September, 2018


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Verifying Twisted Pair Relationships with a Continuity Tester


twists are wired correctly. Some twist errors can be detected with a simple continuity test, while others require a twist wire measurement. The greater the number of


A


twisted pairs in the cable, the greater the probability of a mis-twist error during characterization. A mis-twist- ed wire that passes a continuity test will experience faulty signal trans- mission during its intended opera- tion. To guarantee proper wiring, a twisted pair relationship test should be carried out. Although Ethernet cables are


used here as examples, multiconduc- tor continuity testers, such as CableEye® from CAMI Research, will measure twisted pairs — and even triplets — in any cable.


Why Bother? Twisted pair wires of the type


found in Ethernet cables provide a low-cost approximation of coaxial cable. The more twists per foot, the higher the category number (as in Cat 5 or Cat 6) of the cable, the more it appears like a coaxial cable and the more faithfully it carries the signal without distortion. Unlike coaxial cables, twisted


pair cables can be mis-twisted, lead- ing to incorrect signal transmission. Normally, for each signal line, a com- plementary signal line winds around it to create one pair. Typically, an Ethernet cable contains four pairs and should be wired +Signal1/ –Signal1, +Signal2/–Signal2, etc. If, through mis-wiring, we have two pairs in the cable wired +Signal1/ +Signal2 and –Signal1/ –Signal2, the cable will be faulty and unusable. Unfortunately, this type of error


cannot be determined by continuity measurements alone — the continu- ity would appear correct for either twist arrangement. To arrive at an erroneous twist relationship like this, an identical assembly error would need to be made at each end of the cable, certainly an unlikely sce- nario.


Nonetheless, manufacturers and


cable assembly shops must ensure that the pairs are twisted correctly. This can be accomplished by capaci- tance measurement. Capacit ance meas urements acquired during twist- ed pair check on reels and cables can be used to verify reel-to-reel and cable- to-cable consistency and quality. Ideally, the cable tester will log all con- nection measurements for record keeping and analysis.


Testing Twisted Pair Cables In one twisted pair, the two


wires involved come much closer to each other (two insulation thickness- es) than they do to adjacent pairs. As a result, the capacitance of a twisted pair would be expected to be higher than the capacitance between other wires in the cable. With this informa- tion, the relationship can be deter- mined.


Using CAMI’s CableEye conti-


nuity tester, the “pass” indicator on a twisted pair test shows that the wire resistances comply with the thresh- old and that the wires are paired cor- rectly. If a capacitance value is


twisted cable that passes a continuity test alone is not a fail-safe indicator that wire


By Christopher E. Strangio, President, CAMI Research, Inc.


entered into match data, as well as tolerance, then the values shown in the test data are within the preset tolerance. When connecting both ends of a


cable to the tester, the minimum length to acquire proper pairing for unshielded Ethernet cable is approxi- mately 6 ft (1.8m). For shielded Ethernet cable, the minimum length is a couple of feet longer, because the shield couples with the internal wires. Resistance measurements are


Capacitance relationship in a correctly wired twisted pair cable. Continued on page 67


Page 65


See at SMTAI, Booth 424


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