| Cable Testers and Harness Testers Made Easy!
What are twisted pairs?
Why do they put twisted pairs in cables?
It is an extremely effective way to send high-speed signals down a cable because:
Why do signals leak between wires?
In addition to energy flowing down a wire, it can flow between wires due to the electrostatic and magnetic effects that occur when voltages or currents in the wire change. In understanding electrostatic effects, consider that "insulation between two conductors" is the definition of a capacitor. More surface area (longer cables) means more inter-wire capacitance. Adjacent wires in the same cable behave as though capacitors are connecting them together, thus higher frequency signals can leak, or "cross talk", from wire to wire through this capacitance.
How does twisted pair cabling mininize the effects of capacitance?
A. Cross-talk (leaking signals)
Signals are sent down twisted-pair wires such that when one wire in the pair becomes positive the other wire becomes negative by the same amount. Any other wires close to this pair will be affected by cross-talk equal to the sum of the two signals, so if this sum is zero (or nearly zero) then the affects of cross-talk are eliminated.
Notice in period 1 in the illustration below the signal sent through the orange wire shows up on the black wire. In period 2 the signal sent through the white wire shows up on the black wire. In period 3 the signal sent through the orange wire and it's opposite polarity signal on the white wire cancel each other out, leaving no effect on the black wire.
B. Immunity to Electrical NoiseThe receiving electronics is intended to detect only the difference in polarity between the two wires in the twisted pair. Since electrical noise affects both wires of a twisted pair equally the receiving electronics gets a true signal by rejecting signals on the twisted pair that move toward the same polarity.
Question:If I am using a good, certified, twisted pair cable and I test for opens, shorts and even perform high voltage insulation tests, is this enough to assure the quality of my assemblies?
Continuity, resistance, and insulation tests can tell you if the connections are correct but they can NOT tell you anything about the noise canceling effects provided by twisted pairing. A common and serious error in a twisted pair cable is a "split pair".
This error occurs when one wire from each of two different pairs gets swapped on both ends of the cable. In our example above, you can see that the white wire of the blue & white twisted pair (pin 4) and the white wire of the black & white twisted pair (pin 6) have been swapped on both ends. The result is a cable that will pass a standard continuity test, but will have serious cross-talk problems. Split pair errors can easily happen in twisted pair cables where one wire of each pair is the same color. Some twisted pair cables have wires of all the same color, making this type of error even easier to produce and harder to find. Even when all the colors are different, it is not that hard to make a mis-wire on one end that is caught during a continuity test and then "fix it" at the other end so that the continuity test passes but a split pair error now exists.
What testers detect split pairs?
Some of our testers can detect split pairs by checking that specific wires behave electrically like they are twisted together thus canceling the capacitance coupling effects. Twisted pair testing is available on the following Cirris testers: Signature 500, Signature 1000H+, 1100H+, Touch 1 and easy-wire CH.
Tips to successfully detect Split Pairs
Even with a tester capable of testing for split pairs, there are other challenges when testing twisted pair cables. Test fixturing, twisted pair cable quality and cable length all play an important role in testing for split pairs. Keep in mind the following issues when testing your twisted pair assemblies.
4 Wire Fixturing Issues
If you will be testing using 4-wire fixturing adapter cables that are longer than a few inches:
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