Servomex SERVOPRO - Modbus Protocol

SERVOPRO NOx Analyzers Page
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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.