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4
MODBUS STANDARD FUNCTIONS
4.2 Frame Specifications
Area Name
MBAP header
(MODBUS application
header)
PDU
(Protocol data unit)
4 - 3
The frame specifications of MODBUS/TCP are indicated below.
Transaction ID
Protocol ID
Message length
Module ID
Function code
Data
Area Size
Used by the master for matching of the response
2 bytes
message from the slave.
Indicates the protocol of the PDU (protocol data
2 bytes
unit).
Stores 0 in the case of MODBUS/TCP.
Stores the message size in byte unit.
2 bytes
The message length after this field is stored. (See
the above figure.)
Used to specify the slave connected to the other
1 byte
line, e.g. MODBUS serial protocol.
The master specifies the processing to be performed
1 byte
for the slave.
[When master sends request message to slave]
Stores the requested processing.
1 to 252 bytes
[When slave sends response message to master]
Stores the result of processing execution.
MELSEC-Q
Description
4 - 3
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