14.3 Modbus Protocol
14.3.1 Protocol Description
Brief introduction
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Modbus is a master / slave protocol. Only one device may send commands in
the network at a particular time.
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The master station manages message exchange by polling the slave stations.
Unless being approved by the master station, no slave station may send mes-
sage. In case of an error during data exchange, if no response is received, the
master station will query the slave stations absent from the polling.
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If a slave station is unable to recognize a message from the master station, an
exception response will be sent to the master station.
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Slave stations cannot communicate with each other but through the master’s
software which reads data from one slave station and sends them to another.
There are two types of dialogs between the master station and the slave sta-
tions:
– The master station sends a request to a slave station and waits for its re-
sponse.
– The master station sends a request to all slave stations and does not wait
for their response (broadcasting).
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