twisted pair

Revision as of 01:22, 7 March 2011 by Root (Talk | contribs) (shielded cable)

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to: navigation, search

Twisted Pair cables

Why are long-run data cables twisted?

The reason data cables are twisted is to block noise. But how? Noise usually comes from nearby circuits that generate magnetic fields (EMF). As a changing magnetic field passes through a conductor (your cable) a voltage will be induced. The direction of that voltage (+ or -) depends on the orientation of the conductor to the magnetic field. So a field moving through a straight pair of wires will generate a DC pulse that may be mistaken for a signal. If the orientation of the conductor in the field changes then the induced voltage will reverse. A field moving through a twisted pair of wires will generate both +DC pulses and -DC pulses that will tend to cancel each other out.

Data cables that send multiple signals on multiple pairs of wire (Ethernet) will actually use a different rate of twist for each pair. This is so that one pair will not induce a signal on another pair. If the pairs had the same rate of twist and twists in one pair happened to line up with the twists in a nearby pair then a signal could be induced. Using a different rate of twist for each pair ensures that none of the bundles of pairs can sync up.

shielded cable

Shielded cables add an additional layer of protection. In shielded cables all the pairs are wrapped in a conductive coating. This usually looks like a metal foil wrapped around all the pairs of wire. The shield is grounded on both side of the cable connection. If the cable is routed near a strong magnetic field source then most of the energy will be absorbed by the foil wrapper and shunted to ground. This is like having a Faraday Cage around the signal cables.

Cat5 and Cat5e cables are not shielded. Cat6 cables are shielded. Using an inductive cable tracer (fox and hound or cable toner) on Cat6 shielded cable is much more difficult than Cat5e unshielded cable because any shield that keeps out EMI also keep in EMI. You can still pick up some signal if you hit the cable at the right angle, so all is not lost. It's still worth a shot. The process can be frustrating because you might detect some signal near a bunch of cables, but as you test each cable individually you will get nothing.