Download Process CPU User`s Manual(Function Explanation

4 SEQUENCE PROGRAM CONFIGURATION AND EXECUTION
CONDITIONS
MELSEC-Q
(5) Scan time
(a)
(b)
The "scan time" is a total of following the execution time of the scan
execution type program and END processing.
If multiple scan execution type programs are used, the "scan time" is the
total time required to execute all the programs.
When an interrupt program/fixed scan execution type program is executed,
the value added to the interrupt program/fixed scan execution type
program's execution time will become the scan time.
The scan time current value, minimum value, and maximum value are
measured at the Process CPU, and the results are stored in special
registers (SD520, SD521, and SD524 to SD527). 1
Therefore, the initial scan time can be checked by monitoring the SD520,
SD521, and SD524 to SD527 special registers.
Current value
SD520
SD521
Minimum value
SD524
SD525
Maximum value
SD526
SD527
Stores less than 1 ms initial scan time (unit
s)
Stores the initial scan time in 1 ms units.
If the SD520 value is 3, and the SD521 value is 400, the initial scan time is 3.4
ms.
POINT
1: The accuracy of the scan time stored at the special registers is ± 0.1 ms. The
scan time count will continue even if a watch dog timer reset instruction (WDT)
is executed at the sequence program.
(6) WDT (Watch dog timer)
This is the timer which monitors the scan time, and its default setting is 200 ms.
This WDT setting can be designated within the range of 10 to 2000 ms range at
the "PLC RAS" tab screen in the “(PLC) Parameter" dialog box.
(Setting units: 10 ms)
When using the low speed execution type program, set the WDT greater than the
scan time plus the execution time of the low speed execution type program.
If the scan time (execution time for scan execution type program + low speed
execution type program) exceeds the WDT setting value, a "WDT ERROR (error
code: 5000)" occurs, and Process CPU operation is stopped.
POINT
The WDT measurement error is 10 ms.
Therefore, a WDT setting (t) of 10 ms will result in a "WDT ERROR" if the scan
time is in the following range: 10 ms < t < 20 ms.
REMARK
Use the GX Developer's Program Monitor List to check the execution time of a
program being executed. See Section 7.11.1 for details on the GX Developer's
Program Monitor List.
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