Exclusion circuit in key telephone system

Details Exclusion circuit in key telephone system Exclusion circuit in key telephone system

1974-05-30

1976-06-01

Claffy, Kathleen H.

179 18AD, H04M 168

Patent

active

039611442

ABSTRACT:
In a telephone system of the type in which two or more sets may be keyed into the same line, an exclusion circuit is provided which prevents all other sets from accessing a given line once the line is accessed by any one set. The exclusion circuit eliminates a relatively expensive thyristor or SCR circuit and substitutes a simple flip-flop circuit. The flip-flop circuit controls a relay which in turn controls connection of the line contacts to the speech circuit. When a key is depressed to select a line and when the telephone is off-hook, voltage is supplied to the flip-flop. The latter circuit is structured so that it assumes a first stable state whereby the relay is energized and the speech network is disconnected from the line. The relay also throws a switch which connects the control line associated with the selected line circuit to a trigger input of the flip-flop. If the line has not been previously accessed by another set, the voltage on the control line triggers the flip-flop to a second stable state whereby the relay is reset or deenergized and the speech network is reconnected to the line. However, if the line circuit is already engaged by another set, the voltage on the control line will be substantially zero. This will be insufficient to trigger the flip-flop to the second stable state. Consequently, the relay remains energized and the speech network remains disconnected from the line.

REFERENCES:
patent: 2785229 (1957-03-01), Faulkner
patent: 3299217 (1967-01-01), Burns
patent: 3529098 (1970-09-01), Chapman
patent: 3699265 (1972-10-01), Altenburger
patent: 3725601 (1973-04-01), Jetzt et al.
patent: 3860763 (1975-01-01), Sudoh et al.

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