US 3,961,144 AGranted

Exclusion circuit in key telephone system

AssigneeNippon Electric Co., Ltd.

InventorMasao Hirate

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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.

In a telephone set of the type having, an off-hook switch, and a key operated control for selecting a line circuit in common with other such telephone sets, said line circuit having a control wire corresponding thereto, the improvement being an exclusion circuit comprising:

a. a bistable circuit comprising a pair of transistors having their input and output terminals cross-coupled to provide feedback for holding said bistable circuit in a stable state, and a capacitor connected across the input of a first of said transistors, said bistable circuit being connected via said off-hook switch to a source of power whereby said capacitor causes said bistable circuit to always assume upon initial application of power thereto a first stable state wherein said second transistor conducts and said first transistor is cut off,
b. switch control means comprising a relay connected in the output circuit of said second transistor and responsive thereto for controlling first and second switches to cause said first and second switches to switch from their respective normal positions to their respective alternate positions when said bistable circuit is in said first stable state,
c. said first switch means in its normal position being connected between said control wire and a reference potential via said off-hook switch, and in its alternate position being connected between said control wire and a trigger input to said bistable circuit, and
d. said second switch means in its normal position being connected between said line circuit via said key operated control and a speech network for said set, and in its alternate position disconnecting said line circuit via said key operated control, whereby a non-reference potential on said control wire at the time of switching of said first switch causes said bistable circuit to assume its second stable state, and whereby a reference potential on said control wire at the time of switching of said first switch keeps said bistable circuit in its first stable state thereby to disconnect said speech network of the telephone set from said line circuit.

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Abstract

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