Even here, though, motors, switches, and other electrically
noisy equipment can induce ground noise. If the nodes are in different buildings
or using different power systems, the earth ground is likely to have higher
impedance, and ground currents from other sources might find their way into
Figure 7-14: A 100?„¦ resistor between each node and the ground wire limits the current
in the ground wire when two nodes??™ grounds vary.
Designing RS-485 Links and Networks
139
the cable??™s ground wire. Isolating the link can reduce or eliminate these problems.
TIA-485-A says that RS-485 lines must have a common ground. If you can??™t
guarantee that the grounds of the nodes will be within the components??™ common-
mode limits or if you don??™t want to worry about earth-ground noise, galvanic
isolation is a solution.
Figure 7-15 shows four ways to isolate an RS-485 line.
Figure 7-15A has full isolation. Each node??™s interface has an isolated power supply
and an optoisolated data line. The data lines??™ ground wire has no connection
to any node??™s signal ground or earth ground. This arrangement protects the
data signals from noisy earth grounds and from variations in ground voltage at
different nodes. The isolation also protects the nodes from noise picked up by
the line??™s ground wire. The nodes themselves can share an earth ground or not.
To isolate a line, you can use discrete components or a chip designed for this
purpose.
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