Information Technology Reference
In-Depth Information
ring voltage can approach 90 V AC (both at low current). However, modern data
circuits are fairly immune to these voltages and the fact that the keyed jacks are eas-
ily defeated has led to their disuse.
An alternate style of keyed jack is the MMJ style that was used by Digital
Equipment Corp., described in Chapter 3. This jack has primarily been used for the
connection of RS-232 and RS-422 low-speed devices and is rarely found in other
uses. It is not appropriate for standards-compliant LAN wiring.
Fiber-Optic Outlet Jacks
Fiber-optic technology has become so important that we have devoted an entire
chapter to it, Chapter 11. Outlet jacks for fiber-optic connections have several
unique requirements. The fiber-optic outlet must allow the station cable fibers to be
terminated and connected into a jack plate. In addition, the design of the jack plate
must allow the easy attachment of an optical jumper to the workstation equipment.
A special passive coupling device, called an adapter, is used to connect the two fiber-
optic cables at the outlet jack.
Unlike the termination of metallic conductors, the traditional termination of an
optical fiber can be a delicate and time-consuming process, although the time may
be reduced by using more expensive “quick-termination” connectors. The fiber
must be stripped of its protective coverings, including the binder. It must then be
precisely cleaved before being inserted into the mechanical coupling that is loosely
called the optical connector . Some fiber connectors require the use of an epoxy or
glue to secure the fiber and may use a crimp or clamp-in arrangement in addition
(or in lieu of) the glue. Finally, the connectorized fiber end must be carefully pol-
ished to a smooth surface so that light entering or exiting the fiber is not bent off
axis (the polishing step is not required for a quick-termination connector).
In order for light to couple properly between connectors, the internal fibers
must be carefully aligned and placed virtually in contact by the coupling device. In
fiber optics, this coupling device is called an adapter , rather than a connector,
because it does not actually terminate any fibers. It mounts in the fiber-optic outlet
plate much as a coax bulkhead barrel adapter does in coax systems. The fiber
adapter provides a keyed location where the terminated station fiber connects on the
back side of the outlet plate. The plate is then mounted in the fiber-optic outlet box.
The fiber-optic user cord is connected to the section of the adapter that is on
the front side of the outlet plate. Because of the nature of fiber optic transmission,
two fibers and connectors are generally required for each LAN connection, one for
transmit and one for receive. Keying these two connections, or differentiating them
by color, is used to orient the transmit and receive fibers.
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