On all the sites I have found about implementing RS485 networks the schematics always show two wires (A/B) for the data signal, but never a specific ground connection between two stations on the network. Yes, each station is shown with a local ground connection, but nowhere does it actually tell you if a common ground connection is required between stations.
Only on one site which was about galvanic isolation in rs485 networks did it mention in passing that “since rs485 networks require a common ground…” which makes me think that yes it does.
However, an rs485 connection can run for a few kilometres. Ground potentials at the two sites could vary by quite a large amount. Would a ground wire linking the two sites in that case really be such a good thing?
Given the differential nature of the rs485 signal, the signal can be recovered without any reference to a ground as such - the signal is the difference between the A and B voltages, not the different between one voltage and ground - so is ground really needed?
Experimentally I have proved it works (at least over short distances) with a battery powered sender with no ground connection transmitting to a receiver over just the A/B lines.
Yes, shielding is good (and really needed), but should the shield be grounded at both ends or just one?