Mitsubishi Electric MELSEC iQ-RJ71C24-R4 - MODBUS;TCP Protocol Frame Specifications

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1 FRAME SPECIFICATIONS
1.2 MODBUS/TCP Protocol Frame Specifications
1.2 MODBUS/TCP Protocol Frame Specifications
This section describes the frame specifications of the MODBUS/TCP protocol.
Area name Area size Description
MODBUS application header Communication ID 2 bytes Used by the master to match response messages from slaves.
Protocol ID 2 bytes Indicates the protocol of the protocol data unit.
For MODBUS/TCP, 0 is stored.
Message length 2 bytes Stores the message size in bytes.
The stored message length indicates the length of the message in
areas subsequent to this area.
Module ID 1 byte Used when specifying slaves connected to other lines, such as with
the MODBUS Serial protocol.
Protocol data unit Function code 1 byte Specifies the content of the processing instructed from the master to a
slave.
Data 1 to 252 bytes ■When request message is sent from the master to a slave
Stores the request content of the processing.
■When response message is sent from a slave to the master
Stores the execution result of the processing.
FCSEthernet
header
Application dataIP header TCP header
(Error check)
MODBUS/TCP frame
MODBUS/TCP application data unit
MODBUS application header
Transaction
ID
Protocol ID Message
length
Module ID Function code Data
Protocol data unit