Electronic – RS485 Common Ground: Common Voltage or Common Wire or Both


RS485 connection needs a common ground, but I am very fuzzy about how that works over longer distances and more complex systems.

  • When the transmitter and receiver are about 1000m apart, there will be a voltage drop so the receiver won't have the same voltage on the ground as the transmitter, will it?

  • Say if I power them separately and ensure a 5v ground on both ends, will that work?

  • Or if I run a higher voltage line and ground wire and then step it down to 5v for the receiver and transmitter, technically that will still be the common ground, or will it?

  • Finally, if there are a bunch of devices connected to that common ground, won't that interfere with the signal? If yes then should I have a separate dedicated ground just for the RS485 connection and another ground for everything else?

Best Answer

You have basically two questions in one.

1. Common ground

With 1000m between the nodes it is practically unavoidable to have ground potential out of transceiver's common mode range at some point. Therefore, you need at least one more conductor in your wiring for common ground. This, however, creates ground loop problems. So, you have to make environmental assessment first:

a) If this is permanent indoor installation then TIA/EIA-485-A recommended solution is to connect common ground wire to local ground at each node with 100 Ohm resistor.

b) If this is outdoor installation and especially if hot-plugging of the network nodes is expected, then isolated transceivers should be used, with common wire grounded at one side only.

I strongly recommend reading the RS-485 Design Guide by Texas instruments. Unfortunately, TIA standards are not free, but here you can find very good overview of the key points in TIA-485.

2. Power Supply

I am not sure what you mean when you say "ensure a 5v ground on both ends". A ground is a ground and 5V is 5V. In this context 5V is a supply voltage referenced to the ground. Since you have to run common ground wire between the nodes anyway (see above), an obvious solution would be to add one more conductor with power supply for all the nodes. But this is not a practical solution. The voltage drop and power losses over 1000m will be huge.

So, now again you need to make an assessment of how your network will be actually used:

a) If you go with non-isolated network (case 1.a above) and your nodes require very little current (like various sensors, for example) then one common solution is to have higher voltage power supply line along with data and ground wires. Inject 12-24V at single point and then use POL converters to bring it down to 5V at each node.

b) If you go with non-isolated network but your nodes require local power supplies anyway, then there is no point in added costs of central power distribution. Simply power nodes and their transceivers from local power supplies.

c) In case of isolated network (case 1.b) use isolated transceivers with built-in power converters, like MAXM22511 for example.