Instruction manual

RM-702 TWO-CHANNEL REMOTE STATION
2-3
The RM-702 unit is shipped with the jumper plug over pins 2 and 3 to
maintain the power-channel isolation. Power-channel isolation ensures that
if one channel loses power, the other channel will continue to operate.
5. Replace the cover of the RM-702.
LINE TERMINATION
The fundamental concept of Clear-Com party-line intercom is that all stations
provide high-impedance current-sourced signals into a single common system
termination.
The receive or listen section of stations contain 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.
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.
The RM-702 does not provide termination on the intercom line. Clear-Com
main stations and power supplies 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.
The termination of an intercom line (or channel) is a 220 Ohm resistor in series
with a 4.7 K Ohm that is paralleled with a 10 uF capacitor.
STATION POWERING
Typical Clear-Com systems are powered by a main station or a power supply.
Clear-Com power supplies can be paralleled to increase the number of remote
stations that can be operated in a system.
CABLE CONSIDERATIONS
The Clear-Com intercom line is intended to run on a shielded twisted pair of
cable per channel of intercom. One conductor carries full duplex (“two-way”)
audio, the other conductor carries the DC power for remote stations. The shield
is used for ground return for audio and power. When choosing interconnect
cable, keep the following considerations in mind:
• DC resistance of the ground or common conductor affects crosstalk. For
runs longer than 500 ft. (152.5 m), do not use wire smaller than 20 gauge.
• The capacitance of the interconnect cable affects system frequency response
and sidetone stability. Total capacitance should not be greater than 0.25 uF.