MAGS1 User Guide
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V1.5 22-1-2016
4.6. General Modbus RTU
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 any single point
in 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 is 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 RTU protocol can be found in Reference 1 and 2.
Slave Address Function code Data
Crc
1 Byte
1 Byte
0-252 Bytes
2 Bytes
Unicast mode
Broadcast mode
