Telegram Structure; Error Responses - Siemens SIRIUS 3RW5 Equipment Manual

Description

3.5 Telegram structure

3.5 Telegram structure
3.5.1 Telegram structure Modbus TCP
The data traffic between the client and the server and the corresponding answer from the
server to the client starts with the Address Field, followed by the Function Code. Following
this, the data (Data) are transferred. The structure of the Data depends on the Function
Code used. In the response telegram of the server to the client, the same Address Field and
the same Function Code are entered. The Data are filled according to the requested data.
● Address Field = MBAP Header (Modbus Application Protocol): This address is used to
address a defined server on the bus.
– Transaction identifier (2 bytes)
– Protocol identifier (2 bytes)
– Length (2 bytes)
– Unit identifier (1 byte)
● Function Code: Defines the server function requested with the telegram.
● Data = Telegram data: Function Code dependent administration data and net data. When
transferring the register data, the high byte is always transferred first, followed by the low
byte, in accordance with the Modbus specification.
You can find more information on Function Codes in chapter Function Codes (Page 98).

Error responses

For a negative acknowledgment, the response telegram is equivalent to an Error response.
You can find more information on Error responses in chapter Exception Codes (Page 112).
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SIRIUS 3RW5 Modbus communication module
Equipment Manual, 03/2020, A5E43676481002A/RS-AC/003