8-3
Chapter 8. Modbus TCP Communication Functions
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Message structure
The TCP/IP frame is utilized. A Modbus TCP message is shown in the TCP data
section.
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Modbus TCP
(1)
(2)
(3) (4) (5)
(6)
(1) Transaction Identifier (2 bytes) Not defined particularly.
(2) Protocol Identifier (2 bytes)
0000H for the Modbus protocol
(3) Length (2 bytes)
Shows the number of bytes in (4) to (6).
(4) Unit Identifier (1 byte)
Specify FFH or 00H.
(5) Function (1 byte)
Specify the function code.
(6) Data (n bytes)
Data column depending on the function code
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Detailed data
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Transaction Identifier
A request and its response have the same value.
The host station can use the transaction identifier to confirm that the data is the
response to the request.
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Protocol Identifier
Specify 0000H for the Modbus protocol.
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Length
Shows the data length in bytes based on the unit identifier.
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Unit Identifier
Specify FFH or 00H.
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Function
Specifies the function code.
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Data
Communications data
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Frame detection method
A TCP frame is equivalent to one Modbus TCP frame.
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Port
The Modbus TCP port number is 502 (changeable).
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Function code
Function Codes 3 (03H), 16 (10H), and 6 (06H) are supported.
8 - 2
Message structure
