Mitsubishi Electric MELSEC-Q00U(J)CPU User Manual page 247

(3) Precautions
The following describes precautions when the service processing setting is configured.
• For the following functions, scan time will be increased longer than the specified time during service
processing even if the service processing time specification is set.
• Online change
• Change TC setting
• Local device monitor
• Program memory backup
• Writing/reading data to/from a file register (The scan time will be increased when the write or read
size is large.)
• Writing to/reading from the buffer memory of the intelligent function module (The scan time will be
increased when the write or read size is large.)
• Access to a network module
a) Diagnostic functions (CC IE Control diagnostics, CC IE Field diagnostics, MELSECNET
diagnostics, Ethernet diagnostics, CC-Link/ CC-Link/LT diagnostics)
b) Monitor function (Module access device, Link direct device)
• Note that the scan time will be increased much longer if the CPU module receives multiple requests
simultaneously while the service processing count specification is set many.
• The response performance of service processing significantly decreases in the following cases. Set service
processing time considering the time-out time of the peripheral.
• Service processing time is set much shorter than the scan time.
• Setting "Execute it while waiting for constant scan setting" results in increase in the scan time and
decrease in the service processing time.
• An error of -20µs to +30µs occurs between the actual processing time and the set service processing time.
• When "Specify service process execution counts." is selected and Ethernet communication is performed, the
scan time increases by the service processing time (approx. 500ms). To keep the time increase 500ms or
less, select an item other than "Specify service process execution counts.". (Example: Select "Specify
service process time." and set a desired time value.)
• If communications in the MC protocol are performed with an item other than "Specify service process
execution counts" selected, data inconsistency may occur. To prevent data inconsistency, select "Specify
service process execution counts".
• In the service processing, processing for the requests from each receive port (each intelligent function
module, USB, the RS-232, and built-in Ethernet) is performed one by one. If multiple requests are sent from
a single receive port, the service processing for those requests may not performed in the same scan
regardless of whether or not the service processing time is left. Therefore, when communications are
performed between multiple peripherals via built-in Ethernet port, response time to each peripheral cannot
be shortened even though a longer service processing time is set. In this case, connect peripherals not only
to the built-in Ethernet of the CPU module but also to the Ethernet module to shorten the response time.
• Since the processing for a request including accesses to files takes time, responding to the request
necessarily takes time. The processing for request data is performed one by one in the service processing,
and therefore responses to succeeding request data delays when the CPU module receives a request
including accesses to files. For this reason, set a longer timeout time for peripherals when using a system
that regularly send these requests.
CHAPTER 3 FUNCTIONS
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