In all cases there needs to be something to keep the ground reference between any one RS485 transceiver within a volt or so of any other RS485 transceiver on the network. If the ground potential difference between any two nodes goes outside of the common mode range of the receiver then comminications is sure to fail and the IC's themselves may be damaged and certainly stressed.

There is LOTS of information on this subject in this forum if you search for it!

When I build RS485 networks I send the data lines from node to node down a twisted pair. I then arrange to have a common connection from GND to GND in the same cable on other wires. Then in most situations, particularly for long distances, or high noise, or industrial environments I will shield the cable with a ground connected to the shield at just one end. I have had the most success with the shield being a chassis ground from one end and then having the common connection tie to logic ground on both ends. On some designs where EMI is an issue all the lines are filtered and conditioned with appropriate EMI handling components.

Another forum contributor here, Erik, always recommends that you...

"if you go to links <--- you will find a link to "Best Serial Comm Appnote site". Go there and find the "10 best ways to bulletproof 485" appnote. Nobody should even consider 485 without reading it. Erik"

Michael Karas