Modbus wiring

[StefanA]

Been working on a little projekt with a Nano-10 acting as a RS485 modbus master and a temperature controller as a slave device. When programming and testing on my desk I have only used two wires for communication but what is the right way to wire it up in the field? I attached a picture describing how I think the wiring should be, but I'm not completely sure. I'm planning to replace the RS485 driver to a LT1785AIN8.

[support]

Your connection should work if the two devices share the same power supply or at least they share the same 0V. Otherwise you should connect their 0V together so that they are referenced from the same ground. One way to do this is to tie the shield of the shielded twisted pair wire to the 0V of the power supply of each device so that their 0V are tied together via the shield.

The terminating resistors is needed only if the two devices are located far apart from each other. If they are separated by just a few feet of wire then it is not even necessary to add the terminating resistors.

The installation of the biasing resistors as you did is recommended since this put the RS485 into a known state even when all devices are not actively transmitting data.

[StefanA]

The slave is powered by 230V AC. But if I tie the slave's RS485 COM terminal to the Nano 0V power supply the signal ground should be same?

The distance between the Nano and the slave are about 50 feet so it's maybe a good idea to include the terminating resistor. I've seen some wiring examples with terminating resistors in both end of the network. Is that of any importance?

[support]

If the RS485 has a COM or ground terminal then yes you can connect that to the PLC's power supply 0V to get a common reference.

As for the terminating resistors, the best way is to test it. At 38400 or slower say 9600 bps the resistors may not have any effect over a 50 ft. connection.