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Modbus application data unit on TCP / IP
Modbus requests or responses in a Modbus TCP / IP network
Modbus request / response on TCP / IP
Use a special message header on TCP / IP to identify the Modbus application data
unit. This header is called the MBAP header (Modbus protocol header).
This header provides some differences from the Modbus RTU application data units
used on the serial link:
Replace the Modbus slave address field commonly used on the Modbus serial
link with a single byte unit identifier in the MBAP header. This unit identifier is
used for communication of devices that use a single IP address to support
multiple independent Modbus terminal units, such as bridges, routers, and
gateways.
Use the receiver to verify all the Modbus requests and responses by verifying the
completion of the message. For a function code with a fixed length of the Modbus
PDU, only the function code is sufficient. For a function code that carries a
variable data in a request or response, the data field needs to include the number
of bytes.
When Modbus is carried over TCP, even if packets are divided into multiple
packets, the additional length information is carried on the MBAP packet header
so that the receiver can identify the message boundary. The presence of explicit
and implicit length rules and the use of the CRC-32 error check code (on the
Ethernet) will produce minimal undetected interference to the request or
response message.
MBAP header description
The MBAP header includes the following fields:
Field
Transactio
n identifier
Protocol
identifier
Length
2 bytes
The
MODBUS
response transaction
2 bytes
0 = MODBUS protocol
3onedata proprietary and confidential
Copyright © Shenzhen 3onedata Technology Co., Ltd.
Description
identifier
of
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(I)GW11XX Modbus User Manual
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