Schneider ElectricModbus TCP
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What is a MBAP header and what is it for?
What is a MBAP header and what is it for?
Schneider ElectricModbus TCP
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• 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. 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.
The MBAP (MODBUS Application Protocol) is dedicated header information used for sending MODBUS packages over a UDP in order to identify the MODBUS Application Data Unit.
It is very important to know that all MODBUS/TCP Application Data Units are sent via UDP to registered port 502.
Whenever a MODBUS request or response is sent over in a UDP package, there is a need for this additional information header, MBAP, for the recipient to recognize the message boundaries even if the message has been split into multiple transmissions packets.
Because of the explicit and implicit length rules usage, and CRC-32 error check code on Ethernet, the result shows an extremely low chance of undetected message corruption that can require additional request or response messages.
As presented in the previous answer, it is 7 Bytes long, providing crucial differences compared to the MODBUS RTU Application Data Unit that is used on a regular serial line (RS232C or RS485).
The MBAP header is a part of the Modbus protocol used to transmit data in industrial automation systems. MBAP stands for Modbus Application Protocol and the MBAP header is a 7-byte header that is sent before the data to provide information for transmission.
The MBAP header contains information such as the Transaction Identifier, the protocol, the device address, the function code and the number of data bytes. This information is used to ensure that the transmission of data works properly and that the received data can be correctly allocated.
The MBAP header is an important part of the Modbus protocol and helps to make the transmission of data in industrial automation systems safe and reliable.
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