IE850285L - Telephone subset - Google Patents

Description

580 1 2 The invention relates to circuit arrangements for producing a recall signal in telephone subset apparatus for signalling exchange equipment, particularly Private Branch Exchange (PBX) equipment, connected 5 thereto.

A known method for producing a recall signal is to provide a pushbutton on the subset which, when actuated, applies an earth potential to one leg of the line connecting the subset to the exchange equipment. 10 The applied earth potential is sensed by the associated exchange equipment which thereupon prepares the equipment to receive further dialling signals. This method is known as "Earth Recall".

Another more recent method for producing a 15 recall signal is to provide a signal in the form of a space, that is an interruption of the line current for a precise duration, for example 90 m/s, which is recognised by the associated exchange equipment. The space may be provided by a small electronic circuit mounted in the 20 subset which is triggered by a signal from an actuated pushbutton mounted on the subset. The space is sensed by the associated exchange equipment whereupon the equipment is prepared for receiving further dialling signals. This method of recall is sometimes known as Timed Loop Break 25 (TLB).

The earth recall method is usually associated with older type PBX's, whereas the TLB method is associated with more modern electronic PBX's.

Generally, neither method is compatible with the other, that is the exchange equipment which senses a recall signal produced by earth recall is different from the exchange equipment which senses a recall signal 5 produced by a TLB, end therefore subsets connected to the PBX have to be adapted at the time of installation, to the particular recall system used.

It is an object of the present invention to provide a circuit arrangement which renders a subset 10 compatible with both methods without the need for making any modification to the subset prior to installation.

According to the invention in its broadest aspect there is provided a circuit arrangement comprising a telephone subset circuit having a first and second line 15 terminal means for respective connection to a first and second conductor of an exchange line over which DC current is supplied via associated exchange equipment to the subset circuit, one pole of the DC current supply source being coupled to the first conductor and the other 20 pole being coupled to the second conductor, a third terminal means for connection to the one pole of the DC current supply source extended from the exchange over a third conductor, and a timed loop break means having its switching path serially connected in the main DC current 25 path of the subset between the second line terminal means and the subset circuit, wherein the circuit arrangement comprises a controllable switch means including a first switch element which normally provides a conducting path around the switching path of the timed loop brake means, 30 and a second switch element whose normally non-conducting state provides an interrupted circuit between the third terminal and the second line terminal, whereby upon activation of the controllable switch means the first switch element is rendered non-conducting and the 35 conducting path is removed thereby, and the second switch element is rendered conducting thereby connecting the third terminal to the second line terminal.

Embodiments of the invention will now be described by way of example with reference to the accompanying diagrammatical drawings in which: :> Figure 1 is a simple circuit illustrating the principle of the invention.

Figure 2 is a circuit of a preferred embodiment of the invention.

Figure 3 is a circuit of a Timed Loop Break 10 arrangement which may be used in the circuit arrangement of the present invention.

Referring to Figure 1, there is shown a telephone circuit block designated TC and representing a conventional telephone subset circuit which may comprise 15 a transmission circuit, a tone ringer and a dial signal generator means (which are not shown). Connection to the telephone circuit TC is via line terminals LI and L2. The main DC current path of the subset Includes a timed loop break unit TLB. In parallel with the switching path 20 (not shown) of unit TLB is a short circuit provided by the normally-contacting break contact 3 and lever contact 2 of recall pushbutton Kl. Make contact 1 of K1 is connected to earth terminal E of the telephone circuit 1- Line terminals LI and L2 are respectively connected 25 to the A and B conductors of the exchange line. The A conductor is coupled via exchange line equipment ELE to the positive (earthed) pole of the exchange battery Bl, and the B conductor is coupled, via the equipment 3, to the negative pole of the exchange battery. Earth 30 potential is extended from the exchange over the E conductor and connected to the E terminal for earth recall only.

In operation, after a connection has been established between the telephone circuit and the 35 exchange line equipment and the user of the telephone wishes to recall the exchange equipment, pushbutton Kl is manually operetsd which actuates lever contact 2. Lever contact 2 leaves break contact 3 and contacts make contact 1. If the recall system associated with the exchange line equipment ELE is an Earth Recall system, 5 earth potential extended over conductor E and connected to line terminal E will be connected, via operated Kl contacts 1 and 2, to line terminal L2 and 3 conductor of the exchange line thereby shunting the coil of an auxialiary relay (not shoun) connected in series with the 10 positive pole of the exchange battery and A conductor of the exchange line., This causes the auxiliary relay to drop out and thereby prepare the exchange line equipment for further dialling signals. If, on the other hand, the recall system is a Timed Loop Break system, the 15 disconnection of Kl contacts 2 and 3 when pushbutton Kl is operated simply removes the short circuit from around the switching path of unit TLB thereupon the main DC current path of the telephone circuit TC is interrupted by a relatively high impedance introduced by unit TLB now 20 in circuit. At the same time, unit TLB is energised and, after a predetermined delay, switches on and thereby provides a low impedance path. The precise space provided by unit TLB is sensed by the exchange line equipment 3 whereupon it prepares to receive further 25 dialling signals from telephone circuit TC. In the case of a Timed Loop Break recall system the earth conductor is not provided.

