GB2181625A - Apparatus for connecting and disconnecting telephone equipment - Google Patents

Abstract

A remote isolation device 10 for connecting and disconnecting a customer's telephone equipment 30 from the telephone lines 22, 24 for testing purposes comprises at least one voltage sensitive switch (VSS) 50 connected to each telephone line. The switch is normally non-conductive but becomes conductive upon application of a threshold voltage and remains conductive with a low level of current flow therethrough, whereby the location and magnitude of faults can be determined. The VSS is preferably a sensitive-gate SCR. <IMAGE>

Description

SPECIFICATION Apparatus for connecting and disconnecting telephone equipment This invention relates two a circuit isolation device,which is used to temporarily disconnecttelephone equipmentfrom telephone lines for testing purposes.

With deregulation in the United States, telephone company customers are now able to purchase telephone equipment from a variety of sources. The local telephone companies, however, are not required to and do not service any customer-owned equipment purchased from a supplier other than the telephone company itself.

Consequently, when a service problem arises, the local telephone companywould like to be able to determine, without the expense of a service call, whether or not the problem is caused by the telephone company's equipment and lines or by the customer-owned equipment.

Similar problems arise for British Telecom in respect of equipment not supplied by it.

One device which is used for this purpose is called a remote isolation device, of which there are several types. The remote isolation device is attached to the telephone lines atthe customer's location, and most are used to momentarily disconnect the customer's equipment from the telephone lines. During the brief dis- connection period, the central office of the telephone company runs some tests to determine the source of any problem.Thistesting may also be done as part a periodic, general testing of all telephone lines.

Many prior remote isolation devices, however, usually include electromechanical switches, which besides requiring considerable current, are not very reliable, and may fail in an open position, thereby disconnecting the customer's telephones. Further, these switches do not operate rapidly, and this generally increases the timeittakestoautomaticallytesta number of lines.

Still other remote isolation devices use electrical switches called triacs. These devices, however, also have several serious drawbacks. First, the triacs require fairly large currents in order to "latch" --that is,to switch closed and stay closed. Fortesting purposes, the magnitude ofthe available current is dependent in part (and inversely proportional to) the magnitude of any resistive fault that is on the iine. In practice, faults greaterthan 15K ohms on the customer's side result in an insufficient current to keep the triac closed. Instead, thetriac oscillates. This means that while the fault may be detected because of the oscillation, its resistance cannot bye accurately measured.This is important because in many instances faults between 15K ohms and 50K ohms cause enough of a voltage loss to the system to usually require correction whilefaults greaterthan 50K ohms do not. Thus, with an oscillating triac, the telephone company does not know whether the fault is in the 1 5Kto 50K category or whether it is of a larger resistance and can be disregarded.

The oscillation problem ofthetriacs, however, also causes two other related problems. One isthatwhen automatic test equipment is used to test a large number of lines, the test equipment takes a considerably longer time to analyze an oscillation-type response. This severely slows down the automatictesting process.

Also, there is a similar problem when the testing equipment attempts to identify the presence of a remote isolation device on a particular line. This is done through what is called a signature circuit which is part ofthe device. To detect it, some reverse polarity voltages are sent out by the telephone office equipment. Unfortunately, the current created is so low that the triac oscillates. Because of this oscillation, there is a large "window" ofvalues thatthe test equipment identifies as corresponding to a signature circuit. Unfortunately, the "window" is large enough that certain line faults can be misidentified as a remote isolation device.

Finally, the triac-type devices usually require some associated circuitry in order to operate, which circuitry, if it fails, renders the triactotally inoperative.

We have discovered that by employing a highly reliable remote isolation device which comprises voltage sensitive electrical switches connected to a customer's telephone lines, which switches are normally nonconductive but which become conductive with a threshold voltage and remain conductive with a very low current, this permits accurate measurement of larger resistive faults on the customer's side of the device and optimizes the operation of automatic test equipment. In addition, as a result of the voltage sensitive switches and a signature circuit, the telephone company can correctly detectthe presence of the device on the lines.

The remote isolation device can be selected so as still to operate if the circuitry associated with the voltage sensitive switches fail.

According to a first aspect ofthe present invention, we provide apparatus adapted for use with a customer's telephone equipmentfor connecting and disconnecting that equipmentfrom a telephone system, the appar atus comprising: a pair of telephone lines adapted for direct connection of the said customer's equipmentto the telephone system; and, interpolated in at least one said line, a remote isolation device comprising a voltage sensitive switch having a conductive and a non-conductive state, the arrangement being such that application of a threshold voltage is adapted to cause said switch to become conductive and remain conduct ive atvery low line currents, thereby allowing connection of the customer's equipmentto thetelephone system.

In a second and alternative aspect of the present invention, there is provided apparatus adapted for use with a customer's telephone equipment for connecting and disconnecting that equipment from a telephone system, the apparatus comprising: a pair oftelephone lines adapted for direct connection of the said customer's equipment to the telephone system; and, interpolated in at least one said line, a remote isolation device comprising a voltage sensitive switch having a conductive and a non-conductive state, the arrange ment being such that application of a threshold voltage is adapted to cause said switch to become conductive and remain conductive if a holding current is exceeded, thereby allowing connection of the customer'sequip mentto the telephone system, said switch having a gate and a cathode between which a gate circuit is connected, said gate circuit comprising a means which, when turned on, provides a low resistance between said gate and cathode to control a holding currentthrough said switch, but failure of which means does not disable said switch.

In a preferred embodiment, the remote isolation device comprises two pairs of voltage sensitive switches (VSS),which actually are sensitive-gate, silicon controlled rectifiers (SCRs), each pair being connected to one of two telephone lines to the customer's equipment. All the VSSs are normally non-conductive so thatthe telephone lines are not connected. However, when a threshold voltage is exceeded, such as when the customer liftsthetelephone receiver or under certain testing conditions, the primary VSS of each pair becomes conductive thereby completing the circuit. The primary VSSs remain conductive until the current through them falls below a certain very low level.Fortesting purposes with certain automatic test equipment, voltages above and below the threshold voltage are applied to the lines, and the currentflow is measured for each condition. If the calculated resistance based on the applied voltage and the measured current remains the same, any resistive fault is in the telephone company's lines and not in the customer's equipment. This is because the customer's equipment (and any faults in it) are disconnected from the telephone office when the threshold voltage is not exceeded. On the other hand, if a current is detected when the largervoltage is applied but not the smallervoltage,thefault is on the customer's side.In addition, because ofthe low latching current needed to keep the VSSs conductive, they do not oscillate with faults as large as lOOK ohms, and thus the larger resistive faults ohm can be quickly detected and accurately measured. Also, as part of the device, a signature circuit is connected between the telephone lines which is associated with a secondary pair of VSSs.

Again, because ofthe low current levels needed to keep the secondary VSSs conductive, is there no oscilla tion when the telephone company attempts to detect the presence of the remote isolation device on the lines, and the detection "window" can be narrow. This meansthatthe device can be quickly and accurately identified.

In the second embodiment, the arrangement and operation ofthe switches are similar exceptthata JFET and associated circuitry are connected between the gate and cathode of the VSSsto set the level of current which will keep the VSSs conductive. This embodiment is used with certain manual test equipment in which the threshold voltage for the VSSs is always exceeded. With this equipment, however, the current, however, is limited to an amount belowthatwhich will cause the VSSs of this embodiment to remain conductive, and it operates similarto thetriac-type devices. However, unlike the triac-type devices, if the circuitry associated with the VSSs fails, the device will still operate.

We turn nowto a detailed description of the preferred embodiments, afterfirst briefly describing the drawings, wherein: Figure lisa blockdiagram of a first embodiment of apparatus according to this invention; Figure2is a schematic diagram ofthe preferred embodiment; and Figure3is a schematic diagram of another embodiment.

Referring to Figure 1, the remote isolation device loins connected between a central office 20 ofthetele- phone company and a customer's telephone equipment 30. The remote isolation device 10 is usuallyconnected atthe customer's location, and the actual connection is made in the two lines to the customer's equipment 30, which lines are called the tip 22 and the ring 24 lines respectively. Normally, the lines 22,24 are used to carry calls to and from the customer's equipment. However, testing equipment 40 at the central office 30 can also be connected to the lines 22, 24to checkforfaults, as will hereinafter be explained in more detail.

Referring to Figure 2, the remote isolation device 10 ofthe preferred embodiment comprises two pairs of switches 50,60. The switches 50,60, which are which in simplified form in Figure 1, are identical. Accordingly, onlythe circuitry of switch 50 actually is shown in detail in Figure 2 and need be described herein.

The switch 50 has a pair of voltage sensitive switches (VSS) which are actually sensitive-gate, silicon controlled rectifiers Q1, Q2. The VSSs are connected in parallel. The anode of Q1 ,which is the primaryVSS, is connected to the tip line 22 from the central office 20through resistor Rl. The cathode of Q1 is connected to the tip lead of the customer's equipment 30. Normally, Q1 is non-conductive until it receives a trigger pulse on its gate. As shown in Figure 2, the gate of Q1 is connected through diode D1 and Zener diode D2 to resistor and the telephone company's side of tip line 22.Resistor R2 and capacitor C1, which are arranged in parallel, are connected between the gate and cathode of Q1.

The circuit arrangementofthe other half of switch-50 is similar, with the principal difference being thatthe secondaryVSS Q2 is connected in opposite polarity to Q1. In other words, the cathode of Q2 is connected to the telephone company's side ofthe tip line 22 through resistor R1 while the anode of Q2 is connected to the customer's side of the tip line 22. Diode D3 and Zener diode D4 are connected between the anode and gate of Q2, and the parallel combination of resistor R3 and capacitor C2 are connected between its gate and cathode.

Finally capacitor C3 and resistor R4 in series are connected across both Q1 and Q2. As previously mentioned, switch 60 is identical, and it is connected in the ring line 24.

A signature circuit 70 comprising diode D5 and resistor R5 is connected between the tip and ring lines 22,24 on the customer's side.

The following is a list of values for the elements of the preferred embodiment: Resistors Value (ohms)* R1 0.4 inch (1.016 cm) 36 AWG, Manganin 290 11.6 ohms/ft. (3.53568 ohms/m) annealed .387 ohms nominal (California Fine Wine Co.) R2 2.2M ohms (carbon film) R3 2.2M ohms (carbon film) R4 470 ohms (carbon film) R5 220K ohms 1W (carbon comp. + 5%) (Allen Bradley çGB2245) Diodes Types D1 1N914 D2 1 N5242B Zenerdiode (12V + 5%) D3 1N914 D4 1 N5242B Zenerdiode (12V + 5%) D5 1 N4007 silicon rectifier (1 amp.1000VP.l.V.) Capacitors Value C1 470 picofared (50 WVDC X7R 10%) (glass encap. ceramic) Centralab No. C41 C471 KNP C2 470 picofarad (50 WVDC X7R 10%) (glass encap. ceramic) Centralab No. C4l C471 KNP C3 4.7 microfarad 1 10% (20V KemetT332 B745 K020AS *Unless otherwise noted, all resistors are 1/4 watt, carbon composition resistors with values + 10%.

Silicon controlled rectifiers Type Q1 Teccor No. S422 (sensitive gate) Q2 Teccor No. S422 (sensitive gate) Referring back to Figure 1, the switches 50,60 are normally open, as shown. In other words, with reference to Figure 2, the two voltage sensitive switches Q1 and Q2 for switch 50 are normally non-conductive, as are the two-identical ones (not shown) for switch 60.

The switches 50,60 close whenever a voltage above a certain threshold level is applied to the circuit. Under normal operating conditions, the threshold voltage is applied from a battery at the central office 20 whenever the customer lifts the telephone receiver. Undertest conditions, the testing equipment 40 applies thevoltage, which is usually done when the customer's telephone receiver is not lifted.

