Siemens SITRANS F Modbus RTU RS 485 - Page 6

Features
The SITRANS F Modbus RTU Communication modules comply with the MODBUS Serial Line
Protocol. Among other things this implies a Master-Slave protocol at level 2 of the OSI model. A
node (the master) issues explicit commands to one of the slave-nodes and processes responses.
Slave nodes will not transmit data without a request from the master node, and do not
communicate with other slaves.
Modbus is a mono Master system, which means that only one Master can be connected at the
time.
Communication modes
Two modes of communication are possible, Unicast and Broadcast.
• In unicast mode the Master sends a request to a speci󹪝c Slave device, and waits a speci󹪝ed
time for a response.
response
Master
Slave Slave Slave
request
Figure 1-1 Unicast Communication Mode
• 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.
Master
Slave Slave Slave
request
Figure 1-2 Broadcast Communication Mode
Modbus Frame
The Modbus frame is shown below, and is valid for both requests and responses.
Table 1-1 Modbus Frame
SLAVE ADDRESS FUNCTION MODE DATA CRC
1 Byte 1 Byte 0 ... 252 Bytes 2 Bytes
Introduction
1.3 Modbus RTU technology
Modbus RTU RS 485
6 Operating Instructions, 03/2021, A5E00753974-AB