A TELEPHONE LINE SWITCH

Abstract: A telephone subset line switch (Fig. 1), comprising input terminal means (3, 4) for coupling to the line terminals of a telephone subset circuit, and output terminal means (1, 2) for connection to the transmission circuit of the subset circuit, a first semiconductor switch means (TR 1) having a controllable conductive path serially connected between a first terminal (3) of the input terminal means and a first output (1) of the output terminal means, a second semiconductor device (TR 3) having a controllable conductive path connected between the control element of the first semiconductor switch (TR 1) and a second terminal (4) of the input terminal means, the second input and second output terminals being common, and whose control element forms a first control input for coupling to control means associated with the transmission circuit for controllably switching the telephone subset's loop current circuit.

Description: A TELEPHONE LINE SWITCH Technical Field This invention relates to a telephone subset line switch sometimes known as a 'Line Interface Circui ' , which Incorporates an electronic hook- switch usually in integrated circuit form. Background Art Line switches of the aforementioned type are known and described in detail in, for example, the specification of Australian Patent Application No. 21.775/83. This specification describes a solid state switch for use as a hook-switch, comprising a field effect transistor whose gate/source potential is controlled by a bipolar transistor. Such a solid state hook- switch has an advantage over conventional mechanical hook-switches when "on hook dialling" is required, in that, expensive mechanically interlocked switches can be avoided. The specification of Australian Patent Application No. 57,138/86 de¬ scribes in detail a circuit arrangement in which the outpulsing transistor switch of a telephone subsets' dialling circuit is utilized for generating a timed loop break recall signal. This known circuit arrangement has on the one hand, the advantage of obtaining a significant reduction of compo- nents by the characteristic utilization of the outpulsing transistor switch. But on the other hand, a conventional mechanical hook-switch is incorporated in the subsets' circuit. Suπmary of the Invention This specification discloses a solid state line switch for a telephone subset which may be controlled by one or more of the subset functions. In one embodiment there is provided a telephone subset line switch, comprising input terminal means for coupling to the line terminals of a telephone subset circuit, and output terminal means for connection to the transmission circuit of said telephone subset circuit, a first semiconduc- tor switch means having a controllable conductive path thereof serially connected between a first terminal of the said input terminal means and a first output of said output terminal means, a second semiconductor device having a controllable conductive path connected between the control element of the first semiconductor switch means and a second terminal of the Input terminal means, said second input and second output terminals being cαtimon, and whose control element forms a first control input for coupling to con¬ trol means associated with said transmission circuit for controllably switching said telephone subsets loop current circuit. In another embodiment there is provided a telephone subset circuit ar- rangement incorporating the aforementioned line switch embodiment. Brief Description of Drawings In order that the invention may be readily carried into effect, it will now be. escribed in detail by way of example with reference to the ac¬ companying drawings, in which: Pig. 1 is a circuit diagram of one form of a line switch of the pres¬ ent invention; Pig. 2 shows a circuit diagram of another form of a line switch of the present invention; Pig. 3 shows a circuit diagram of a telephone subset incorporating a line switch of the present invention. Pig. - shows a polarity guard incorporating the line switch. Best Mode of Carrying out the Invention Referring to Pig. 1, there is shown a line switch circuit comprising output terminals 1 and 2 for connection to a telephone subset's line termi¬ nals via a polarity guard device (not shown) which ensures correct polarity irrespective of line polarity. The circuit further comprises input termi¬ nals 3 and connected to the telephone subset's transmission circuit TX. Between terminals 1 and 3 is connected the collector/emitter path of a PNP transistor TR1 which is the main switching transistor. A resistor Rl is serially connected between terminal 1 and the collector element. The base element of transistor TR1 is coupled via resistor R2 to the