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An RS-485 line can extend for thousands of feet. The differential drivers cause
equal and opposite return currents that essentially cancel each other, and thus it
may seem that RS-485 has no need for a ground connection at all. With few
exceptions, however, the entire line should share a ground connection. Chapter
5 introduced the topics of power supplies, grounding, and isolation. This chapter
looks at grounding and isolation as it relates to RS-485 lines, which may
extend much farther than RS-232.
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The currents in RS-485 balanced lines are nearly but not exactly equal. The
currents differ slightly due to imbalances between the components and noise
that isn??™t exactly equal in both wires. The current in the ground wire may be
very small but isn??™t zero. If there is no ground connection, the energy in the
return current has to dissipate somehow, possibly as radiated energy that shows
up as EMI.
Designing RS-485 Links and Networks
135
In some RS-485 circuits, all of the nodes and the cable that connects them
share a common ground. In other circuits, the line is isolated from the nodes it
connects to. In either case, all of the drivers and receivers should share a ground
connection, which can have any of several sources. Most obviously, the RS-485
cable can include a wire that connects to signal ground at each node.
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