Clear-Com CS-702 - Manual (Page 17)

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INSTALLATION
INSTALLATION OVERVIEW
The CS-702 is a combination of a very versatile intercom station and a system
power supply. Installations can vary depending on what features are used. The
fundamental concept of the Clear-Com party-line intercom is that all stations
provide high impedance current sourced signals into a single common system
termination.
The high DC resistance allows a call voltage to be placed on the line without
drawing too much current. The DC source for this call signal is also a current
source thereby providing a high impedance and not affecting the audio signal.
The receive or “listen” section of stations contains a hybrid null circuit that
attempts to reject (null) any talk signal being sent by that station on that
channel. The hybrid null circuit depends on a known impedance on the
intercom line to accomplish this. Variations in impedance on the line upset the
null.
Clear-Com main and some remote stations provide switch-selectable
termination networks on all intercom output lines. It is up to the user to
determine where the termination will be provided. An unterminated line will
cause excessive levels, possible oscillation of line drivers, and severe
unbalance of hybrid null networks. A double- or multiple-terminated line will
cause low levels and severe unbalance of hybrid null circuits.
Caution: All Clear-Com intercom lines must be terminated. Care must be
taken NOT to fail to terminate or to “double-terminate” a line. All
unused intercom inputs must be terminated to keep the line drive
circuits stable.
CLEAR-COM STATION TYPES
Clear-Com party-line intercom stations all fall into one of four distinct
categories that relate to system powering and termination. The four types are
as follows:
A. MAIN STATION:
• Supplies system power for external stations.
• Provides switch-selectable terminations.
B. MASTER STATION:
• Contains a power supply just for itself.
• Provides switch selectable line terminations.
C. REMOTE STATION: