Grounding in CAN / RS485 Busses
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.

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.

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.