Mitsubishi Q2ACPU User Manual page 217

12. OVERVIEW OF PROCESSING PERFORMED BY THE QnACPU
(5)
(6)
Scan time
• The scan time is a total of the scan execution type program execution time, the
END processing time, and either the low-speed program execution time or the
constant scan waiting time.
When more than one scan execution type program is executed, "the execution
time of the scan execution type program" is the time required for completing
execution of all these programs.
*1 Refer to Section 12.1.3.
• The QnACPU measures the present, minimum, and maximum values for the scan
time and stores them in special registers SD520, SD521, and SD524 to SD527.
The scan time can be checked by monitoring these special registers.
Example:If "3" and "400" are stored in SD520 and SD521 respectively, the scan
time is 3.4ms.
*2 The accuracy of each scan time stored in the special registers is
the watchdog timer (WDT) reset instruction is executed in the sequence program,
measurement of each scan time is continued.
WDT (watchdog timer)
This is a timer that monitors the scan time; and 200ms is set as a default value.
WDT is set within the range of 10ms to 2000ms in "PLC RAS" in the parameter
mode. (Unit: 10ms).
When using a low-speed execution type program(s), make sure that the specified
WDT value is greater than the sum of the scan time and the low-speed execution
type program execution time.
If the scan time (total of execution times for scan execution type programs and low-
speed execution type programs, END processing time, and low-speed END
processing time) exceeds the time set for WDT, a "WDT ERROR" occurs and the
QnACPU stops its operation.
POINT
The WDT measurement error is 0 to 10ms.
Because of this, when WDT (t) is set to 10ms, a WDT ERROR may not occur
even if the scan time exceeds the limit within the range of 10ms t 20ms.
*1
12 - 8
*2
0.1ms. Note that, even if