Proline Promass 83E/F/O Modbus RS485 Operating Instructions

Proline Promass 83 Modbus RS485OperationMaster/slave communicationA distinction is made between two methods of communication with regard to master/slavecommunication 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).Fig. 31: Modbus RS485 polling data traffic1 Modbus master (PLC, etc.)2 Modbus RS4853 Modbus slave (measuring devices, etc.)a Request telegram to this one specific Modbus slaveb Response telegram to the Modbus master• Broadcast messageBy means of the global address 0 (broadcast address), the master sends a command to all the slavesin 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.Fig. 32: Modbus RS485 polling data traffic1 Modbus master (PLC, etc.)2 Modbus RS4853 Modbus slave (measuring devices, etc.)a Broadcast message - command to all Modbus slaves (request is executed without a response telegram to the master)Issue/Rev. 0.2 (12/12) MN0M030 • Page 43