Siemens SITRANS F Operating Instructions Manual page 9

Communication modes
Two modes of communication are possible, Unicast and Broadcast.
● In unicast mode the Master sends a request to a specific Slave device, and waits a
specified time for a response.
Image 1-1
● In Broadcast mode the master sends out a request to address "0", which means that the
information is for all Slave devices on the network. In Broadcast mode there is no
response from the Slave devices.
Image 1-2
Modbus Frame
The Modbus frame is shown below, and is valid for both requests and responses.
Table 1- 1
SLAVE ADDRESS
1 Byte
MAG 8000 Modbus RTU
Operating Instructions, 04/2016, A5E03409989-AB
Unicast Mode
Broadcast Mode
Modbus Frame
FUNCTION MODE
1 Byte
1.2 Modbus RTU technology
DATA
0 ... 252 Bytes
Introduction
CRC
2 Bytes
7