Key telephone system
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United States of America Patent
Stats
-
Mar 3, 1981
Issued Date -
N/A
app pub date -
Dec 6, 1978
filing date -
Dec 26, 1975
priority date (Note) -
Expired
status (Latency Note)
Importance
Abstract
A key telephone system, in which a key service unit is provided for selectively connecting a plurality of key telephone sets to a CO line by two control lines and a talking line. The key service unit is comprised of a DC power source having two terminals of a positive output voltage and a negative output voltage, a first detector connected to one of two control lines for detecting a state where one of the two output voltages is applied across the two control lines to flow a current of predetermined value therethrough, a first switch for connecting the other output voltage across the control lines in response to the output of the first detector, a second detector connected to the other of the two control lines for detecting a state where the other of the output voltages is applied across the two control lines to flow a current larger than a certain value therethrough, and a second switch for connecting the talking line to the CO line in response to the output of the second detector. Additional circuitry is further provided to perform the intercomm. talking operation, the holding operation for the CO line, and the privacy operation and other functions.
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Family
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