Key telephone system

Abstract

A key telephone system is disclosed that includes a clock for generating both flash and wink lamp signals and digital timing signals. The system also includes a plurality of line control circuits, each of which is associated with an individual telephone line and a plurality of key telephone sets, each of which is associated with one or more telephone lines. Thus the key telephone systems includes a plurality of subsystems, each comprising a telephone line and the line circuit and elements of the key telephone sets associated with that telephone line. Each line circuit includes an electronic ringing detector and hold bridge circuit and a magnetic line current sensor. In addition, each line circuit includes several counters, and each counter is activatable to a plurality of states, one of which corresponds to a particular operational mode of the subsystem. A first group of logic gates responds to the occurrence of conditions associated with a particular operational mode (i.e., presence of loop current and absence of A lead current associated with hold mode) by applying the digital timing signals of the clock to one of the counters and/or enabling the counter to be advanced in response to such signals toward the state corresponding to that particular mode. A second group of logic gates respond to the counter reaching the corresponding state, indicating the continuous existence of the associated conditions for a predetermined period of time, by placing the subsystem in the particular mode (i.e., applying the hold bridge to the line and wink lamp signals to the sets).

What is claimed is: .[.1. A telephone line control circuit for use in a telephone system comprising a telephone line, the line control circuit, and at least one telephone set associated with the telephone line, the system having a plurality of operational modes, the line circuit comprising: 
     
        lamp..]. 8. A control circuit .[.as in claim 7 wherein.]. .Iadd.for use in a telephone system comprising a telephone line, the control circuit, and at least one telephone set including a ringer and a signal lamp, the system having operational modes of idle, ringing, in-use, and hold, the ringing mode being characterized by flashing of the signal lamp and periodic operation of the ringer responsive to the periodic application of ringing voltage to the telephone line, the control circuit comprising: counter means activatable to a plurality of states, one of which corresponds to the idle mode;   means responsive to the cessation of ringing voltage for a continuous period of time exceeding the time period of the silent intervals between each application of ringing voltage for advancing the counter means to the state corresponding to the idle mode; and   idle mode logic means responsive to the counter means advancing to said corresponding state for terminating the flashing of the signal lamps, .Iaddend.the idle mode logic means .[.comprises.]. .Iadd.comprising .Iaddend.a first logic gate that has a unique output only in response to the counter means advancing to said corresponding state, the unique output serving to disable a second logic gate having as one of its inputs a clock   
     
     
        signal that provides the timing of the flashing of the signal lamp. 9. A control circuit .[.as in claim 1 wherein the.]. .Iadd.for use in a telephone .Iaddend.system .[.has.]. .Iadd.having .Iaddend.operational modes of idle, ringing, in-use, and hold .[.and the set includes.]..Iadd., the telephone system comprising a telephone line, the control circuit, and at least one telephone set including .Iaddend.a signal lead, a speech network, and switch hook contacts for connecting the speech network to the associated telephone line, current being present on both the signal lead and the associated telephone line when the system is in the in-use mode, and the signal lead being interrupted when the system is to be placed in the hold mode, .[.and wherein the condition responsive means includes.]. .Iadd.the control circuit comprising: counter means activatable to a plurality of states, one of which corresponds to the hold mode; .Iaddend.   means responsive to the presence of line current on the associated telephone line and the cessation of signal lead current when the system is in the in-use mode for .[.advancing.]. .Iadd.activating .Iaddend.the counter means.[...]. .Iadd.to advance toward the corresponding state; and   means responsive to the advancement of the counter means to the corresponding state for placing the system in the hold mode. .Iaddend.   
     
