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MELSEC-F
COMMUNICATION USING MC PROTOCOL8
8 - 3
8.1.3 Access Timing on the PLC Side
The following diagram illustrates the access timing on the PLC side when an external
device accesses the PLC via the Ethernet module.
1) A command message is transmitted from the external device to the PLC
side.
2) Upon receiving a command message from the external device, the
Ethernet module on the PLC side sends a data read/write request to the
PLC according to the content of the command.
3) The PLC reads/writes data according to the request from the external
device at the time of executing the sequence program's END instruction,
and passes the processing result to the Ethernet module.
4) Upon receiving the processing result from the PLC, the Ethernet module
sends a response message including the processing result to the
external device that originated the request.
*1 The ACK response shown in the diagram is sent/received between the
Ethernet module and the external device respectively when the access
via the Ethernet module is performed by TCP/IP communication.
It is different from the response to the processing requested by the
external device in the command message (the processing result).
When access via the Ethernet module is performed by UDP/IP, the ACK
response indicated by *1 is not sent.
NOTE
Regarding the scan time of the PLC
1) While the PLC is performed by required command, access command to the
Ethernet module and the PLC 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 the Ethernet module at
the same time, the processing requested from the external devices may have to
wait for multiple END processing executions depending on the timing of the
requests.
(*1)
(*1)
STEP 0
END STEP 0 END STEP 0 END STEP 0 END
External device
Read/write command
(command)
Ethernet module
ACK
ACK
Reply to the
command (response)
4)1)
3)2)
PLC
END processing END processing END processing
Processing of the command from the external device