Information Technology Reference
In-Depth Information
DECconnect
Created by Digital Equipment Corporation, the DECconnect system supports four
communication technologies in an attempt to provide a global wiring system. The
four technologies are voice, video, low-speed data, and network. DEC's approach
uses five different types of cable that are run individually to the DEC faceplate at
the communications outlet. The five cable types are listed in Table 3.4, along with
the preferred connector.
The complete DECconnect system is rarely used because of its separable cable
technologies and its unusual Data Connector. If you must run a different cable type
to each workstation, you might as well keep each technology totally separate. The
only advantage the DEC system offers to a mixed media user is a common faceplate
mounting for the connectors.
The system uses a unique MMJ connector with an offset locking tab and key-
way, shown in Fig. 3.5. Unlike a normal keyed jack, with an extra keyway at the
side of the opening, the MMJ will not accept a non-MMJ plug at all. It is totally
incompatible with all other modular devices, including connectors, jacks, adapters,
fan-out cables, patch panels, and test equipment. Special MMJ devices must be used,
or an adapter cable must be fashioned. (The first M may stand for “maddening.”)
Because of the shortcomings of the DECconnect system, DECconnect wiring is pri-
marily used to connect legacy DEC asynchronous computer terminals using a vari-
ant of RS-232/RS-422 wiring. These async terminals use an MMJ connector for the
interface. The DECconnect system specifies a clever symmetrical wiring pattern for
the Data Connector that avoids the failure that would result from reversing the
order of pin wiring, a common error. Ironically, the most frequent source of this type
of error would come from the use of standard telephone cables that flip pins from
end to end. These are the very cables that will not plug into the MMJ outlets.
TABLE 3.4
DECconnect Cable Types
DECconnect Type
Cable
Connector
Voice
4-pair
RJ-45
Video
75-ohm coax
F-type
Low-speed data
2-pair
MMJ (modified RJ-45 type)
Network
50-ohm coax
BNC
62.5/125 micron fiber
ST or SMA
