The EIA / TIA 568 standard for LAN cables only uses 4
of the wires in the eight wire cable. The other 4 wires in
the cable may have faults, but these will not affect the
operation of the LAN. The XR-5 tests all of the wires in
the LAN cable, and identifies the faults, even though
these wires may not be being used in the LAN system.
Not all LAN cables are wired "straight through".
Crossover cables used on LANs are purposely
"miswired", with their Receive and Transmit wires
"crossed over". The XR-5 will test this as a bad cable, but
it may work just fine as a Crossover cable.
7.6.2 The WireMaster XR-5 says my cable is good, but it
doesn't work on my LAN:
Many cable testers like the
XR-5 only perform continuity style tests (Open, Short,
Crossed, etc.). Ethernet LAN cables are constructed in a
special way. The 8 wires inside are grouped into 4 pairs
of 2 wires each. Not only must the 8 wires connect
from end to end of the cable, the pairs in the cable must
connect to specific pins of the RJ-45 connectors. An
installer can crimp the RJ-45 plugs onto the ends of a
cable, ignoring the pairing (as described in EIA / TIA
568), and the cable will test OK. But when the cable is
tried on the LAN, it doesn't work. This is because the
lack of proper pairing causes excessive crosstalk in the
cable, preventing the LAN from working. This type of
cable fault is sometimes called a "split pair" or "double
split pair". To detect this type of fault, a more
sophisticated tester capable of performing a NEXT test
(Near End Cross Talk) must be used. Both the Triplett
PairMaster and LAN TDR can perform NEXT tests.
Note:
Crosstalk increases with the length of the cable. A LAN system
will tolerate a certain amount of crosstalk. A short cable (10ft
or less) that is improperly paired may work just fine. However,
longer cables, paired in exactly the same way, may not work.
This explains why an installer can make short jumper cables
that work (although they are improperly paired), but when he
installs RJ-45's in exactly the same way on a longer cable, the
cable does not work.
7.6.3 The WireMaster XR-5 says says my phone cable is
bad, but my phone works OK.
Most single line telephones only use 2 wires in a modular
cable. The modular cable, which terminates in RJ-11
connectors, could have as many as 6 wires in it. The
unused wires may have faults, which the XR-5 will
identify, but these faults may have no effect on the
working 2 wire telephone circuit.
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