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8. Modbus protocol
8. 1. Principle
The Modbus protocol is a master-slave protocol.
Direct slave-to-slave communications are not possible.
For slave-to-slave communication, the master’s application software must therefore be designed to interrogate one slave and send back
data received to the other slave.
Two types of dialog are possible between master and slaves:
• The master sends a request to a slave and waits for it to respond.
• The master sends a request to all slaves without waiting for them to respond (broadcasting principle).
Note: The Modbus server on the "Controller Inside" card cannot be accessed via the Modbus card.
Only one device can transmit on the line at any one time.
The master manages the exchanges and only it can take the initiative.
It interrogates each of the slaves in succession.
No slave can send a message unless it is invited to do so.
In the event of an error during data exchange, the master repeats the question and declares the interrogated
slave absent if no response is received within a given time period.
If a slave does not understand a message, it sends an exception response to the master.
The master may or may not repeat the request.
Master
Slave i
Slave k
Slave j