Description
3.3 Modbus RTU technology
FC410 with Modbus
24
Operating Instructions, 02/2016, A5E33120874-AC
Features
The SITRANS F Modbus RTU communication complies 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 at a time can be
connected.
Unicast communication mode
In unicast mode (master/slave mode) the master sends a request to a specific slave device
and waits a specified time for a response.
Image 3-4
Unicast Mode
Modbus Frame
The Modbus frame is shown below and is valid for both requests and responses.
Table 3- 1
Modbus Frame
SLAVE ADDRESS
FUNCTION MODE
DATA
CRC
1 Byte
1 Byte
0 to 252 Bytes
2 Bytes
References
For further information, please refer to the following specification and guidelines available at
the Modbus Organization (
) website
1.
Serial Line Specification & Implementation guide
2.
Application Protocol Specification