'# -$
TIA-485-A doesn??™t recommend a specific cable type, but twisted-pair cable is
inexpensive and performs well in RS-485 circuits. A twisted pair consists of two
insulated conductors that spiral around each other in a double helix (Figure
7-12). The pairs typically have one or two twists per inch. Catalogs may list this
type of cable as network cable or alarm wire. In a twisted pair, much of the
noise that couples into the wires cancels out.
Another option is triaxial cable, which is like coaxial cable except with two conductors
rather than one surrounded by a shield. Triaxial cable is expensive, however,
compared to twisted pair.
Thus, here is guideline #5 for RS-485:
Use twisted-pair cable.
Chapter 7
132
9 - .*
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 circuit.
The noise can enter a wire in many ways, including by conductive, common-
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.
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