General Modbus Rtu - Siemens SITRANS F M MAGFLO series Operating Manual

Modbus RTU RS-485

2.1 General Modbus RTU

2. Technical data
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.
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.
SFIDK.PS.023.D2.02
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