     
            A control circuit as in claim 9 wherein the counter means is advanced to a state corresponding to the hold mode responsive to the presence of line current and absence of signal lead current for a continuous period of time exceeding the maximum time period that line current continues after the speech network is disconnected from the associated telephone line by the opening of the switch hook contacts while the system is in the in-use 
     
     
        mode. 11. A control circuit as in claim 10 wherein the counter responsive means includes hold mode logic means responsive to the counter means advancing to said corresponding state for applying a hold bridge to the 
     
     
        associated telephone line that shunts the speech network of the set. 12. A control circuit as in claim 11 wherein the telephone set further includes a signal lamp and the hold mode logic means initiates winking of the signal lamp responsive to the counter means advancing to said selected 
     
     
        state.  13. A control circuit as in claim 12 wherein the hold mode logic means comprises a first logic gate having a unique output responsive to the counter means advancing to said corresponding state, the unique output serving to apply the hold bridge and enable a second logic gate having as one of its inputs a clock signal that provides the timing of the winking of the signal lamp.  .[.14. A control circuit as in claim 1 wherein the system has operational modes of idle, ringing, in-use, and hold and wherein the condition responsive means includes means responsive to the cessation of line current on the associated telephone line while the system is in the hold mode for advancing the counter means..]. .[.15. A control circuit as in claim 14 wherein the line current cessation means comprises means for detecting line current on the associated telephone line, the line current detecting means providing a first logic signal when line current is present and a second logic signal when line current is absent, and logic means that activates the counter means responsive to the absence of the first logic signal and the presence of the second logic signal..]. .[.16. A control circuit as in claim 15 wherein the counter means is set at zero count responsive to the first logic signal of the line current detecting means and the counter activating logic means is permitted to advance the counter means responsive to the absence of the first logic signal and the presence of the second logic signal..]. .[.17. A control circuit as in claim 1 wherein the telephone set includes a speech network and a signal lamp and the system has operational modes of idle, ringing, in-use, and hold, the hold mode being characterized by winking of the signal lamp and the presence of a bridging path across the associated telephone line that shunts the speech network of the telephone set, and wherein the counter means is advanced to a state corresponding to the idle mode responsive to the termination of line current on the associated telephone line for a predetermined period of time while the set is in the hold mode..]. .[.18. A control circuit as in claim 17 wherein the counter responsive means includes idle mode logic means responsive to the counter means advancing to said corresponding state for terminating the winking of the signal lamp and removing the bridging path from across 
     
     
        the associated telephone line..]. 19. A line circuit for a telephone system which includes a telephone line formed by a pair of line conductors interconnecting a central office with a telephone set, the telephone set having means for completing a path between the line conductors to permit line current to flow, the circuit comprising: a magnetic structure having three magnetic flux paths connected in parallel with one another;   a control winding disposed about the middle magnetic flux path of the magnetic structure, the control winding being dimensioned so that the flux generated thereby when current the magnitude of the line current flows therethrough saturates the magnetic structure;   means for connecting the control winding in series with one of the line conductors whereby line current flows through the control winding when the line current is present on the line conductors;   a sense winding disposed about each of the outer flux paths of the magnetic structure, the sense windings both having approximately the same number of turns and being poled oppositely to and connected in series with one another;   means for connecting the input of the sense windings to a signal source; and   means connected to the output of the sense winding for responding to the   
     
     
        signal of the signal source. 20. In a line circuit for a telephone system which includes a telephone line formed by a pair of line conductors interconnecting a central office with a telephone set, a circuit for applying a bridging path across the line conductors in parallel with the telephone set when the telephone line is placed on hold, the circuit comprising: a transformer having a first and second winding magnetically coupled to one another;   an oscillator including the first winding of the transformer, the oscillator when turned on generating an ac voltage that is coupled to the second winding of the transformer;   means for rectifying the voltage induced in the second winding of the transformer;   a transistor having its base connected to the output of the rectifying means and its emitter-collector path connected in series with an impedance, the transistor being turned on responsive to dc voltage provided by the rectifying means; and   means for connecting the emitter-collector impedance path across the line conductors of the telephone line whereby when the oscillator is turned on, the transistor is turned on and the impedance is connected across the line conductors of the telephone line.