Clear-com RM-702 User Manual download pdf (Page 15)

RM-702 TWO-CHANNEL REMOTE

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.

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Clear-com RM-702 User Manual Page 15