Modbus/Tcp Communication Message Frame - Mitsubishi Electric MR-JE-C Series Instruction Manual

3. Modbus/TCP

3.5 Modbus/TCP Communication Message Frame

Query Messages sent from the client (controller) and Response Messages sent from the servers (servo
amplifiers) are both sent in the following message frame format. Messages are sent by using TCP/IP.
Client
(controller)
A message frame is composed of six message fields.
When a server (servo amplifier) received a Query Message sent from the client (controller) without errors,
the function code in the Query Message is copied into the Function in a Response Message. When a server
(servo amplifier) received a Query Message with an error, the server returns a value obtained by adding
"80h" to the function code value in the Query Message to the client. Judge the occurrence of an error by
checking the function code in the Response Message on the controller side.
Message frame
Transaction Protocol
Length Field
Identifier Identifier
2 × 8 bits 2 × 8 bits
Message field
Transaction Identifier
Protocol Identifier
Length Field
Unit Identifier
Function
Data
Unit Identifier
Function
Data
Query Message
Response Message
Unit Identifier
Function
Data
Unit
Function
Identifier
2 × 8 bits 8 bits 8 bits
Communication
Size
2 × 8 bits Client → Server This is the data to be added by the client for transaction
Server → Client Transaction Identifier received from the client (controller) is
2 × 8 bits Client → Server The value is fixed to "0".
Server → Client The value "0" is returned.
2 × 8 bits Client → Server Set the byte length from Unit Identifier to Data.
Server → Client The byte length from Unit Identifier to Data is returned.
8 bits Client → Server The value is fixed to "255".
Server → Client Unit Identifier received from the client (controller) is copied and
8 bits Client → Server Set a function code to request to the server.
Server → Client Send the function code requested by the client.
n × 8 bits Client → Server The format changes depending on the function code selected.
Server → Client
Server
(servo amplifier)
Data
n × 8 bits
path
management. Set a value available for transaction
management.
copied and returned.
If the server receives a value other than "0", the received value
is discarded and no value is returned.
If the server receives a value other than "255", the received
value is discarded and no value is returned.
returned.
When a communication error has occurred, send a value
obtained by adding "80h" to the function code requested by the
client.
Refer to section 3.6 for details.
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Description