collector ele ment of an NPN transistor TR3 which is the control transistor. The emitter of transistor TR3 is connected to terminal 2 and the base element is cou¬ pled to control means (not shown) associated with transmission circuit TX. The base element of TR3 is also connected to the collector element of an NPN transistor TR2 which is the protection transistor. The base element of transistor TR2 is coupled to terminal 1 via resistor R3 and zener diode Zl. The conductive path of transistor TR1 is in series with the loop circuit of telephone transmission circuit TX and may be controllably switched by con- trol transistor TR3 with signals applied to the latter transistor base ele¬ ment by the control means. Over current protection for elements of the transmission circuit TX is provided by resistor Rl, typically 4.7 ohms, and diodes Dl and D2. Diode D2 compensates the emitter/base voltage drop of transistor TR1 so that the voltage drop across resistor Rl is equal to the voltage drop across diode Dl. Thus, when the current exceeds a value de¬ termined by the values of resistor Rl and diode Dl, the base current of transistor TR1 is reduced, causing the conductance of the emitter/collector junction to decrease thereby limiting the loop current. Over voltage pro¬ tection for the main switching transistor TR1 is provided by protection transistor TR2. Should the voltage across terminals 1 and 2 rise above a predetermined level typically 150V, zener diode Zl begins to conduct and switches on transistor TR2 which in turn switches off control transistor TR3 and consequently transistor TR1. Transistor TR1 is thereby prevented from operating outside its safe operating area (SOA) . Referring to Pig. 2, the line switch comprises a main switching tran¬ sistor arrangement consisting of a complementary pair of transistors TR1 and TR4 whose switching is controlled by a control transistor TR3. The line switch includes overcurrent protection elements identical to those shown in Pig. 1, but omits the over voltage protection elements. Such a line switch may be used where overvoltage protection is provided by a clamping device across terminals LI and L2, or where a suitable high volt ¬ age transistor is used for transistor TR1, such as, for example a high- voltage DMOS device, or a high voltage bipolar device. Referring now to Pig. 3, the telephone subset circuit comprises line terminals LI and L2 connected across the AC points of a polarity guard de¬ vice in the form of a diode bridge PG comprising diodes D3, D4, D5 and D6; It will be understood, however, that transistors, particularly field effect transistors, could substitute some or all of the diodes in the bridge, in which case it would be necessary to incorporate a polarity sensing control means. A tone ringer (not shown) is also connected across the line termi¬ nals. Across the DC points of bridge PG is a telephone transmission cir¬ cuit TX having a load impedance resistor R6. Also connected across the DC points of bridge PG and in parallel with the transmission circuit TX is a conventional ten number repetory tone/pulse dialler chip, such as for exam¬ ple, a SHARP LR4806 designated RD, although a microprocessor could be uti¬ lized. Dialler RD includes a dial pulse output A, a mute signal output B, a power terminal C, common voltage rail terminal D, dual tone multifre- quency (DTMP) output E, an earth recall/ground star output P, a DC charac¬ teristic adjustment signal or "kick-start" output G, a plurality of inputs 1-28 from the key-pad (not shown) associated with the subset, and inputs X and Y from the associated hook-switch HS. Also connected across the DC points of bridge PG and in parallel with repetory dialler RD is a storage capacitor Cl which provides power for repetory dialler RD during decadic dialling. A current bleed path is provided by resistor R2 and diode D7 connected between the DC positive point of the polarity guard PG and output C of dialler RD to provide power for the dialler when the subset is in the- on-hook mode. Serially connected between the DC positive point of polarity guard PG and the transmission circuit TX is the main switching path of the line switch circuit of Fig. 2, comprising a main switch in the form of a comple- mentary configuration of transistors TR1 and TR2, a control transistor TK3 whose base element is connected to output A of dialler RD and the over- current protection elements resistor Rl and diodes Dl and D2. Connected across the loop circuit is the main switching path of a shunt circuit for "kick start", comprising a main switch in the form of a complementary configuration of transistors TR7 and TR8, and a control tran¬ sistor TR9 whose control element is connected to output G of dialler RD. Viewed from