Grounding in CAN / RS485 Busses

Question

I'm interested in how to properly realize Grounding in a CAN / RS 485 bus setup.

From my understanding, when non-isolated transceivers are used, in addition to the differential signal lines, a third ground wire should be used in order to eliminate potential differences between the nodes, that is, in order to avoid excess common-mode voltage at the transceivers' inputs.

In contrast, when isolated transceivers are used, no problems caused by different ground potentials will occur as the isolated nodes are floating.

My questions are:

1) Is the above assumption valid?

2) Assuming a CAN bus setup with only two transceivers: It is sufficient to just use one isolated transceiver while the other transceiver does not need to be isolated? From my understanding, this should be the case since no harmful potential differences may occur. Do I need the ground wire in such a setup?

3) If my CAN bus has only isolated transceivers connected, is there any need to provide e.g. a high impedance ground connection at one transceiver in order to deal with static charging (or some other unexpected problem I am not think about)?

Many thanks in advance!

Answer 1

1. No, assumption is not valid. RS-485 and CAN buses need a common reference, so that the voltages are within a certain operating range. If they don't have a common reference, the voltages may go beyond operating range, and the logic level cannot be determined properly. They also tolerate a certain maximum range after which there might be damage. For RS-485, the common mode voltage, or grounds between tranceivers, must be within 7V and for CAN bus within 2V.

In fact, it is the opposite, if you have two RS485 devices, and they both already share a common reference, like both tranceivers have their grounds already connected to same potential via mains plug earth for example, then they already share a reference, and a separate ground wire between devices should not be connected to prevent ground loops.

2. See 1, yes, tranceivers need common ground reference. For example, isolated RS-485/CAN tranceivers have the isolated RS-485/CAN side which contains the differential data wires and the ground reference for them, and the grounds must be within the common mode voltage range.

3. See 1, yes, even if there are two isolated tranceivers, both isolated, they still need the ground reference for the bus to keep the bus common mode voltage range within limits. Since the bus ground reference is floating, i.e. not connected to anything, it usually is tied to some potential like earth/0V/ground/chassis etc at some point, sometimes not directly for setting the DC level, but via capacitor to have a low impedance for high frequencies to suppress electromagnetic interference.

Basically, if a single isolated RS-485 tranceiver allows for 1kV of isolation between MCU side and RS-485 side, you can have MCU ground to be 0V, bus ground to be 1kV, and the other MCU ground at 2kV potential.