Schneider Electric LXM32M manual Mbap header, Field Length Description

3 Basics LXM32M

3.2.1MBAP header

The MBAP header contains the information allowing the recipient to uniquely identify a message. This is even possible if a message is split into several packets for transmission.

Explicit and implicit length rules as well as the use of a CRC-32 error check code (on Ethernet) results in an infinitesimal chance of undetec- ted corruption to a request or response message.

Design The MBAP header has a length of 7 bytes and contains the following fields:

Field	Length	Description
Transaction Identifier	2 bytes	Identification of a Modbus
		request or Modbus response.
Protocol Identifier	2 bytes	Value 0 means Modbus proto-
		col.
Length	2 bytes	Number of bytes to follow.
Unit Identifier	1 byte	Identification of a slave con-
		nected to another bus via a
		serial line.

•Transaction Identifier

The field "Transaction Identifier" is used for "Pairing". The server copies the "Transaction Identifier" of the Modbus request to the Modbus response.

•Protocol Identifier

The field "Protocol Identifier" is used to identify the protocol. The Modbus protocol is identified by the value 0.

•Length

The "Length" field is a byte counter for the following fields ("Unit Identifier", "Function Code" and "Data").

•Unit Identifier

The field "Unit Identifier" is used to identify the server in the slave.

0198441113843, V1.01, 01.2012

18 Modbus-TCP module