(I)GW11XX Modbus User Manual
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received request
Length
2 bytes The number of bytes
below
start up
(request)
Server (response)
start up
Unit
identifier
1 byte
The serial number of the
remote slave connected
to the serial link or other
bus
start up
The server is
replicated from the
received request
The message header is 7 bytes long:
Transaction Identifier: Used for transactional pairing. In response, the Modbus
server replicates the transaction identifier of the request.
Protocol identifier: multiplexing within the system. The Modbus protocol is
recognized by a value of 0.
Length: The length field is the number of bytes in the next field, including the cell
identifier and the data field.
Unit identifier: Use this field for intra-system routing. Dedicated to communication
over Modbus or serial link slaves via a gateway between an Ethernet
TCP-IP network and a Modbus serial link. The Modbus client sets this field in the
request, and the server must return the domain with the same value in the
response.
11.5 Modbus basic architecture diagram
Modbus Basic Architecture Figure is shown below:
