GB2149274A - Testing subscriber lines - Google Patents

Abstract

A high speed, highly reliable remote isolation device for temporarily disconnecting a customer's telephone equipment from the telephone lines for testing purposes comprising at least one solid state impedance changing element located in a switching circuit 60 connected to each telephone line, the element being normally conductive but which can selectively be changed to a high impedance state thereby effectively disconnecting the equipment, the element later returning to a conductive state to reconnect the equipment. <IMAGE>

Description

SPECIFICATION Remote isolation device This invention relates to a circuit isolation device, which is used to temporarily disconnect telephone equipment from telephone lines for testing purposes.

Since telephone customers are now able to purchase telephone equipment from a wide variety of sources, it may be desirable for the telephone company to determine whether a fault is in their equipment and lines or in the customer-owned equipment.

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 at the customer's location and it is activated by a signal from the central office of the local telephone company. When activated, the device momentarily disconnects the customer's equipment from the telephone lines, and during this disconnection period, the central office of the telephone company runs some automatic tests which determine the source of the problem. This disconnecting and testing may also be done as part of a periodic, general testing of all the telephone lines.

The 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 5 telephones. Further, these switches do not operate rapidly, and this greatly increases the time it takes to automatically-test a number of lines.

Summary of the Invention We have discovered a highly reliable, fast-operating remote isolation device which comprises impedance-changing elements connected to the telephone lines, which elements though normally conductive may be selectively changed from their conductive state to a high impedance state which effectively disconnects the customer's equipment from the telephone lines, the elements later returning to the conductive state, thereby reconnecting the customer's equipment.

In the preferred embodiment, the pulse-operated remote isolation device of this invention comprises solid state FETs which are connected to the two telephone lines to a customer's location. The FETs are normally conductive so that they do not interfer in any way with the normal signals over the lines. However, when detection circuitry in the remote isolation device detects a 1 30 volt command pulse from the telephone company's central office, a control circuit in the device is activated, which by means of a transformer circuit, changes the state of the FETs. The FETs then place a high impedance on both telephone lines thereby effectively disconnecting the customer's equipment.After a fixed period of about 1 6 to 18 seconds and without another command signal, the control circuit automatically allows the FETs to return to their normal conductive state thereby reconnecting the customer's equipment. The remote isolation device of the preferred embodiment also includes a signature circuit, which is connected across the telephone lines and which allows the central office to detect the presence of the device on the lines.

The remote isolation device ov this invention is highly reliable, and should a failure occur, the FETs will tend to remain conductive thereby keeping the customer S telephone connected to the telephone lines. The device also uses little current and switches very quickly to permit the automatic testing of a large number of lines to be done rapidly.

Description of the Preferred Embodiment Drawings I turn now to a detailed description of the preferred embodiment, after first briefly describing the drawings.

Figure 1 is a block diagram of the remote isolation device of this invention; Figure 2 is a schematic diagram of the device of Fig. 1; Figure 3 is a representation of a command pulse for this invention; Figure 4 is a schematic diagram of an alternate signature circuit; and Figure 5 is a schematic diagram of another alternate signature circuit of this invention.

Structure Referring to Fig. 1, a pulse-operated remote isolation device of this invention is shown at 10.

The device 10 generally comprises a pulse detection circuit 20, a command detection circuit 30, a control circuit 40, an isolation circuit 50, and a switch circuit 60. The device 10 is connected so that the switch circuit 60 is connected to the two standard telephone lines, called the tip lead and the ring lead, from the telephone company to the customer. When the device 10 is inactive, it is virtually "invisible" to these lines, adding only 10 ohms of resistance to each. As will be explained in more detail, however, this changes when the device 10 receives a command pulse from the telephone company side of the network.

As shown in Fig. 1, when the device 10 is in place, the pulse detection circuit 20 is connected between the tip lead and the ring lead from the telephone company's central office, and the basic function of the pulse detection circuit 20 is to initally detect the command pulse from the telephone company, pass it on to the command detection circuit 30 and use it to activate a power supply 36 of the device 1 0. As shown in Fig. 2, the basic pulse detection circuit 20 comprises a high voltage diode D7 connected in series with a resistor R23, and a high voltage diode D8 connected in series with a resistor R22, each diode-resistor series tapping off one of the two telephone lines. With this basic arrangement, not only is a very high impedance maintained between the tip and ring lines, but a command pulse can be detected on either one.Further, any two of these basic components may short out (or any one may open) without disabling the device 10 or the telephone lines.

The remainder of the pulse detection circuit 20 comprises a pair of resistors R20 and R21, which are parallel to the basic resistors R22 and R23. An output line 22 for the command detection circuit 30 is connected between these parallel resistors R20 and R21. An output line 24 for the power supply 36 is connected from between the basic resistor pair R22 and R23, through carbon composition (current limiting) resistor R24 and forward biased silicon diode D9.

As will later be explained in more detail, the pulse detection circuit 20 passes along the detected command pulse to the command detection circuit 30. The command detection circuit 30 attenuates any high voltage pulse it receives on output line 22 from the pulse detection circuit 20, and it sends this reduced voltage signal to the control circuit 40.

The command detection circuit 30 comprises a resistor R1 9 which is connected to the line 22 from the pulse detection circuit 20. The opposite end of resistor R1 9 is connected to the cathode of a 10 volt zener diode D5. The anode of the zener diode D5 is connected to resistor R9 and resistor R 11. A 10 volt zener diode D2 is connected across resistor R11, as is a voltage divider comprised of resistors R7 and R8. The output line 32 from the command detection circuit 30 is from the voltage divider. A return line 26 from the control circuit 40 is connected to the resistors R7 and Ri 1 and the anode of zener diode D2.

The power supply 36, which received the other output line 24 from the pulse detection circuit 20, supplies power in two steps to the control circuit 40 for a limited period of time (although longer than the duration of the basic command pulse from the telephone company which lasts about one second). The output line 24 from the pulse detection circuit 20 is connected to two portions of the power supply 36. On connection is to the cathode of 62 volt zener diode D11, the anode of which is connected to the return line 26 from the control circuit 40 and a ground isolation circuit 34, (which merely comprises forward biased silicon diode D12, 43 volt zener diode D13 and carbon composition resistor R25 which is connected to ground).Apart from zener diode D11, the other portion of the power supply 36 connected to output line 24 comprises resistor R17, which in turn is connected to resistors R16 and R48. Resistor R47 and capacitor C15 in series are connected across resistor R 1 6. Resistor R48 is connected to junction FET Q2 which is connected to the voltage input of the control circuit 40 through output line 38.

A p-channel power MOSFET Q1 is connected with FET Q2 as a sort of amplifier pair. FET Q2 and MOSFET Q1 not only provide the initial voltage to the control circuit 40 but also regulate that voltage.

The same power supply connection which goes to resistor Ri 7 also goes to resistor R 18, which is in series with aluminium electrolytic capacitor C5. The capacitor C5 is connected to MOSFET Q3, which in turn is connected through filter capacitor C4 and protection zener diode D4 to line 38 and the voltage input of the control circuit 40. MOSFET Q3 is also connected to return line 26 and an ECDS output line 42, both from the control circuit 40. Generally, this energy storage capacitor C5 and associated elements provide power to the control circuit 40 during the latter portion of the disconnect cycle, after the command pulse has gone but while the disconnect cycle is still in effect.

The control circuit 40 provides the timing means and pulse means to drive the isolation circuit 50, which in turn opens the switches of the switch circuit 60. In the preferred embodiment, the control circuit 40 is a Teradyne Integrated Circuit 940-300-00, idenfified here as ICI.

However, any integrated circuit having a suitable clock, comparator circuits and drive circuits, as will hereinafter be explained, would be suitable, and it also is suitable to hard-wire such components together to obtain an equivalent circuit.

The control circuit 40 is actually only powered by the power supply 36 during a command pulse, and immediately thereafter during the disconnect period. Thus, it and the remainder of the device 10 require no current during passive periods. During operation, however, the power supply 36 supplies voltage to the control circuit 40 by output line 38, which is connected to pin 16, designated VDD, of ICI. The return is from pin 8, designated Vss, froth which the return line 26 originates. As previously indicated, the return line 26 is connected back to the command detection circuit 30 and the power supply 36. Pin 1 of the IC1, designated ECDS, is connected to the ECDS output line 42, and provides a means for the control circuit 40, once it determines the pulse received is a proper command pulse, to activate the capacitor portion of the power supply 36.This line is also connected through diode D28 and resistor R49 to the output line 32 from the command detection circuit 30.

The basic input to the control circuit 40 is the voltage on output line 32 from the command detection circuit 30. This voltage output is received by IC1 on pin 7, designated TDET, where it is internally compared at various times with the voltages on pins 5 and 6, designated TGDH and TGDL, respectively. These comparison voltages are set by the resistor divider of resistor R 12, R13 and R14, and for the preferred embodiment, they are 1.5 volts on pin 6 and 2.9 volts on pin 5. As the input voltage from the command detection circuit 40 exceeds each threshold, different operations commence.

The comparison is made with the higher threshold first. Once the higher threshold is exceeded, the internal oscillator of IC1 starts, and in the preferred embodiment, the oscillator runs at 1220 Hz with a cycle of 819 microseconds. This interval is established by resistor R1 and capcitor C1 connected to pins 1 3 and 14 of IC1. Feedback resistor R2 completes the external circuitry for the oscillator by coupling this combination to pin 1 5. Precision, low current RC values are used here, as this single oscillator generates all the other timing periods, and thus the components setting the timing must remain stable.

As for the remainder of the control circuit 40, capacitor C2 is the external component for a power-on reset circuit in IC1, which circuit restricts the operation of the other chip elements if the input voltage falls below a certain level.

The output of the control circuit 40 is on lines 44 and 46 to the isolation circuit 50, and these lines are internally connected in IC1 to two CMOS driver sets in push-pull operation. The timing and rise and fall time of the driver sets depend upon resistors R3 and R4, diode D1 and capacitor C2.

The isolation circuit 50, which drives the switch circuit 60 and isolates the remainder of the circuitry from the telephone lines, comprises in part a transformer 52, a primary winding 54 of which is connected to the output lines 44, 46 from the control circuit 40. Current to the winding 54 is limited by resistor R5 in output line 46.

The transformer 52 has two main secondary windings 56, 58, each of which is connected to identical gate drive circuits 57, 59. The gate drive circuit 57, which also provides isolation from the telephone lines for the switch circuit 60 should the transformer 52 short out, has a capacitor C14 and a diode D23 connected to the winding 56. MOSFET Q8 is connected across the diode D23, and diode D22, shunted by capacitor C17, along with resistor R41 are connected across the MOSFET Q8. Capacitor C13 is connected across resistor R41. Resistor pair R42 and R43 with capacitor Ci 2 between them are connected across capacitor C13. Zener diode D20 (16 volts) completes the drive gate circuit 57.Gate drive circuit 59 is identical, except that it is connected to the transformer 52 in a mirror image fashion, its input capacitor corresponding to C14 being connected to the end of its secondary winding 58 opposite the end to which capacitor C14 is connected.

The switch circuit 60, which is an integrated circuit, Teradyne Part No. 323-200-00, hereinafter IC2, receives the outputs from the gate drive circuit 57, 59. The circuit IC2 consists of MOSFET devices, which are connected to the telephone lines so that there are two of the FETs in series in each line. Either FET can block its line. The MOSFETS of the preferred embodiment are depletion devices, which eliminates the need for a gating voltage to turn them on (make them conductive). Thus, they would tend to remain conductive at all times. A voltage, however, is needed to turn them off. The volage is supplied, when appropriate, by the two gate drive circuits, on pins 3 and 4 for circuit 57 and pins 5 and 10 for circuit 59. Any pair of depletion mode FETs may be used instead of IC2.

The connection of the telephone lines to the FETs is as follows. For the tip line, the path is first through inductor L1, which is shunted by resistor R29, and which inductor prevents fast rising transients from reaching IC2. From inductor L1 the path is into pin 1 of IC2. The path passes through one FET and out pin 2 from which it reenters IC2 at pin 1 2 which is connected to pin 2 through resistor R35. After passing through the second FET, the path comes out pin 14, from which it is directed to the customer's equipment.

Te return ring lead is similar. The ring line from the customer returns to pin 8 of IC2 from which it goes through a third FET and then out pin 9. Pin 9 is connected through resistor R40 to pin 6, which is the input for the fourth FET. The output of the last FET is from pin 7, through inductor L2, shunted by resistor R30, and back to the central office and the negative side of the 50 volt exchange battery (not shown).

in the preferred embodiment, the lines have additional isolation protection in the form of a pair of tranzorbs TR1 and TR3, which clip off excess voltages, and a pair of gas tubes G1 and G2 which provide a low voltage, high energy short for transients.

The final portion of the device 10 is a signature circuit 70, which generally enables the telephone central office to identify the presence of the device 10 because of its impedance across the lines (which does not affect the telephone lines themselves). The basic signature circuit is a diode D14 and R26 in series between the telephone lines. Optionally, the signature circuit 70 may also include other signature circuits. One is a fixed delta passive polarized signature, as best shown in Fig. 4. There, the basic diode-resistor signature circuit 80 is shunted by two pair of diode-resistors 82, 84. The center tap from these pairs would be to ground.

The second additional signature circuit is shown in Fig. 5, and it is used in the circuit of Fig.

2. Basically, it merely replaces the center tap to ground from the fixed delta signature circuit with a center tap to a switch 72. In the preferred embodiment, the switch 72 comprises a series-connected pair of MOSFETS Q4 and Q5 (having a tranzorb TR2 for voltage protection).

The MOSFETS Q4 and Q5 are driven by a gate drive circuit 74, almost identical to the gate drive circuits 57. The only differences are that the equivalent of capacitor C14, diode D23 and MOSFET Q8 are omitted, and the zener diode corresponding to diode D20 is a 5.6 volt zener.

The gate drive circuit 74 is connected to an additional secondary winding 76 of the transformer 52.

The following is a list of values for the elements of the preferred embodiment.

Resistors Value (ohms)* R1 4.22N + 1% (Precision film) TempCO= + 150 PPM/C R2 47K (carbon film) Unless otherwise indicated, all resistors are k watt, carbon composition resistors with values + 10%. Resistors designated carbon film resistors have values j 5%.

Resistors Value (ohms) R3 100K (carbon film) R4 330K (carbon film) R5 1K (carbon film) R7 3.30M + 2% (Precision film) TempCQ = + 150 PPM/C R8 3.30M + 2% (Precision film) TempCO = + 150 PPM/C R9 5.6M # 5% (carbon comp.) R11 l.00M # 2% (Precision film) TempCO = # 150PPM/C R12 3.6M # 2% (Precision film) TempCO = + 150 PPM/C R13 2.70M + 2% (Precision film) TempCO = + 150 PPM/C R14 2.70M + 2% (Precision film) TempCO = + 150 PPM/C R15 15M (carbon film) R16 - 3.3M R17 10K (carbon film) R18 22K (carbon film) R19 0 ohm jumper (Allen Bradley) R20 330K R21 330K R22 39K hW R23 39K 1/2W R24 33K 1/2W R25 6.8K R26 100K 1W R27 100K 1W R28 100K 1W R29 100 R30 100 R31 100 (carbon film) Resistors Value (ohms) R35 0 ohm jumper (Allen Bradley) R36 100 (carbon film) R37 100 (carbon film) R38 100 (carbon film) R39 100 (carbon film) R40 0 ohm jumper (Allen Bradley) R41 22M (carbon film) R42 1M (carbon film) R43 1M (carbon film) R47 47K (carbon film) R48 1.5M (carbon film) R49 5.60M + 2% (Precision film) TempCO = + 150 PPM/C Diodes Type D1 1N914 (small signal) D2 1N5240B (Motorola) lOV Zener (+ 5%) D4 1N5240B (Motorola) lOV Zener (+ 5%) D5 1N5240B (Motorola) 10V Zener (+ 5%) D7 1N4007 (H.V.) D8 1N4007 (H.V.) D9 FDH6626 (Fairchild) Dil 1N5265B (Motorola) 62V Zener (# 5%) D12 1N4007 (H.V.) D13 1N5260B (Motorola) 43V Zener (+ 5%) D14 1N4007 (H.V.) D15 1N4007 (H.V.) D16 1N4007 (H.V.) D18 FDH6626 (Fairchild) D20 1N5246A (Motorola) 16V Zener (+ 1090) Diodes Type D22 IN914 (small signal) D23 IN914 (small signal) Capacitors Value C1 100 picofarad i 1% (50V) C2 100 picofarad + 20% (50V) C3 220 picofarad i 20% (50V) C4 .1 microfarad + 20% (50V) C5 6.8 microfarad + 10% (75V) C12 .01 microfarad + 20% (50V) C13 .01 microfarad i 20% (50V) C14 .01 microfarad + 20% (50V) C15 .01 microfarad + 20% (50V) C17 100 picofarad + 20% (500V) Transistors Type Q1 VP0109N (Supertex) Q2 2N3436 (Motorola) Q3 VN1310N3 (Supertex) Q4 VN0120N3 (Supertex) Q5 VN0120N3 (Supertex) Q8 VN1310N3 (Supertex) Miscellaneous Type TR1 1.5SE82C 1.5 kw (82V) (Semicon) TR2 1.5KE220CA 1.5 kw (220V) (General Semiconductor) TR3 1.5SE82C 1.5 kw (82V) (Semicon) G1 Y085-90 (90V) Sankosha G2 Y085-90 (90V) Sankosha Miscellaneous Value L1 100 microhenri (133 mA) 5 ohm Dc max resistance L2 100 microhenri (133 mA) 5 ohm DC max resistance Operation As previously explained, the device 10 is invisible to the telephone lines during normal operation. The FETs of the switch circuit 60 are all conductive (no voltages being received on their gates), and the remainder of the circuit is passive. When the telephone company wishes to perform a test, the device 10 disconnects the customer's equipment from the lines.

To start the test, the telephone company sends a + 1 30 volt command pulse, shown in Fig.

3, to the device 1 0. This command pulse is the same type used to trigger coin-returns in pay telephones. It has fixed rise and fall times and is at peak for about one second. The pulse may be sent over either the tip load or the ring lead. The reason for this is that the particular problem or fault may be that one of the leads is shorted out, and if so, the pulse must travel over the non-shorted one in order to reach the device 10. Accordingly, the basic diode-resistor arrangement (D8-R22 or D7-R23) of the pulse detection circuit 20 is such that it will pull the command pulse off either lead. At the same time, the polarity of the diodes D7 and D8 blocks current flow between the leads.

When the leading edge of the command pulse is detected, the voltage into the pulse detection circuit 20 increases linearly over time, as represented by the rise time section of the command pulse shown in Fig. 3. This voltage is sent on line 22 to the command detection circuit 30 and one line 24 to the power supply 36.

As for the pulse going on line 24 from the pulse detection circuit 20 to the power supply 36, the initial path in the power supply is through zener diode D11 to the ground isolation circuit 34 and to ground. As the pulse voltage increases, however, the voltage generated across the power supply diode D11 increases.When this voltage reaches 51 volts + 2 volts, it permits current to flow through resistor R1 7 and the MOSFET Q1 and junction FET Q2 portion of the circuit. (The current does not flow to resistor R18 and its portion of the cicuit at this time because MOSFET Q3 is not biased at this point.) Junction FET Q2 then begins supplying voltage to the control circuit 40 at pin 16, VDD, and the return path from circuit 40 is from pin 8, Vss, and return line 26. At this time, the power-on reset function is performed by the circuit IC1 of the control circuit 40.This is an internal function which prior to this condition prevented operation of any of the circuitry of IC1, and which will again prevent operation of that circuity if the VDD voltage falls below a level of about 3 volts. The circuitry of IC1 is now enabled at this point.

Referring again to the power supply, as the command pulse voltage increases, the junction FET Q2 approaches its cutoff voltage. The resulting increase in voltage across the resistors R48 and R16 turn MOSFET Q1 on, so that it provides additional voltage for the control circuit 40.

The overall voltage to the control circuit 40, however, is regulated because once junction FET Q2 cuts off entirely, MOSFET Q1 will lack the gate voltage to supply anything further to the control circuit 40. The MOSFET Q1 and junction FET Q2, however, do provide the power to the control circuit 40 for the first portion of the operation of the device 10.

Soon after the control circuit 40 has been turned on, the command detection circuit 30 is also turned on. When the command pulse reaches a level of 53 volts + 2 volts, there is sufficient voltage on line 22 to breakover zener diode D5 of the command detection circuit 30. This results in current through and voltage across the resistor R11 and the divider resistors R7 and R8. Thus, there is an output to the control circuit 40 on line 32. The level of the command detection circuit current is kept fairly low (10 microamps) by zener diode D5 and the associated resistors. It is also limited in part by zener diode D13 of the ground isolation circuit 34.In any event, the voltage level on pin 7 of IC1, which is connected to the command detection circuit output line 32, is only a volt or so, although it rises as the voltage level of the command pulse rises.

The voltage from the command detection circuit 30 is compared internally by IC1 of the control circuit 40 with the 2.9 volts on pin 5 (TGDH) of IC1. This fixed comparison voltage comes from the power supply 36. When the command pulse has risen to 100 volts i 10 volts, the comparison is such that it enables IC1 to clear all of its internal counters. This is called master reset. At the same time, this marks the beginning of timing period T1, shown in Fig. 3.

As indicated earlier, IC1 has an internal oscillator which runs at 1220 Hz with a cycle of 819 microseconds. This oscillator began running after the power-on reset function occurred. The control circuit IC1 now begins to measure the timing period T1 by counting the oscillator pulses. In the preferred embodiment, 1024 pulses are counted, which corresponds to .84 seconds. IC1 monitors the voltage level of the command pulse (by both the power-on reset and the command detection circuit output comparison) during this entire period R1 in order to determine whether or not a true command pulse is being received. This eliminates possible false disconnect signals caused by high voltage short transients and by lower voltage long transients.

Another purpose of the timing period T1 is to allow sufficient time for activation of the second portion of the power supply 36, which portion powers the control circuit 40 after the command pulse is no longer received. At the completion of the master reset function, IC1 applies a voltage on line 42 (ECDS) to MOSFET Q3 of the power supply 36. During the T1 period, the capacitor charges to 40-45 volts, which is sufficient to power IC1 for 1 6 to 1 8 seconds.

As shown in Fig. 3, the timing period T1 ends while the command pulse is still at peak 1 30 volts. The control circuit continues to draw operating current from the power supply 36 (and the command pulse) and the power supply capacitor C5 continues to charge, or if it has already done so, it remains charged. The device 10 will remain in this state indefinitely so long as the command pulse voltage level remains at peak.

Once the voltage of the command pulse falls, however, the second timing period T2 (or disconnect phase) begins. This second timing period is again begun by a comparison of the now falling voltage from the command detection circuit on line 32 with the 1.5 volts on pin 6 of IC1. Once the command detection output falls below the 1.5 volt level, which occurs when the actual command pulse voltage drops to about 80 volts + 7 volts, lC1 starts counting its oscillator pulses again. This time 20480 are counted, which makes the period T2 about 1 6 to 18 seconds long.At the same time, with MOSFET Q3 of the power supply 36 still enabled, the capacitor C5 begins discharging through the regulator circuit of junction FET Q2 and MOSFET Q1, both of which are rapidly losing the current originally supplied by the now-decaying command pulse.

During this disconnect period T2, control circuit 40 begins driving its two CMOS drivers (internal to IC1), and they provide an alternating signal to transformer primary 54. In the preferred embodiment, line 44 is switched high and line 46 low for 10 microseconds. Both leads are then high for 1 630 microseconds, followed by line 44 switched low and line 46 switched high for 10 microseconds. Both lines are then kept low for 1630 microseconds. This sequence repeats for 5120 cycles at 305 Hz. The result is that the transformer 52 operates at an equivalent frequency of 50 KHz while only drawing current for .5 percent of the cycle.

During the T2 period when the primary is being driven as described, the secondaries 56, 58 of the transformer 52 have a square wave, with a short on-time, which is sent to the gate drive circuits 57, 59. Initially, referring to circuit 57, capacitor C14 charges during the positive swing. On the negative swing, capacitor C14, through MOSFET Q8 and diode D22 adds its voltage to the voltage from the transformer 52 to charge capacitor C13, so that capacitor carries double the voltage on the secondary 56.

Capacitor C12 and resistors R42 and R43 filter the voltage, and the zener diode pair D20 and D21 clamp the voltage in either direction to prevent voltage spikes to the switching circuit 60.

Resistor R41 is used to decay the voltage on the capacitors C12 and C13 when the operation is complete. The resulting output voltage from the gate drive circuit is applied to the gates of two of the FETs of IC2 thereby changing thie state to a high impedance one. The gate drive circuit 59 does the same thing for the other pair of FETs of IC2. At the end of the T2 period, as determined by the count of IC1, the voltages from the gate drive circuits cease, and the FETs revert back to their normal, conductive condition. At the same time, capacitor C5 has discharged, and the control circuit 40 reverts to its passive state.

The signature circuit 70 provides a means for the telephone company's central office to identify the presence of the device 10 on the particular telephone lines because of its unique impedance between the lines. The basic signature circuit of D14 and R26 perform this function.

This signature circuit also permits some testing to be performed without actually disconnecting the customer's equipment.

The second signature circuit shown in Figure 4, is the fixed delta polarized signature, which also allows for testing of a number of specific faults without disconnecting the customer's equipment.

The third signature circuit of simplified Fig. 5, which is actually incorporated in the preferred embodiment, uses the pair of MOSFETS Q4 and Q5 as its switch, which are enabled (by the gate drive circuit 74 at the same time the switching circuit 60 disconnects the customer's telephones. The switched delta polarized signature is used if the particular circuit cannot have the fixed delta signature on the line during normal operating conditions.

Other Embodiments Other embodiments of the invention are possible. One such embodiment involves the use of enhancement mode MOSFET switches to disconnect the lines rather than the depletion mode MOSFETs of the preferred embodiment. This substitution would, of course, require the circuity to be revised somewhat so as to apply gate voltage (which keeps the enhancement mode MOSFETs conductive) during normal telephone operation.

Also, it is possible to use other solid state devices, as switches. Any such device, which can be alternated between a normal low impedance and a high impedance condition could be used.

Voltage sensitive switches, for example, could be placed in the telephone lines, and made to become high impedance devices by dropping the normal line voltage below a threshold level.

Claims (27)

1. A remote isolation device for selectively disconnecting a customer's telephoe equipment from telephone lines, comprising: at least one solid-state, impedance-changing element having a high impedance state and a low impedance state, at least one said element being connected to one of the telephone lines to the customer's telephone equipment whereby selective activation of said element so that it changes to a high impedance state effectively disconnects the customer's telephone equipment.

2. The device of claim 1 wherein said element is a field effect transistor.

3. The device of claim 2 wherein four of said field effect transistors are used, two in series in each telephone line.

4. The device of claim 2 wherein said field effect transistor is a depletion mode device which is conductive without a gate voltage, said transistor being selectively activated to a high impedance state by application of a gate voltage.

5. The device of claim 2 wherein said field effect transistor is an enhancement mode device which is conductive when a gate voltage is applied, said transistor being selectively activated to a high impedance state by removal of said gate voltage.

6. The device of claim 1 wherein said element is a voltage sensitive switch.

7. The device of claim 6 wherein a pair of voltage sensitive switches are used, one in each telephone line.

8. The device of claim 6 wherein said switch is normally conductive when the usual operating voltage is applied to the telephone lines, said switch being selectively activated to a high impedance state by removal of said normal line voltage.

9. The device of any one of claims 1-8 further comprising a means for selectively activating said element, said means including a control circuit.

10. The device of claim 9 wherein said control circuit is activated by a command pulse, and when said control circuit is activated, the state of said element is changed to a high impedance state for a disconnect period.

11. The device of claim 10 wherein said command pulse is a + 1 30 volt pulse with a peak duration of approximately one second.

1 2. The device of claim 10 wherein said control circuit includes an oscillator, said oscillator generating pulses, the count of which determines the disconnect period.

1 3. The device of claim 1 2 wherein said control circuit also includes a drive circuit, said drive circuit providing a signal to selectively activate said element, the signal being provided during the disconnect period.

14. The device of any one of claims 10-13 wherein said means further comprises an isolation circuit, said isolation circuit being activated by said control circuit, and when activated, said isolation circuit changing said element to a high impedance state.

1 5. The device of claim 14 wherein said isolation circuit comprises a transformer to isolate said element from said control circuit, and at least one gate drive circuit to activate said element.

16. The device of any one of claims 10-15 further comprising command pulse detection means for detecting the command pulse and applying it to activate said control circuit.

1 7. The device of claim 1 6 wherein said detection means includes a pulse detection circuit, said pulse detection circuit comprising a diode-resistor pair to draw the command pulse from either of the two usual telephone lines.

1 8. The device of claim 1 7 wherein said detection means also includes a command pulse detection circuit which reduces the voltage level of the command pulse detected by said pulse detection circuit and sends a reduced-level voltage corresponding to the command pulse to said control circuit.

19. The device of claim 1 6 further comprising a power supply means, said power supply means providing power to said control circuit for the period when said Command pulse is applied and also for the disconnected period.

20. The device of claim 1 9 wherein said power supply means includes a voltage regulator circuit for supplying power to said control circuit when said command pulse is received, and a voltage storage circuit for supplying power to said control circuit for the disconnect period.

21. The device of claim 20 wherein said voltage storage circuit is charged by the command pulse.

22. The device of claim 10 further comprising a ground isolation circuit which isolates the device from earth ground.

23. The device of any one of claims 1-22 further comprising a signature circuit, said signature circuit being connected across the telephone lines and providing an unusual impedance between the lines which identifies the presence of said device on the lines.

24. The device of claim 23 wherein said signature circuit comprises a first diode and resistor series.

25. The device of claim 24 wherein said signature circuit further comprises a second diode and resistor series and a third diode and resistor series, both said second and third series being connected across said first diode and resistor series, the midpoint of said second and third series being connected to ground.

26. The device of claim 25 wherein a switch is included between the ground and the second and third series, said switch being closed when the device is activated to disconnect the customer's equipment and open during normal telephone operation.

27. A remote isolation device for selectively disconnecting a customer's telephone equipment from telephone lines substantially as herein described with reference to the drawings.