Modbus Protocol; Protocol Description; Brief Introduction - Bosch Rexroth VFC 3210 Operating Instructions Manual

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Communication

14.3 Modbus Protocol

14.3.1 Protocol Description

Brief introduction

Modbus is a master / slave protocol. Only one device may send commands in
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the network at a particular time.
The master station manages message exchange by polling the slave stations.
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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.
If a slave station is unable to recognize a message from the master station, an
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exception response will be sent to the master station.
Slave stations cannot communicate with each other but through the master's
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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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