Mitsubishi MELSEC Q Series Reference Manual page 33

(1) Note that when data is read/written from/to the external device while the
(2)
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2 DATA COMMUNICATION USING THE MELSEC COMMUNICATION PROTOCOL
POINT
programmable controller CPU is in operation, the scan time of the
programmable controller CPU increases for the amount of time it takes to
process the command from the external device.
If the increase in scan time affects the operations of the programmable
controller CPU, access the CPU module multiple times by dividing the points.
An access request from the external device to the buffer memory of the
C24/E71 is immediately acknowledged. Also, it does not affect the scan time
of the programmable controller CPU.
REMARKS
Regarding the scan time of the programmable controller CPU
(1) While the programmable controller CPU is performed by required command,
access command to the C24/E71 and the programmable controller CPU is
processed during every END processing.
(The scan time becomes longer according to the amount of time it takes to
process the command.)
(2) When multiple external devices issue access requests to a single station at the
same time, the processing requested from the external devices may have to
wait until END processing has been executed multiple times depending on the
timing of the requests.
Multiple command requests can be processed within one scan by using any of
the following methods. Note that, however, these methods are not available for
safety CPUs.
(a) Using the COM instruction
On the sequence program, execute the COM instruction.
For details of the COM instruction, refer to the Programming Manual
(Common Instructions) for the CPU module used.
(b) Using the Time reserved for communication processing (SD315) (for the
Basic model QCPU, High Performance model QCPU, Process CPU, and
Redundant CPU)
With the Time reserved for communication processing (SD315), set a time
value for communication processing within the range of 1ms to 100ms.
(c) Using the service processing setting (for the Universal model QCPU and
the LCPU)
Under the "Service processing setting" on the PLC system tab of PLC
parameter, set a time value for service processing within the range of 1ms
to 100ms.
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