Key telephone system

Details Key telephone system Key telephone system

1979-08-16

1983-10-18

Brigance, Gerald L.

179 99LC, 179 17E, H04Q 520, H04M 906

Patent

active

044107676

ABSTRACT:
A key telephone system in which a key service unit is provided for selectively connecting a plurality of key telephone sets to at least a pair of CO lines through at least one control line provided in correspondence to the CO lines and a common line. The key service unit comprises a DC power source for producing two outputs respectively having certain positive and negative potential differences with respect to a reference potential, at least one first detector provided in correspondence to the CO lines and selectively connected to the control line for detecting the state in which one of the two output voltages is applied across the control line and the common line to flow a current of predetermined value, at least one second detector provided in correspondence to the CO lines and selectively connected to the control line for detecting the state in which the other of the two output voltages is applied across the control line and the common line to flow a current larger than a certain value, and at least one switching means provided in correspondence to the CO lines for disconnecting the first detector from the control line and for connecting the second detector to the control line when the first detector produces an output.

REFERENCES:
patent: 3210479 (1965-10-01), Engelschall
patent: 3450844 (1979-06-01), MacDowell
patent: 3649778 (1972-03-01), Suntop et al.
patent: 3865995 (1975-02-01), Kerman et al.
patent: 3906168 (1975-09-01), McEowcn
patent: 4022985 (1977-05-01), Tanoka

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