| 摘要 |
<p>A means for checking the integrity of a cable pair dedicated to a public address system and having a plurality of loudspeakers bridged thereacross is disclosed. The integrity checking circuit comprises a line supervisory sensor which will provide an indication if there is an open in the dedicated cable pair and/or if there is a shunt below a predetermined minimum value. The loop is monitored through a D.C. circuit and an end of line impedance is provided for circuit continuity. A device, such as a capacitor, is used in series with each loudspeaker bridged across the line for blocking D.C. current. The line supervisory sensor has three states; one of which indicates that there is continuity in the dedicated line and that there is no shunt across the line which is below an acceptable minimum; another of which indicates that the dedicated line has an open; and the last of which indicates that there is a shunt across the line which has a value below an acceptable minimum. The line supervisory sensor may comprise a two-step relay, a plurality of relays, an electronic circuit, or other techniques which may be convenient and expedient.</p> |
