Modbus/Tcp Protocol Frame Specifications - Mitsubishi Electric MELSEC iQ-R Series Reference Manual

1.2

MODBUS/TCP Protocol Frame Specifications

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