Consider this arrangement:
A daisy chain of Modbus/RTU slaves that will require to communicate simultaneously with 2 master devices using different protocols (Bacnet IP and Modbus/RTU).
Is it possible ?
Is there a device that can act as a gateway using two protocols ?
The MangoES can act as a Modbus RTU Master and then simultaneously expose the data as Modbus IP and BACnet IP as a slave. https://infiniteautomation.com/mango-es/
If you needed to have the same data also available to another Modbus RTU master I would use a product like the Nano 10 PLC and push the data via Modbus IP and then let the other Modbus RTU master read it from this PLC over RTU. http://www.triplc.com/nano10.htm
There is no way to have two modbus RTU masters on the same network.
This would be a pretty cost effective solution that would also be pretty flexible in other ways.
You could also use this PLC: http://www.triplc.com/fx1616ba.htm which has 2 RS-485 ports to poll on one RS-485 and be a slave on the other and also be a Modbus IP slave but it doesn't support BACnet IP so this is where the MangoES really shines.
The reason Modbus RTU is "one master only" is that there is no control over the 485 bus (other than a single master) so that messages don't collide with one another.
Assuming there's a BacnetIP/ModbusRTU gateway,
- Have the Modbus RTU master poll the Modbus RTU slaves.
- Have the Modbus RTU Master write that newly acquired data (from the slaves) to the Modbus RTU slave side of a gateway.
- Have the Bacnet IP master read the Bacnet side of the gateway.