Designing RS-485 Links and Networksthe cable’s ground wire. Isolating the link can reduce or eliminate these prob-
lems.
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’ com-
mon-mode limits or if you don’t want to worry about earth-ground noise, gal-
vanic 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 sup-
ply and an optoisolated data line. The data lines’ ground wire has no connec-
tion 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. Maxim’s MAX1480 is a complete, isolated RS-485 interface that con-
tains a tiny transformer that isolates the line’s power supply and has optical iso-
lators for the signal lines.
Partial isolation can be cheaper or more convenient than full isolation and in
some cases is sufficient.
In Figure 7-15B, the nodes and the line are isolated from earth ground, but the
RS-485 line isn’t isolated from the nodes it connects to. The power supplies can
be batteries or floating AC supplies. This arrangement is useful if the nodes’ cir-
cuits are relatively quiet but you want to isolate the nodes and line from varia-
tions in earth ground. A system where each node is battery powered has this
type of isolation.
In Figure 7-15C, the data lines are isolated, but the grounds aren’t isolated. This
partial isolation offers some protection to the nodes if a voltage surge hits the
line. Because the line shares its ground with the nodes, the grounds must be
within the common-mode limits of the components. If for some reason the line
can have only two wires, the line can use a common earth ground instead of a
ground wire as the return path.
Figure 7-15D shows another partially isolated line. The line shares its ground
with just one node, while all of the other nodes are isolated from the line.
Because the line has a single ground connection, the common-mode voltage is
small.