Chapter 7
9
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Understanding how a twisted pair cancels noise requires knowing something
about how noise couples into a wire. Noise is any signal you don’t want in a cir-
cuit. The noise can enter a wire in many ways, including by conductive, com-
mon-impedance, magnetic, capacitive, or electromagnetic coupling.
Conductive and common-impedance coupling require direct contact between
the signal wire and the wire carrying the noise. Conductive coupling occurs
when a wire brings noise from another source, such as a noisy power-supply
line, into a circuit. Common-impedance coupling occurs when two circuits
share a wire, such as a common ground return. In RS-485, the differential sig-
nals 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 poten-
tials, 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 capaci-
tive, or electric, coupling. Current in a wire causes the wire to emanate a mag-
netic 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.