the line terminals Ll and L2, the shunt circuit is connected behind the line switch and before the transmission circuit TX. It will be understood, however, that the shunt circuit can be connected across the loop between the polarity guard PG and the line switch. Connected to earth terminal E is the main switching path of the earth recall/ground start switching circuit, comprising a main switch in the form of a complementary configuration of transistors TR4 and TR5, and a control transistor TR6 whose base element is connected to output P of dialler RD. In operation, when brought into the off-hook mode, the operation of hook switch contacts HS is sensed by dialler RD whereupon a mute signal is applied to output B which mutes the transmission circuit TX; a signal Is applied to output G which turns on the control transistor TR9 of the shunt circuit which in turn switches on transistors TR8 and TR7; and a signal is applied to output A which turns on the control transistor TR3 of the line switch which In turn switches on transistors TR2 and TR1. Upon operation of transistor TR1 a current loop is provided via line terminal Ll, positive point of polarity guard PG, resistor Rl, collector/emitter junction of transistor TR1, collector/emitter junction of the shunt circuit transistor TR7, negative point of polarity guard PG to L2. The shunt circuit which effectively shunts the transmission circuit TX reduces the voltage drop of the loop circuit thereby providing a higher loop current to ensure exchange line relay operation on long line conditions. After a predetermined pe- riod, typically 400 m/s, the signal applied to output G of dialler RD is changed and control transistor TR9 of the shunt circuit is switched off which in turn switches off transistor TR8 and transistor TR7 whereupon the shunt circuit is removed and transmission circuit TX substitutes the shunt circuit in the current loop. The signal applied to output B of dialler RD is changed and the muting is removed from the transmission circuit TX. As ¬ suming that the dialler RD has been set in the pulse dialling mode, upon dialling, dial signals from the key pad are converted by dialler RD into corresponding trains of pulses which are applied via output A to the base of control transistor TR3 of the line switch, thereby pulsing the main switch transistors TR2 and TRl. Transistor TRl makes and breaks the cur¬ rent loop to operate switching equipment at the exchange. To provide more reliable dial pulsing it can be arranged for the shunt circuit to be switched on by the dialler RD during dialling. The transmission circuit TX is muted during dialling by a signal on output B of dialler RD. If the ex¬ change system to which the subset is connected provides a timed loop break (TLB) recall system, after the call has been established and the user de¬ sires to recall the exchange euipment, a designated key on the key pad is selectively operated. Its operation is sensed by dialler RD and the signal applied to output A of dailler RD is changed for a predetermined period, typically 90 m/s, which causes the control transistor TR3 of the line switch to switch off for 90 m/s which in turn switches off transistors TR2 and TRl. Transistor TRl breaks the loop circuit for 90m/s which Is sensed by the exchange equipment, whereupon the exchange equipment is prepared for receiving further dialling signals. If the dialler RD has been set in the DTMF mode, signals from the keypad cause dialler RD to generate a coded VF signal representing the dig¬ its keyed. The coded VF signal is applied to output E of dialler RD which is coupled to the line terminals Ll and L2 by a current source means (not shown) In the transmission circuit TX. 7 If the recall system associated with the exchange line equipment is <___. * earth recall system, upon actuation of a designated recall key its opera¬ tion is sensed by dialler RD which thereupon applies a signal to output P for at least 100 m/s. This signal switches on the control transistor TR6 of the earth recall/ground stand switching circuit, which in turn switches on transistors TR5 and TR4. Earth extended from the positive pole of the exchange battery to terminal E is thereby connected via the collector/emitter junction of transistor TR4 to the negative point of po¬ larity guard PG and line terminal L2, and is sensed by the exchange equip- ment, whereupon the exchange equipment is prepared for further dialling signals. A timed loop break as described above will also be applied during earth recall. Under certain circumstances the additional TLB signal actu¬ ally assists the earth recall operation. In some telephone systems a ground start is required to sieze the ex¬ change equipment prior to dialling. This is accomplished by