Information Technology Reference
In-Depth Information
Cross-connect wire pairs are introduced to the row in the same way, through the slot
just above each contact row, but punched down on the inner contact of each clip.
The cross-connect wire overlaps the station wire that was punched into the outer
contact, but the color code of each should still be visible. When punching, take care
that the punchdown tool is turned the correct way so that the excess wire, and not
the intended connection, gets trimmed off.
The tendency to fan the wires out, and the amount of untwist that exists where
the wires of a pair pass through the side slots, creates a problem for higher wiring
categories, such as Category 5e and 6. The contact clip also is a rather large piece
of metal that can contribute to impedance mismatch and crosstalk at higher fre-
quencies. For that reason, old-style standard 66M blocks are not suitable for use
above Category 3.
Most manufacturers have introduced low-crosstalk 66 blocks that are rated to
Category 5e, but that's about as far as the venerable 66 block will go. Even with
these special blocks, the normal wiring technique of one-wire-per-slot can still cause
an unacceptably high amount of crosstalk. If special 66 blocks are used for Category
5e, the wire pair should be inserted intact through the side slot
between
the two tar-
get contact rows, rather than split and run through two side slots.
The wires of each pair are then wrapped up or down (whichever is appropri-
ate) into the contact and terminated. You must be meticulous about rotating the
punchdown tool, or you will accidentally
cut
the wire you are attempting to termi-
nate, as well as jamming the remaining stub of wire that should have been cut off
into the jaws of the contact. However, this method maintains the amount of untwist
well under the 1/2-in maximum, and it can help you achieve the category of opera-
tion you seek. Unfortunately, this technique also makes reading of the color stripes
on the wires difficult after termination, so you should be very methodical when
wrapping the wires to be sure you do it right the first time. A subsequent termina-
tion on the inner contact will make the color stripes of the wires on the outer con-
tact of that row almost impossible to discern.
Many accessories, including adapters, bridging clips, and plug-on jacks, exist
for the 66M block. Virtually all of these are inappropriate for use in permanently
installed LAN wiring systems. The most stable connection system for LAN wiring
is to use cross-connect wires between punchdown blocks and patches, or to termi-
nate directly on the patch and skip the punchdown block altogether.
A version of the 66 block incorporates 8-pin modular jacks into the assembly.
The jacks are mounted in groups of four or so at the side of the mounting bracket.
This type of block may eliminate the need for a separate patch panel. However, it is
more difficult to see and access the jacks on the side of the block, and potential wire
management methods are poor. In addition, you must be certain that the assembly