The specific operation ofthe circuit is best described with reference to Figure 2. The Zener diode D2 and the reverse polarity blocking diode D1 set the threshold voltage forthe VSS Q1 at about 12 volts in the preferred embodiment. (Thethreshold voltage is usually set between 12.0volts and 20.0 volts.) When as voltage above the threshold voltage is applied, the gate of Ol receives currentthrough resistor R1 and diodes D1 and D2.As a result, 01 begins to turn on or, in other words, become conductive.

Once turned on, Q1 will remain conductive as long as the current flowing through it exceeds a certain very small amount called the holding current. When the currentfallsto a point where Ql starts to turn off, theVSS Q1 may actually oscillate, that is turn on and off. The value of the holding current depends in part on capacitor C3 and resistor R4, and it is much less than thatforthetriac devices of the prior art. The latter capacitorresistor series combination also provides transient immunity and a low impedance path for lowfrequency, low amplitudetest signals for some types of test equipment.Resistor R2 and capacitor C1 also provide additional transient immunity for Q1 And resistor R1 protects the circuit from large long term transient voltages and currents (e.g., power line cross).

Referring backto Figure 1, possible resistive faults are shown by the boxes numbered oneto six. Thefaults numbered one to three are on the telephone company side (and arethe telephone company's responsibility), while the faults numbered fourto six are on the customer side (and are the customer's responsibility). Assum ing a fault exists on the customer's side, the test equipment 40 (automatictest equipment is used with this embodiment) applies a voltage exceeding the threshold voltage forVSS Ol and the comparable VSS of switch 60. With a resistive fault, at least one ofthese VSSs turns on, and the current flows through thatVSS and the fault. lffaultfour exists, for example, the flow is through Q1 and the fault.If fault six exists, the flow is through the primary VSS of switch 60 and that fault If fau It five is present, the flow is through both primary VSSs (Q1 of switch 50) and the fault. Accordingly, the current is measured, and as the applied voltage is known, the fault resistance is calculated.

The test is then repeated with the automatic testing equipment applying a voltage below the threshold voltage. In this instance, the VSSs remain open, disconnecting the customer's equipment. The current is then measured. If there is no current, the fault lies on the disconnected customer's side. If there is current and the calculated resistance remains the same, the fault is located on the telephone company side. The location of the fault depends upon which VSS is conducting (or both), and the exact value of the fault on the customer side can be determined by the caiculation. For example, regardless of which fault exists, the value of current flow depends upon the vaiue of the resistive fault. When the resistive fault is very high (a minorfault),the holding current is very low.Nevertheless, even with such faults up to 100K ohms, the holding currentis sufficientto keep the VSSs conductive, and the value ofthesefaults can be accurately measured using Ohms Law, as the applied voltage is known. On the basis of this measurement, the telephone company can determine whether or not a correction is required (e.g., as may be the case as with a fault of 30K ohms). If the resistive fault is above lOOK ohms, the holding currentwill be reduced so that the associated VSS will not stay latched but instead will oscillate. The presence of the fault can still be detected, butthevalue is of little importance as the resistance is so high and the fault considered so minorthat it would generally not require correction.

As for the signature circuit 70, the secondary VSS Q2 and its corresponding secondaryVSS in switch 60 are used. The signature current is low, but it is above the latching currentforthose VSSs, and the secondaryVSSs become conductive when a voltage which exceeds the threshold voltage of both VSSs and the signature circuit 70 is applied. This is about 25 volts in the preferred embodiment. The signature circuit 70 allowsthe telephone company to verify the presence of the device 10 by applying this reverse polarity voltage and detecting the signature current.Once again, as only a very low current is needed to keep the VSSs conductive, the telephone equipment can more accurately and more quickly detect the presence of the remote isolation device than would be possible if the signature switches only oscillated.

Another embodiment is shown in Figure 3, and it is primarily intended to be used with certain manual testing equipment. As in the preferred embodiment, it comprises two voltage sensitive switches 80 and 90. As before, the switches are identical. Also, a signature circuit 100 is connected between the two.

