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Modbus RTU RS-485
5
SFIDK.PS.023.D2.02
The module complies with the Modbus serial line protocol [Reference 1].
Among other things this implies a
master-slave
protocol at level 2 of the OSI model. One 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.
Two modes of communication are possible,
Unicast
and
Broadcast
.
Unicast
mode is where the
master
sends a request to one
slave device
, and waits a specified time for a response. 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 are no response from the
slave
devices
.
2.1 General Modbus RTU
The Modbus frame is shown below, and is valid for both requests and responses.
Further details of the Modbus protocol can be found in Reference 1 and 2.
2.
Technical data