Designing RS-485 Links and NetworksWhen 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 electro-
magnetic coupling. An example of electromagnetic coupling is when a wire acts
as a receiving antenna for radio waves.1 "
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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. Any noise that magnetically couples into one wire is canceled in
the next twist by an equal, opposite noise in the other wire.
If the twisting isn’t perfectly uniform, the canceling will be less than 100%, but
the noise will be much reduced. The twisting is most effective in reducing mag-
netic coupling of low-frequency signals, such as 60-Hz power-line noise. For a
similar reason, twisted pairs also reduce the electromagnetic radiation emitted
by a pair.
The magnetic field emanating from a circuit is proportional to the area between
the conductors. Twisting the wires tightly reduces this area, and thus the size of
the magnetic field and the amount of noise that couples into it.
With cable containing two twisted pairs, you can use one pair for the RS-485
signals and the other for a ground connection. Connect both wires in the pair
to ground.!
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IBM and the EIA have published specifications for cable types. The specifica-
tions help in obtaining cables of known quality. Each cable type has a defined
characteristic impedance and maximum bit rate. The propagation velocity may
be specified as well. Manufacturers also publish specifications for cables
designed for use in data links.
Many RS-485 links use 120Ω cable, but cables with higher characteristic
impedance can also work. IBM Type 1 cable contains shielded twisted pairs, is
rated for use at up to 16 Mbps, and has a characteristic impedance of 150Ω.
Some cable intended for data, including the popular Category 5 and Category
6 network cables, use 100Ω, unshielded twisted pairs. These cables are fine for
1-way or full-duplex RS-485 or RS-422 lines. RS-422 allows just one driver
and often uses 100Ω cable with a single 100Ω termination. A 2-way, half-duplex
RS-485 line would need two 100Ω resistors in parallel, which brings the parallel