In the switch 80, the anode ofvoltage sensitive switch (VSS) Q3 is connected to the tip line 22 from the central office 20through resistor R6. The cathode of 03 is connected to the tip line of the customer'sequip menu 30. As before, Q3 is non-conductive until it receives a trigger pulse on its gate. The gate of Q3 isconnected through diode D6 and Zener diode D7 to resistor R6 and tip line 22. Resistor R7 and capacitor C4 in parallel are connected between the gate and cathode of 03.

A p-channei JFET Q4 is also connected to VSS Q3. Specifically, the drain of the JFET Q4 is connected as shown to the gate of VSS Q3. The gate ofJFET 04 is connected through resistor R8 to the cathode of diode D6.

The source of Q4 is connected through resistor R9to the cathode of VSS 03. Resistor R10 is also connected between the gate ofthe JFET 04 and the cathode of 03.

The circuit arrangement of the other half of switch 50 is similar, again with the principal difference being that voltage sensitive switch Q5 is connected in the opposite polarity to Q3. In othe words, the cathode of 05 is connected to the telephone company side of the tip line 22 through resistor R6 while the anode of OS is connected to the customer's side ofthetip line 22. Diode D8 and Zener diode D9 are connected between the anode and gate of Q5, and the parallel combination of resistor R11 and capacitor C5 are connected between the gate and cathode of Or. A p-channel J FET Q6 is connected to the OS circuit.The gate of J FET Q6 is connected through resistor R12 to the cathode of diode D8. The source of JFET Q6 is connected through resistor R1 3 to the cathode of 05. Resistor R14 is also connected between the gate ofJFET andthecathode of Q5. Finally, capacitor C6 and resistor R1 5 are connected across the VSSs Q3 and 05. The switch 90 is identical.

Signature circuit 100 is comprised of diodes D10 and Dl 1 in series with resistor R1 6.

The general operation of the basic portion of switch 80 is the same as that of switch 50 of the preferred embodiment, exceptthatthe threshold voltage is set at 17.5 volts instead of 12 volts by using a differentZener diode. However, the JFET Q4 has been added to the basic switch. When Q3 is conductive, JFET O4wilI also turn on, and there will be a low resistance between the gate and cathode of 03. The value of this resistance, which comprises Q4 and R9, determines the value ofthe current which allows the VSS O3 to turn off. This holding current is higher than that in the preferred embodiment so that the device is operationally the same as the existing triac devices. However, resistors R8 and R10 serve to bias Q4 off at voltage between 2 volts and 17.5 volts, which increases the sensitivity of Q3 to turn on at low values of gate current.

This embodiment is generally intended to be used with manual rather than automatic test equipment, and the faults are located as in the preferred embodiment. In the case of this embodiment, however, faults of greaterthan 15K ohms will causetheVSSsto oscillate because of a holding current that is too low. Faults with values less than 15K ohms will latch the VSSs and allow calculation oftheirvalues by measuring the current flow, as in the previous embodiment with fault of up to 100K ohms.Also, while some manual test equipment always applies a voltage above the threshold, it limits the currenttotheVSSsto about 1 milliamp atwhich level the VSSs will not remain closed.The resulting osciilation will identify a fault on the customer's side.

However, although this operation is deliberately comparable to the existing triac-type devices, unlike those devices, if any of the JFET circuitry should fail here, this device remains fully operational.

Finally, the signature circuit 100 creates a signaturecurrentwhich is set bythevaluesofDlO, Do 1, and R16, and of course, the applied voltage. The signature current chosen is below the latching currentforthe secondary VSSs Q5 and the corresponding device (not shown) of switch 90. As a result, the VSSs oscillate when a voltage exceeding the threshold of both VSSs plus the signature circuit 100 is applied. In this embodiment, that threshold is approximately 56 volts (17.5 volts for each VSS and 21 volts forthe signature circuit 100). The signature current allows verification of the device's presence for one polarity since Dl 1 prevents opposite current flow.

