Mitsubishi Electric MELSEC-Q00U(J)CPU User Manual page 95

(b) Initial scan time
Initial scan time is the sum of the execution time of initial execution type program and the END processing time.
When multiple programs are executed, the initial scan time will be the sum of the time required for completing
all the initial execution type program execution and the END processing time.
• Initial scan time storage location
The CPU module measures the initial scan time and stores it into the special register (SD522 and
SD523).The initial scan time can be checked by monitoring SD522 and SD523.
SD522
If the stored values in SD522 and SD523 are 3 and 400 respectively, the initial scan time is 3.4ms.
Ex.
• Accuracy and measurement of the initial scan time
Accuracy of the initial scan time stored in the special register is ±0.1ms. Even if the WDT instruction
(instruction that resets the watchdog timer) is executed in the sequence program, the measurement of the
initial scan time continues.
• Execution of an interrupt program or fixed scan execution type program
When an interrupt program or fixed scan execution type program is executed before completion of the
initial execution type program execution, the execution time of the executed program will be added to the
initial scan time.
(2) Precautions on programming
Initial execution type programs do not support the instructions that require several scans (instructions with
completion device).
SEND, RECV, and similar instructions
Ex.
(3)
Initial execution monitoring time setting
Initial execution monitoring time is a timer for monitoring initial scan time. Set a time value in the PLC RAS tab of
the PLC parameter dialog box. The setting range is 10 to 2000ms (in increments of 10ms). No default value is set.
(a) When the initial scan time exceeds the preset initial execution monitoring time
"WDT ERROR" occurs and the CPU module stops program operations.
Set a time value so that the initial execution monitoring time becomes longer than actual initial scan time.
An error of the measurement value is 10ms for the initial execution monitoring time setting.
If the initial execution monitoring time (t) parameter is set to 10ms, "WDT ERROR" occurs when actual initial scan time is
within the range of 10ms < t < 20ms.
SD523
Stores the initial scan time of 1ms or less (unit: s).
Stores the initial scan time. (unit: ms).
CHAPTER 2 APPLICATION OF PROGRAMMING
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