Q:

RS-485 over many 1000s ft, ground wire and termination always needed?

I've only ever used RS-485 over short connections before.  I may have a project coming up where the cables will run several 1000 ft.  My questions are:

Is the gnd wire needed?
Should the shield in the cable be used as the gnd wire?
When must terminator resistors be used?

A:

I would think it would be asking for trouble if you did not use termination and grounding. See Jan Axelson's write up. Also the further your cable extends, the lower the baud rate will be. At 4000' the maximum baud rate is 90kbps. I think using the shield as the ground is okay, but you need to isolate each node with 100 ohm resistors. I have used RS485 for short hops (~100' or so) without using termination or grounds, but I think for K's of feet you would be wise to stick to the rules.

A:

For 3. you need termination resistors to avoid reflections, especially at distance or high speed. This is even more true for RS-485 where the signal can be driven from the middle (or at least not at the end).