The values of the components of basic switch of the preferred embodiment and corresponding components of the circuit of this embodiment are the same exceptforthe diodes D7 and D8. Those and the additional components of the circuit of this embodiment are as follows: Resistors Value (ohmsJ R8 Same as R2 R9 Same as R4 R10 SameasR2 R12 SameasR2 R13 SameasR4 R14 Same as R2 Diodes Type D7 lNS247BZenerDiode 17V + 5% D8 IN 52478 Zener Diode 1 7V + 5% Dl 0 N 5250B Zener Diode 20V + 5% Junction Field Effect Transistors Type Q4 P-Channeljl76(National Semiconductor) Q6 P-ChannelJ176 (National Semiconductor) Silicon Controlled Rectifiers Type Q3 Teccor No. 412 (sensitive gate) Q5 Teccor No. 412 (sensitive gate) Othervariationswill occurtothoseskilled in the art.

Claims (16)

1. Apparatus adapted for use with a customer's telephone equipment for connecting and disconnecting that equipment from a telephone system, the apparatus comprising: a pair of telephone lines adapted for direct connection of the said customer's equipment to the telephone system; and, interpolated in at least one said line, a remote isolation device comprising a voltage sensitive switch having a conductive and anon- conductive state, the arrangement being such that application of a threshold voltage is adapted to cause said switch to become conductive and remain conductive at very low line currents, thereby allowing connection of the customer's equipmentto the telephone system.

2. Apparatus adapted for use with a customer's telephone equipment for connecting and disconnecting that equipmentfrom a telephone system, the apparatus comprising: a pair of telephone lines adapted for direct connection of the said customer's equipment to the telephone system; and, interpolated in at least one said line, a remote isolation device comprising a voltage sensitive switch having a conductive and anon- conductive state, the arrangement being such that application of a threshold voltage is adapted to cause said switch to become conductive and remain conductive if a holding current is exceeded, thereby allowing con nection of the customer's equipment to the telephone system, said switch having a gate and a cathode between which a gate circuit is connected, said gate circuit comprising a means which, when turned on, provides a low resistance between said gate and cathode to control a holding currentthrough said switch, but failure of which means does not disable said switch.

3. Apparatus according to Claims 1 or 2, wherein said switch is a sensitive-gate, silicon controlled rectifier.

4. Apparatus according to Claim 3, wherein fourof said rectifiers are used, two in each telephone line.

5. Apparatus according to Claims 1 or 2, wherein four of said switches are used, with a primary switch and a secondary switch in each telephone line.

6. Apparatus according to Claim 5, wherein said primary and secondary switches are both in the same telephone line but arranged in opposite polarity so that for a threshold voltage of one polarity, only one of said switches becomes conductive.

7. Apparatus according to Claim 1, further comprising a gate circuit for activating said switch, said gate circuit comprising a diode means.

8. Apparatus according to Claim 2, wherein said gate circuitfurther comprises a diode means.

9. Apparatus according to Claims 7 or 8, wherein said diode means comprises a reverse blocking diode and a Zener diode in series.

10. Apparatus according to Claim 9 as appendentto Claim 1,wherein said switch is a sensitive-gate, silicon controlled rectifier having a gate to which said gate circuit is connected.

11. Apparatus acccording to Claim 2 or any claim appendentthereto, wherein said switch is a sensitivegate, silicon controlled rectifier having a gate and cathode and said means is a JFETconnected between said gate and cathode of said rectifier.

12. Apparatus according to Claim 11, wherein said JFETturns on when said rectifier becomes conductive.

13. Apparatus according to Claim 1 or any claim appendentthereto, further comprising a signature circuit disposed between the telephone lines.

14. Apparatus according to Claim 13, wherein said signature circuit comprises at least one diode and resistor in series.

15. Apparatus according to Claim 14, wherein said signature circuit includes a Zenerdiode.

16. Adapted for use with a customer's telephone equipment for connecting and disconnecting that equipmentfrom a telephone system, apparatus substantially as hereinbefore described with reference to and as shown in the accompanying drawings.