3 Basics | LXM32M |
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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
Design The MBAP header has a length of 7 bytes and contains the following fields:
Field | Length | Description |
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Transaction Identifier | 2 bytes | Identification of a Modbus |
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| request or Modbus response. |
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Protocol Identifier | 2 bytes | Value 0 means Modbus proto- |
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| col. |
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Length | 2 bytes | Number of bytes to follow. |
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Unit Identifier | 1 byte | Identification of a slave con- |
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| nected to another bus via a |
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| serial line. |
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•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
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