CAI 700M - MBAP Header Description

700M HFID 6-2-15
178
MBAP Header Description
The MBAP Header contains the following fields:
Field
Length
Description
Transaction Identifier
2 Bytes
Identification of a Modbus request / Response transaction
Protocol Identifier
2 Bytes
0 = Modbus
Length
2 Bytes
Number of following bytes
Unit Identifier
1 Byte
Identification of a remote slave connected on a serial line or
on other buses
The header is seven bytes long:
• Transaction Identifier – Used for transaction pairing, the Modbus server copies in the
response the transaction identifier of the request.
• Protocol Identifier – 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. It is typically used to
communicate to a Modbus+ or a Modbus serial line slave through a gateway between an
Ethernet TCP-IP network and a Modbus serial line. This field is set by the Modbus client
in the request and must be returned with the same value in the response by the server.
All Modbus/TCP ADU are sent via TCP to Registered Port 502.
The different fields are encoded in Big-endian. The 700M HFID analyzer uses only the
Length bytes from the MBAP section.