Operation Proline Promag 53 MODBUS RS485
62 Endress+Hauser
Master/slave communication
A distinction is made between two methods of communication with regard to master/slave
communication via MODBUS RS485:
• Polling (request-response-transaction)
The master sends a request telegram to one slave and waits for the slave's response telegram.
Here, the slave is contacted directly due to its unique bus address (1 to 247).
a0004401
Fig. 44: MODBUS RS485 polling data traffic
1 MODBUS master (PLC, etc.)
2 MODBUS RS485
3 MODBUS slave (measuring devices, etc.)
a Request telegram to this one specific MODBUS slave
b Response telegram to the MODBUS master
• Broadcast message
By means of the global address 0 (broadcast address), the master sends a command to all the slaves
in the fieldbus system. The slaves execute the command without reporting back to the master.
Broadcast messages are only permitted in conjunction with write function codes.
a0004402
Fig. 45: MODBUS RS485 polling data traffic
1 MODBUS master (PLC, etc.)
2 MODBUS RS485
3 MODBUS slave (measuring devices, etc.)
a Broadcast message command to all MODBUS slaves (request is executed without a response telegram to the
master)
a
b
2
33
3
1
3
a
2
33
aa
1
3
3
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