momentarily applying an earth, extended from the earthed pole of the exchange battery, to a line terminal having the opposite pole potential, thereby unbalancing the line. Using the circuit of the present invention the ground start sig¬ nal may be provided in a manner similar to the provision of the earth re- call described above with a key designated for earth start. On the other hand the dialler RD could be programed to provide the earth start signal at output P upon closing the contacts of the hook switch. Fig. 4 shows an embodiment of the line switch incorporating the polar¬ ity guard function. This arrangement is similar to a bridge diode in which one diode in each DC circuit has been replaced by a controllable switch (5,6) each of which may perform any or all of the functions previously de-- scribed. The output of the bridge is fed to terminals 3 and 4 across which is a polarity sensing circuit which switches the switches 5 and 6 on and off such that when terminal 1 is positive switch 5 is on and 6 is off and conversely if 2 is positive 6 is on and 5 is off. The polarity guard may Incorporate a time delay to allow fleeting test reversals of the line volt¬ age by the exchange test and supervisory equipment. An additional terminal 10 is provided to give the desired telephone signalling controls. This may be done through polarity sensor 9 as shown or by further connections to switches 5 ad 6, not shown. In a further embodiment diodes 7 and 8 may be replaced by further switches and controlled so the switch replacing 7 is switched synchronously with 6, and 5 with 8. Polarity sensor 9 may be applied across the output of the bridge as an alternative arrangement. An arrangement such as shown in Pig. 4 may be used to replace the bridge rectifier of Pig. 3. While the present Invention has been described with regard to many particulars it is to be understood that equivalents may be readily substi¬ tuted without departing from the scope of the invention. Industrial Applicability The afore described invention may be advantageously applied in the manufacture of telephone subset apparatus.

Claims: The claims defining the invention are as follows: 1. A telephone subset line switch whose switching element is formed by a conductive path of a controllable semiconductor switch means having a con¬ trol element for coupling to a first output means of a control means asso¬ ciated with a telephone subset circuit for controllably switching the said subsets loop current circuit when said switching element is serially con¬ nected in said loop current circuit, in response to a hook- switch signal and at least one other subset function signal coupled to input means of said control means. 2. A telephone subset line switch comprising input terminal means for coupling to the line terminals of a telephone subset circuit, and output terminal means for connection to the transmission circuit of said telephone subset circuit, a first semiconductor switch means having a controllable conductive path thereof serially connected between a first terminal of the said input terminal means and a first output of said output terminal means, a second semiconductor device having a controllable conductive path con¬ nected between the control element of the first semiconductor switch means and a second terminal of the input terminal means, said second input and second output terminals being coπrnon, and whose control element forms a first control input for coupling to control means associated with said transmission circuit for controllably switching said telephone subsets loop current circuit. 3. A telephone subset line switch as claimed in claim 2, including an over-current protection arrangement comprising a first resistor means seri " ally connected between said first terminal of the input terminal means and the said conductive path of the first semiconductor switch means, and a first and a second diode means serially connected between the first termi¬ nal of the Input terminal means and the said control element of the first semiconductor switch means, whereby when the value of current determined by the resistive value of the first resistor means and the first diode means is exceeded, the conductance of the controllable conductive path of said first semiconductor switch means is decreased. 