Siemens SITRANS F Modbus RTU RS 485 Operating Instructions Manual page 6

Introduction
1.3 Modbus RTU technology
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 specific Slave device, and waits a specified
time for a response.
Figure 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.
Figure 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
6
Master
Slave Slave
Unicast Communication Mode
Master
Slave Slave
Broadcast Communication Mode
Modbus Frame
FUNCTION MODE
1 Byte
Slave
Slave
DATA
0 ... 252 Bytes
Operating Instructions, 03/2021, A5E00753974-AB
CRC
2 Bytes
Modbus RTU RS 485