6: M
ODBUS
C
OMMUNICATION
6-28 FC6A S
ERIES
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ICRO
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MART
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IN
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OMMUNICATION
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ANUAL
FC9Y-B1730
Modbus TCP communication format in the Modbus RTU pass-through function
The Modbus RTU pass-through function deletes the Modbus TCP headers from the Modbus TCP communication frame received by
the Modbus TCP server and sends the frames with a CRC added to the "Unit ID", "Function code", and "Description" frames to the
Modbus RTU slave specified by the unit ID.
The Modbus TCP communication format in the Modbus RTU pass-through function is as follows.
Transaction ID
The Modbus TCP server returns the transaction ID from the client as is. The client can confirm to which request the response was
returned. Enter 0 when there is no particular check to perform.
Protocol ID
This number indicates the Modbus TCP protocol and is 0.
Message Length
Represents the length of the message that follows in bytes.
Unit ID
The ID for identifying the device. It stores the slave number of the Modbus RTU slave. The Unit ID and subsequent frames are
passed through to the Modbus RTU network.
If 255 is specified for the unit ID, the frames are processed by the Modbus TCP server (local station) and not passed through to
the Modbus RTU slave.
Function Code
The function number such as reading or writing.
Description
The data required for processing.
Transaction ID Protocol ID
Message Length (bytes)
Unit ID Function Code Description
2 bytes 2 bytes 2 bytes 1 byte 1 byte N bytes
Slave No. Function Code Description CRC
2 bytes1 byte 1 byte N bytes
“Idle”
3.5 characters
RTU Mode Communication Format
Modbus TCP Header
Modbus TCP Communication Format
“Idle”
3.5 characters
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