4. A telephone subset line switch as claimed in claim 2 or 3, including an over-voltage protection arrangement compirsing a third semiconductor switch means a controllable conductive path of which is connected between the control element of said second semiconductor switch means and the com¬ mon connection of the second input and second output terminals, and whose control element is connected to said first Input terminal means via a cir¬ cuit which includes a voltage reference means arranged such that when a predetermined voltage level occurs across said first and second input ter¬ minal means, said voltage reference means renders said third semiconductor switch means non-conducting cuasing in turn said second and said first semiconductor switch means to switch off. 5. A telephone subset line switch as claimed in claim 4, wherein said voltage reference means is a zener diode arranged such that when said pre¬ determined voltage level occurs, said zener diode conducts thereby render¬ ing said third semiconductor switch means non-conducting. 6. A telephone subset line switch as claimed in any one of claims 2, 3, 4 or s including fourth semiconductor switch means having a controllable conductive path thereof connected across said output terminal means, and a control element of which forms a second control input for coupling to con¬ trol means associated with said transmission circuit for providing a con¬ trollable shunt circuit around said transmission circuit. 7. A telephone subset line switch as claimed in claim 6, wherein the con¬ trollable conductive path of said fourth semiconductor switch means is con¬ nected across said input terminal means. 8. A telephone subset line switch as claimed in any one of claims 2 to 7, including fifth semiconductor switch means, a controllable conductive path of which is connected between said second input terminal means and a fourth input terminal means, and a control element of which forms a third control input for coupling to control means associated with said transmission cir¬ cuit for controllably coupling the second input terminal means to the fur¬ ther input terminal means. 9. A telephone subset line switch as claimed In any one of claims 2 to 8, wherein said input terminal means are operatively connected across the con¬ stant polarity DC output means of a polarity guard device whose AC input means are for connection to said line terminals of said telephone subset circuit. 10. A telephone subset line switch as claimed in claim 9 , wherein said po- larity guard device comprises a full diode bridge arrangement. 11. A telephone subset line switch as claimed in claim 9, wherein said po¬ larity guard device comprises a rectifier bridge at least two arms of which include a controllable semiconductor switch means. 12. A telephone subset line switch as claimed in claim 11, wherein said controllable semiconductor switch means is at least one field effect tran¬ sistor. 13. A telephone subset line switch as claimed in any one of claims 2 to 12, wherein each said semiconductor switch means comprise at least one bi¬ polar transistor whose collector/emitter junction forms the controllable conductive path thereof and whose base element forms the control element thereof. 14. A telephone subset line switch as claimed in claim 13, wherein each said semiconductor switch means comprise a complementary configuration of an NPN and a PNP transistor the collector/emitter junction of the NPN tran¬ sistor forming said controllable conductive switching path thereof and the base element of the PNP transistor forming the control element thereof. 15. A telephone subset line switch as claimed In claim 13, wherein said at least one semiconductor switch means is a high voltage bipolar device. 16. A telephone subset line switch as claimed in any one of claims 2 to 1 , wherein at least one semiconductor switch means is a DMOS device. 17. A telephone subset line switch as claimed in any one of the preceding claims in integrated circuit form. 18. A telephone subset line switch, substantially as herein described with reference to Pigs. 1 and 2 of the accompanying drawings. 19- A telephone subset circuit arrangement comprising a first and a second line terminal means for respectively connecting to conductors of an ex¬ change line, a telephone transmission circuit means connected between said line terminal means, and a line switch whose switching element is formed by a conductive path of a controllable semiconductor switch means having a control element coupled to a first output of a control means associated with said transmission circuit, said switching element being serially con¬ nected in the subset circuits' loop current circuit, said control means having a first input means coupled to hook-switch means for applying hook- switch mode signals to said first output for switching said switching ele¬ ment accordingly. 20. An arrangement as claimed in claim 19, wherein said control means has a second input means coupled to dial means, whereby pulse signals corre¬ sponding to dial signals from said dial means are applied to the first out¬ put to pulse said switching element. 