Key telephone system comprising only one control lead per outsid

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H04Q 520

Patent

active

041684036

ABSTRACT:
A key telephone system for a predetermined number of outside lines comprises only one control lead shared per outside line by cables connected between a key service unit and key telephone sets, respectively. Each telephone set has manually operable keys for supplying a higher voltage to a relevant control lead while an outside line is selected for an answer or a reanswer to a call present thereon and substituting a lower voltage for the higher voltage when it is desired to hold an answered call. Associated with each outside line and with the control lead therefor, the service unit comprises a unit comprising, in turn, a signal producing circuit for producing a first and a second intermittent signal while a call present on the associated line is not yet held and placed on hold, respectively, a supply circuit supplied with each intermittent signal for supplying a corresponding one of a flashing and a winking signal to the associated lead, a detector for detecting noncorrespondence of a signal supplied to the supply circuit and a signal supplied to the associated lead to produce a control signal, and a hold circuit responsive to the lower voltage supplied to the associated lead for holding the call. The control signal stops production of the first intermittent signal and takes the call off hold when produced while the call is not yet held and placed on hold, respectively. Outside line status indicating lamps are connected in each telephone set to the respective control leads.

REFERENCES:
patent: 3453396 (1969-07-01), Lacey et al.
patent: 3739104 (1973-06-01), O'Neill
patent: 3748403 (1973-07-01), Schartmann et al.
patent: 3860763 (1975-01-01), Sudoh et al.
patent: 4024353 (1977-05-01), Hijikata
patent: 4081625 (1978-03-01), Hamatani et al.

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