Information Technology Reference
In-Depth Information
Cable Wire Map Testers
A very good device to test an installed cable link is the wire map tester. These little
testers give an indication of cable continuity and proper pin-out. They are generally
used for installed cables or assembled cords.
One type has a main tester and a remote unit. The two units communicate
across the cable to perform a wire map test and, of course, a basic continuity test.
Most of these testers can easily find all of the standard connection errors and prob-
lems, such as opens, shorts, reversed wires, and crossed connections. (These cable
faults are all explained in the next section.) Simple miswirings are probably the most
common cable installation problem, and the easiest to fix.
The simple wire map tester performs a simple DC continuity test and checks
for undesired connections. It does not check for proper pairing, as with a split-pair
fault. A
split pair
is a condition where one of the two wires in one pair is acciden-
tally exchanged for one of the wires in another pair. For example, the white-green
and the white-brown might be exchanged. This actually means that two pairs are
mixed, and the balance and self-shielding properties of the paired wires is lost. In
this type of miswire, DC connectivity tests fine and it would pass a simple wire map
test, but the cable would fail if tested for AC signal balance by a cable scanner. In
many instances, a cable with a split-pair fault will cause a LAN link to fail, which
is why it is important to test for this with the more sophisticated scanners.
A specialized version of the wire map tester is used to test assembled cables.
This cable continuity tester has two modular jacks where each end of the cable is
plugged in. When a button on the tester is pressed, LEDs light to indicate continu-
ity of each wire in the cable, one by one.
A Pass is indicated by observing that the LEDs each light up in order through
the number of pins in the cable.
This type of tester may be used for 6-pin modular connectors in addition to the
8-pin connectors used for LAN wiring. Frequently, the same test jack is used for
both sizes of connectors. This creates a potential false-failure problem for pins 1 and
8 of the 8-pin plug. Although the 6-wire plug will fit into the jack of the tester, the
plastic sides of the plug may permanently bend the jack's first and eighth pins, mak-
ing them no longer contact Pins 1 and 8 of an 8-position plug and giving a false indi-
cation of failure. The solution is to never use 6-pin plugs in a tester you intend to
use for 8-pin plugs! If you believe a false failure has occurred, you can usually form
a small stiff wire (such as a paper clip) to bend the jack wires back into position.
Then, test the cable again.
