Considerations - Mitsubishi Electric MELSEC IQ-R series Reference Manual

3.2

Considerations

The following are the considerations when performing data communication.
When accessing CPU module
The following are the considerations when accessing a CPU module from an external device via a supported device.
■Processing timing of CPU module
Processing for a request message is performed during an END processing of CPU module.
External device
Request message
Supported device
Step 0
END
CPU module
END processing
1.
Send a request message from the external device to a supported device.
2.
When the request message is received from the external device to the target device, a data read request or write request
is issued to the CPU module.
3.
In accordance with the request issued from the external device during END processing from the external device, CPU
modules reads/writes data and then sends the processing result back to the supported device.
4.
Once the supported device receives a processing result from the CPU module, send a response message including the
processing result to the external device.
■Read or write data while CPU module is in RUN
• Due to process the request from the external device, scan time of the CPU module will be extended. If the extension of the
scan time affects the control, access the CPU module several times with less number of points.
• Before writing, check whether the online change is enabled (such as whether the system protection is unlocked or not) on
the CPU module side.
■When the access target CPU module is protected by system protection
An error occurs at the access target, and an abnormal response is sent back to the external device. Unlock the system
protection of the CPU module side, and resend the request message.
■When access requests are sent to one station from several external devices at the same time
Depending on the request timing, the processing requested from the external device may be on hold until several END
processing take place. By using either of the following methods, multiple requests can be processed in one scan.
• Execute COM instruction by program.
• Ensure 1 to 100 ms of service processing time using "Service Processing Setting" of Engineering tool.
External device
Response message
1. 4.
2. 3.
Step 0 Step 0
END
to to
END processing
Processing of the request message from the external device.
Step 0
END END
to
END processing
3 COMMUNICATION PROCEDURE
3.2 Considerations
3
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