Lesson 2/Learning Event 2
a. Main
Distributing Frames.
An MDF always mounts terminals for
outside cable
pairs and protector groups on the vertical side, and mounts
terminals for
inside cable pairs on the horizontal side. Access to patch
bays or other
equipment is through the IDFs and the inside cable.
b. Intermediate Distributing Frame.
An IDF is usually a grouping of
terminal boards located near or within equipment items and connected to the
MDF by inside multipair cable. Interconnections between equipment and the
inside multipair cable are made by jumpers at the IDFs. When an IDF serves
trunk lines it is called a trunk intermediate distributing frame (TIDF) and
is usually mounted on the vertical side of the main-frame assembly.
Protector groups are not essential on these trunks because all wiring is in-
hours; protector groups are essential and are always used on external lines.
c. Combined Distributing Frame. A CDF, is a combination of an MDF and
an IDF, having the functions of both.
Thus, in smaller centers, the CDF
eliminates the need for a separate MDF and IDF. Protector groups are used
on the CDF only when the cables are externally routed.
Learning Event 2: MAIN-FRAME CONSTRUCTION
All main-frames use similar construction features, notified for the
application.
As shown in figure 2-3, open-frame construction is used.
There are normally at least two frame sections to provide mechanical
rigidity. A number of horizontal channels and insulated rings are fastened
to each vertical angle iron. A horizontal fanning strip is fastened to the
front of a horizontal channel, and the terminal boards are fastened to this
fanning strip. Some terminal boards are furnished with an integral fanning
strip; such boards are mounted on the frame assembly as needed. The back,
or vertical, side of the frame assembly supports either a framing strip and
terminal board for IDF use, or a fanning strip and protector assembly for
MDF use.
Terminal boards are also known as terminal strips and terminal
blocks.
a. Terminal Blocks.
Most terminal blocks used on the MDF or IDF are
similar.
Each terminal board consists of terminals mounted in a wood or
plastic block as shown in figure 2-4.
The terminals appear in different
lengths or tiers, arranged in a "Christmas tree" fashion.
The purpose of
using different lengths of terminals is to minimize soldering errors. Since
the terminals are unmarked, pair numbers are stenciled on the block.
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