21. An arrangement as claimed in claim 19 or 20, wherein said control means has a third input means coupled to key means associated therewith whereby a pulse of a predetermined duration is applied to the first output to render said switching element non-conducting for said duration. 22. A telephone subset circuit arrangement including a line switch as claimed in any one of claims 2 to 18, comprising a pair of line terminals, a telephone transmission circuit, a control means having a first input means coupled to hook-switch means, and a second Input means coupled to dial means, wherein the first and second terminals of the line switch input terminal means are respectvely connected to the line terminals of the cir- cult arrangement and the first and second terminals of the line switch out- put terminal means are connected into said transmission circuit, whereby at least hook-switch mode signals are applied to the first output of said con¬ trol means for switching said switching element accordingly. 23. A circuit arrangement as claimed in claim 22, wherein pulse signals corresponding to dial signals from said dial means are applied to the first output of said control means to pulse said switching element. 24. A circuit arrangement as claimed in claim 20 or 23, wherein said con¬ trol means includes a third input means coupled to a timed loop break sig¬ nal means associated therewith, whereby a signal therefrom causes said control means to apply a pulse of a predetermined duration to the first output thereof to render said switching element non-conducting for said du¬ ration. 25. A circuit as claimed in claim 24, wherein the duration of the prede¬ termined pulse is 90 m/s. 26. A circuit arrangement as claimed in any one of claims 22 to 25, wherein said control means includes a fourth input means coupled to shunt signal means associated therewith, whereby a signal from said shunt signal means causes the control means to apply a signal of a predetermined dura¬ tion to the control element of the fourth semiconductor switch means thereby providing a shunt circuit around said transmission circuit for said duration. 27. A circuit arrangement as claimed in claim 26 wherein the shunt signal means is a key means. 28. A circuit arrangement as claimed in claim 26 wherein the shunt signal means is actuated by the hook- switch means. 29. A circuit arrangement as claimed in claim 26 wherein the duration of • the predetermined signal is 400 m/s. 30. A circuit arrangement as claimed in any one of claims 22 to 29, wherein said control means includes a fifth input means coupled to earth recall/start means associated therewith, whereby a signal therefrom causes the control means to apply a signal to the control elements of the fifth semiconductor switch means thereby coupling a line terminal of the subset circuit arrangement to the further input terminal means of said line cir¬ cuit. 31. A circuit arrangement as claimed in claim 30, wherein said further in¬ put terminal means is connected to an earth terminal of the subset circuit arrangement. 32. A circuit arrangement as claimed in claim 31 , wherein the line termi¬ nal of the subset circuit arrangement connected to the fourth input termi- nal is connected to that exchange line conductor which has the same polarity as that of the exchange battery earth. 33. A circuit arrangement as claimed in any one of claims 19 to 32, wherein said control means is a ten number repetory tone/pulse dialler. 34. A circuit arrangement as claimed in any one of claims 19 to 32, wherein said control means is a microprocessor. 35- A telephone subset circuit arrangement substantially as herein de¬ scribed with reference to Fig. 3 of the drawings. 36. A telephone subset electronic line switch to switch subset loop cur¬ rent in response to one or more control functions of the subset. 37. A line switch as claimed in claim 36, wherein one said control func¬ tions is the hook-switch function. 38. A line switch as claimed in claim 36 or claim 37, wherein one said control functions Is the dial pulsing function. 39. A line switch as claimed in any one of claims 36 to 38, wherein an¬ other said control function is a loop break for a predetermined duration. 40. A telephone line Interface comprising a polarity guard in the form of a bridge recti ier in which in at least one leg of each output loop there is a line switch as claimed in any one of claims 1 to 19 or 37 to 39.

Inventors: GOODE PETER ANTON, STEVENSON DAVID RUSSELL, GOODE, PETER, ANTON, STEVENSON, DAVID, RUSSELL
Applicants: GOODE PETER ANTON, INT STANDARD ELECTRIC CORP, STEVENSON DAVID RUSSELL, GOODE, PETER, ANTON, INTERNATIONAL STANDARD ELECTRIC CORPORATION, STEVENSON, DAVID, RUSSELL
Application year: 1986
Main classification: H04Q3
IPC codes: H03K17, H04M1, H04M19, H04Q1