Constant Scan - Mitsubishi Electric MELSEC iQ-R Series User Manual

Constant scan

Scan time is different for each scan because its processing time varies depending on whether instructions used in a program
are executed or not. By setting constant scan, the I/O refresh interval can be kept constant even when the program execution
time varies because the program can be executed repeatedly by keeping the scan time constant.
• When constant scan is set (Setting value = 7ms)
Program
END processing
END 0 END 0
Waiting time
5ms
2ms
7ms
• When constant scan is not set
Program
END 0 END 0
5ms
Setting constant scan
Constant scan can be set.
[CPU Parameter]  [RAS Setting]  [Constant Scan Setting]
Window
Displayed items
Item
Constant Scan
The setting time for the constant scan must be a value that satisfies the relational expression below.
"Watchdog timer setting time" > "Constant scan setting time" > "Maximum scan time of the program"
If the maximum scan time of the program is longer than the constant scan setting time, it is assumed as an
error to ignore the constant scan, and the scan time of the program is applied.
END 0 END
5ms
6ms 1ms
7ms 7ms
END processing
END 0 END
6ms 5ms
Description
Sets the constant scan time.
0
2ms
0
Setting range
0.2 to 2000ms (unit: 0.1ms)
Default

8 RUNNING A PROGRAM
8.2 Scan Time
8
85