Concept Of Scan Time - Mitsubishi Electric MELSEC-Q Series User Manual

4 SEQUENCE PROGRAM CONFIGURATION & EXECUTION
CONDITIONS

4.2 Concept of Scan Time

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(1) Scan time
(a) The "scan time" is a total of following the execution time of program and
END processing.
When an interrupt program is executed, the value including the execution
time of the interrupt program will be the scan time.
(b) The scan time present value, minimum value, and maximum value are
measured at the Basic model QCPU, and the results are stored in special
registers (SD520, SD521, and SD524 to SD527).
The scan time can therefore be checked by monitoring the SD520, SD521,
and SD524 to SD527 special registers.
Current value SD520
SD524
Minimum value
Maximum value SD526
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 watchdog time reset instruction
(WDT) is executed at the sequence program.
(2) Constant scan setting:
When constant scanning is designated, the main routine program is executed at
each designated constant scan period.
(3) WDT (Watchdog timer)
This is the timer which monitors the scan time, and its default setting is 200 ms.
This WDT setting can be designated in a 10 ms to 2000 ms range in the PLC
RAS settings of the PLC parameter.
(Setting units: 10 ms)
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
1: The "constant scan" function executes the scan type program repeatedly at
regular intervals.
For details regarding of the constant scan, refer to Section 7.2.
SD521
SD525
SD527
Stores less than 1 ms initial scan time (unit
Stores the initial scan time in 1 ms units.
2
MELSEC-Q
1
s)
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