This question came up in another group, and I wasn't sure what the answer should be.
Let's say you've set up a system using an RS-485 communication bus, with differential transmission in both directions over a single wire pair. Then you decide that you want to connect a new unit to the existing network, and it's some distance away. It's going to run off an independent power supply, maybe just a wall wart. You run a twisted pair to carry the signals to the new unit. But is that enough, or should there also be a ground wire to tie the grounds of the two systems together? Or maybe the two grounds should each be tied to earth ground, rather than be left floating, and that would link them to the same voltage level. But is that legal and safe?
Let's say you've set up a system using an RS-485 communication bus, with differential transmission in both directions over a single wire pair. Then you decide that you want to connect a new unit to the existing network, and it's some distance away. It's going to run off an independent power supply, maybe just a wall wart. You run a twisted pair to carry the signals to the new unit. But is that enough, or should there also be a ground wire to tie the grounds of the two systems together? Or maybe the two grounds should each be tied to earth ground, rather than be left floating, and that would link them to the same voltage level. But is that legal and safe?