2. MODBUS/TCP PROTOCOL
2 - 2
Message frame
Transaction
Identifier
Protocol
Identifier
Length Field
Unit
Identifier
Function Data
2 × 8 bits
2 × 8 bits
2 × 8 bits
8 bits
8 bits
n × 8 bits
Message field
Size
Communication
path
Description
Transaction Identifier
(Transaction identifier)
2 × 8 bits
Client
→
Server This is the data to be added by the client for transaction
management. Set a value available for transaction
management.
Server
→
Client Transaction Identifier received from the client (controller) is
copied and returned.
Protocol Identifier
(Protocol identifier)
2 × 8 bits
Client
→
Server The value is fixed to "0".
If the server receives a value other than "0", the received value
is discarded and no value is returned.
Server
→
Client The value "0" is returned.
Length Field (Message length)
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.
Unit Identifier (Unit identifier)
8 bits
Client
→
Server The value is fixed to "255".
If the server receives a value other than "255", the received
value is discarded and no value is returned.
Server
→
Client Unit Identifier received from the client (controller) is copied and
returned.
Function
(Function field)
8 bits
Client
→
Server Set a function code to request to the server.
Server
→
Client Send the function code requested by the client.
When a communication error has occurred, send a value
obtained by adding "80h" to the function code requested by the
client.
Data (Data field)
n × 8 bits
Client
→
Server The format changes depending on the function code selected.
Refer to section 2.3 for details.
Server
→
Client
Summary of Contents for MELSERVO-JE MR-JE-C
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