
5ms
END 0
5ms
6ms
• Scan time with constant scan setting (7ms)
Program execution
END 0
END processing
END
0
END
Waiting time
5ms
2ms
6ms
7ms
END
0
5ms
1ms
7ms
0
2ms
7ms
• Scan time for multiple programs with constant scan setting (10ms)
Execution of program A
Execution of program B
Execution of program C
END processing
Waiting time
8ms
2ms
10ms
9ms
1ms
10ms
(2) Constant scan time setting
The setting range is from 0.5 to 2000ms (in increments of 0.5ms).
When not using the constant scan function, leave the setting box blank.
Project window
[Parameter]
[PLC Parameter]
[PLC RAS]
Enter a value.
The constant scan accuracy is 0.01ms. However, constant scan time may increase when an interrupt program or
a fixed scan execution type program is being executed.
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