In RS-485, the differential signals
cancel much of these types of noise.
The other types of coupling result from interactions between the electric and
magnetic fields that emanate from the wires themselves or that couple into the
wires from outside sources.
Capacitive and inductive coupling are a source of crosstalk, where voltages in
one wire couple into another. When two wires carry charges at different potentials,
an electric field exists between the wires. The strength of the field varies
with the distance between the wires. This electric field is the source of capacitive,
or electric, coupling. Current in a wire causes the wire to emanate a magnetic
field. Inductive, or magnetic, coupling occurs when magnetic fields of two
wires overlap, causing the energy in one wire??™s field to induce a current in the
other wire.
Figure 7-12: Many RS-485 systems use twisted pair cable for the data lines.
Designing RS-485 Links and Networks
133
When wires are greater than 1/6 wavelength apart (1/4 mile at 10 MHz), the
effects of the capacitive and inductive fields are considered together as electromagnetic
coupling. An example of electromagnetic coupling is when a wire acts
as a receiving antenna for radio waves.
1 "
4
Twisted pairs are effective at canceling low-frequency noise caused by magnetic
coupling. In a twisted pair, each twist of the cable swaps the physical positions
of the wires.
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