Modbus Rtu Telegram Format - Siemens SIRIUS 3RW5 Equipment Manual

3.5.2

Modbus RTU telegram format

The data traffic between the master and slave and the corresponding answer from the slave
to the master 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. The CRC-CHECK is transmitted at the end of the telegram. In the response telegram
of the slave to the master, the same Address Field and the same Function Code are entered.
The Data are filled according to the requested data.
● Address Field = Slave address (parameter "Station address"): This address (1 to 247) is
used to address a defined slave on the bus. The broadcast address "0" is used to
address all slaves that are subordinate to the master.
● 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.
● CRC CHECK = Telegram checksum: The end of the telegram is identified by the CRC-16
checksum of 2 bytes in length.
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).
SIRIUS 3RW5 Modbus communication module
Equipment Manual, 03/2020, A5E43676481002A/RS-AC/003
Description
3.5 Telegram structure
29