The preferred circuit shown in Figure 2 is virtually the same as that of Figure 1 except that it 30 includes a constant polarity bridge Ml comprising diodes Dl, D2, D3 and D4. The junction of diodes Dl and D2 and the junction of diodes D3 and D4 are respectively connected to line terminals L2 and LI. A constant negative polarity is provided at the junction of diodes 35 Dl and D3 while a constant positive potential is provided at the junction of diodes D2 and D&. The telephone circuit TC is connected across the constant polarity junctions and lever contact 2 (Figure 1) is connected to the constant negative point of the bridges Ml, that is to the pole opposite to that of the exchange earth.

It will be understood that the contacts Kl (of 5 Figure 1) associated with a push-button may be replaced by a spring set associated with a controlled electromagnetic relay. Further, a controllable semiconductor switch arrangement could replace the contacts, controlled, for example, by a microprocessor or 10 other control circuitry.

Referring to Figure 3, there is shown a Timed Loop Break circuit comprising time constant resistor R1 and capacitor CI, switching transistor TR1 and a latch consisting of transistors TR2 and TR3. Capacitors C2 and 15 C3 provide a bypass for any transient voltages which are not absorbed by varistor Ml to prevent premature switching of transistors TR2 and TR3. Resistors R2 provides a leakage current path around capacitor C2 to prevent premature switching of transistor TR2. Resistor 20 R3 has a similar function in regard to capacitor C3 and transistor TR3.

In operation, when contacts of pushbutton Kl are opened, the circuit TLB presents a high impedance. Current immediately begins to flow into capacitor CI via 25 resistor R1 and continues until the voltage across the capacitor equals the knee voltage of Zener diode Dl plus the Vbe of transistor TR1 whereupon transistor TR1 turns on. Transistors TR2 and TR3 are then switched on and provide a low impedance path until contacts controlled by 30 Kl are restored

Claims (9)

CLAIMS:

1. A circuit arrangement comprising a telephone subset circuit having a first and a second line terminal means for respective connection to a first and second conductor of an exchange line over which DC current is 5 supplied via associated exchange equipment to the subset circuit, one pole of the DC current supply source being coupled to the first conductor and the other pole being coupled to the second conductor, a third terminal means for connection to the one pole of the DC current supply 10 source extended from the exchange over a third conductor, and a timed loop break means having its switching path serially connected in the main DC current path of the subset between the second line terminal means and the subset circuit, wherein the circuit arrangement comprises 15 a controllable switch means including a first switch element which normally provides s conducting path around the switching path of the timed loop break means, and a second switch element whose normally non-conducting state provides an interrupted circuit between the third 20 terminal and the second line terminal, whereby upon activation of the controllable switch means the first switch element is rendered non-conducting and the conducting path is removed thereby, and the second switch element is rendered conducting thereby connecting the 25 third terminal to the second line terminal.

2. A circuit arrangement as claimed in claim 1, wherein the controllable switch means comprises a changeover spring set, the first switch element comprising the spring sets' lever and break spring, and 30 the second switch element comprising the lever and make spring, the make spring being connected to the third terminal and, activation of the controllable switch means actuating the lever spring.

3. A circuit arrangement as claimed in claim 2, 35 wherein the controllable switch means comprises a manually-actuated switch. - 7 -

4. A circuit arrangement as claimed in claim 3, wherein the controllable switch means comprises an electromagnetic relay.

5. A circuit arrangement as claimed in any one 5 of the preceding claims, wherein a constant polarity device is interposed between the line terminals and the serially connected time loop break means and subset circuit, the first and second line terminals being connected to the input means of the device and the 10 serially connected timed loop break means and subset circuit being connected to the constant polarity output of the device, the distal end of the serially-connected timed loop break means remote from the subset circuit bein

6. A circuit arrangement as claimed in claim 5, wherein the constant polarity device comprises a diode bridge arrangement.

7. A circuit arrangement as claimed in any one 20 of the preceding claims, wherein the timed loop break means comprises an input means across which is connected an R/C network comprising a series-connected resistance and capacitance whose junction is connected to the control element of a first semiconductor switch means 25 whose conductive path circuit includes a zener diode means, an element of the conductive path being connected to the control element of a second semiconductor switch arrangement whose conductive path is also connected across the input means, the second semiconductor switch 30 arrangement having a current-controlled negative resistance characteristic and latching upon the application of a signal to its control element but which can be turned off only by reducing the current therethrough to a value below that required to maintain 35 conduction, the first semiconductor switch means being rendered conducting when current flowing via the input means and the resistor into the capacitance causes the voltage across the capacitance substantially to equal the knee voltage of the zener diode means, whereupoon the conducting first semiconductor switch means applies a signal to the control element of the second semiconductor 5 switch arrangement &hich is rendered conducting thereby providing a low resistance path across the input means via the conduction path of the second semiconductor switch arrangement.

8. 3. A circuit arrangement as claimed in claim 7, 10 wherein the second semiconductor switch arrangement comprises a pair of bi-polar transistors in i^hich the input elements of one are connected to the output elements of the other.

9. A circuit arrangement as claimed in any one 15 of the preceding claims, therein che one pole of the DC current supply source is at earth potential. 10» A circuit arrangement substantially as „ described wich reference to the accompanying drawings. Dated this 6th day of February 1985. BY: ttoflKIMS & CO., Applicants' Agents, (Signed) 5, Dartmouth Road Dublin 6. - 9 -