Mitsubishi Electric MELSEC-Q00U(J)CPU User Manual page 123

(4) Constant scan accuracy
The constant scan accuracy is 0.01ms.
However, the constant scan time may increase in the following cases.
(a) When interrupt program or fixed scan execution type program is executed
Interrupts are disabled while an interrupt program or fixed scan execution type program is executed.
Even if the constant scan time runs out during execution of an interrupt program or fixed scan execution type
program, the constant scan cannot be finished.
In this case, the constant scan time may exceed the time set and increase by the time of the program
executed.
(b) When service processing is performed just before the end of constant scan
The constant scan time may exceed the time set and increase.
CHAPTER 3 FUNCTIONS
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