GN-9289 User Manual
Page 33 of (70)
G-series GN-9289 Modbus TCP_UDP Rev. 1.01.docx
8.1.2.
MODBUS TCP/ UDP MBAP Header
The MBAP (MODBUS Application Protocol) header contains the following fields.
Fields
Length
Description
Client
Server
Transaction
Identifier
2bytes
Identification of a MODBUS
Request /Response transaction.
Initialized by the
client
Recopied by the server from
the received
Protocol
Identifier
2bytes
0 = MODBUS protocol
Initialized by the
client
Recopied by the server from
the received
Length
2bytes
Number of following bytes
Initialized by the
client (Request)
Initialized
by
the
server
(Response)
Unit
Identifier
1byte
Identification of a remote slave
connected on a serial line or on
other buses.
Initialized by the
client
Recopied by the server from
the received
Transaction Identifier - It is used for transaction pairing, the MODBUS server copies in the response the
transaction identifier of the request.
• Protocol Identifier – It is used for intra-system multiplexing. The MODBUS protocol is identified by
the value 0.
• Length - The length field is a byte count of the following fields, including the Unit Identifier and data fields.
• Unit Identifier – This field is used for intra-system routing purpose. Typically MODBUS server must
be returned with the same value set by MODBUS client.
