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4.2. Protocol
Description
The Modbus protocol defines a simple protocol data unit independent of the
underlying communication layers. The mapping of Modbus protocol on network can
introduce some additional fields on the application data unit.
Modbus/TCP Application Data Unit
MBAP Header
Function
Code
Data
Protocol Data Unit
MBAP
The Modbus/TCP extension includes 7 additional bytes to the original Modbus
protocol, which allows for transport over the TCP/IP layers.
A dedicated header is used on TCP/IP to identify the Modbus Application Data Unit.
It is called the MBAP Header (MODBUS Application Protocol Header). The MBAP
Header consists of 7 bytes of information:
Field
Length
Description
Transaction
Identifier
2 bytes
Identification of Request/Response
transaction
–
Copied from request to response
Protocol Identifier
2 bytes
0 = Modbus protocol
Length
2 bytes
Number of following bytes - Includes the Unit
Identifier
Unit Identifier
1 byte
Identification of remote slave
Summary of Contents for OME-ET-7000 Series
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