WO2000035174A1

WO2000035174A1 - Fast continuous telephone line fault detection

Info

Publication number: WO2000035174A1
Application number: PCT/US1999/026421
Authority: WO
Inventors: Nahum Gitnik; Amos Intrater
Original assignee: Infogear Technology Corporation
Priority date: 1998-12-09
Filing date: 1999-11-09
Publication date: 2000-06-15
Also published as: AU1521600A

Abstract

A telephone-line-connection monitoring device comprises a switch to disconnect the ringing circuit of a subscriber telephone and a ring detector circuit to monitor the central office connection for ring indications. When a ring indication is recognized, the ringing circuit switch is closed to allow ringing currents to reach the bell. The ringing circuit switch in its open condition allows a battery-voltage threshold monitor across the subscriber line connection to quickly detect a line fault. Otherwise, the large capacitor in the ringing circuit would artificially prop up the voltage and cause a slow bleed-down to the threshold voltage after a fault occurred.

Description

Fast Continuous Telephone Line Fault Detection

BACKGROUND OF THE INVENTION

TECHNICAL FIELD

The present invention relates to detection methods and devices to monitor the operational health of telephone line connections, and more particularly to making such connections more easily monitorable by open-circuiting the DC blocking capacitors in the ring circuits until line ringing from the central office actually commences.

DESCRIPTION OF THE PRIOR ART

When a telephone is "on-hook", the direct current (DC) in the subscriber loop is blocked by a large value capacitor that will permit alternating current (AC) ring signals to pass through uninhibited. Such capacitors are often as large as one microfarad (1.0 UF). The central office (CO) will see a large increase in DC subscriber loop current when the ringed phone goes "off-hook". A battery voltage, usually greater than forty-eight volts, is maintained across the subscriber loop to power the telephone set and to help sense at the CO the on-hook and off-hook conditions of the subscriber set. The DC blocking capacitor charges to the battery voltage during on-hook conditions. Off-hook, the subscriber loop current increases and the voltage at the subscriber set can dip as low as twelve volts.

The monitoring of the health of a telephone connection to the CO should therefore be a simple one of looking for the battery voltage across the line pair. In reality, it's not quite that simple. If and when the connection to the CO were to break, the large DC blocking capacitor would hold the battery voltage across the broken line pair for quite some time. The voltage would have to find some path to bleed off until the voltage dropped to less than three volts, the typical fault threshold. If the break occurred in the idle on-hook condition, the bleed down to three volts may have to start from a high of 42-105 volts. In such case, it could be even several minutes before the monitoring circuits could detect a fault had occurred.

United States Patent 4,710,949, describes a fault-locating device for determining on the central office side whether a problem exists on the customer side or the central office side of a telephone line. A voltage and current sensitive switch is connected in series with each of the ring and tip lines near the customer side. The voltage and current sensitive switches are activated by a current demand only h the presence of a "threshold voltage" across the telephone line. The switches include a triac connected in series with each telephone line and a bilateral (bipolar) avalanche device connected to the gate of the triac (bipolar thyristor) and the telephone line. The switches will not turn on, and the customer will remain disconnected from the central office if the voltage is below the breakdown voltage of the avalanche device. A distinctive termination circuit is connected across the tip and ring lines on the customer side of the voltage and current sensitive switches. The termination circuit conducts only when the ring is positive and the tip is negative. This provides a signature to the central office, indicating the presence of a fault locating device in the customer loop. A test voltage is applied below the threshold voltage of the voltage sensitive switches to test the line with the customer side disconnected to determine which side of the device a fault is located.

The differences between the prior art and the present invention are as follows: < a fault location occurs not on the central office side but on the customer side; < the present invention does not require any change in operation mode of the central office (special line voltages, loop polarity reversals, etc.).

SUMMARY OF THE INVENTION

A telephone-line-connection monitoring device embodiment of the present invention comprises a switch to disconnect the ringing circuit of a subscriber telephone and a ring detector circuit to monitor the central office connection for ring indications. When a ring indication is recognized, the ringing circuit switch is closed to allow ringing currents to reach the bell. The ringing circuit switch in its open condition allows a battery-voltage threshold monitor across the subscriber line connection to quickly detect a line fault. Otherwise, the large capacitor in the ringing circuit would artificially prop up the voltage and cause a slow bleed-down to the threshold voltage after a fault occurred.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a functional block diagram of a telephone system that includes a telephone-line-connection monitoring device embodiment of the present invention;

Fig. 2 is a schematic diagram of a telephone-line-connection monitoring device embodiment of the present invention;

Fig. 3 is a diagram of a telephone system in a prior art configuration in which a ringer is always connected to the telephone subscriber line through a series capacitor;

Fig. 4 is a diagram of a telephone system in an embodiment of the present invention in which a ringer is not always connected to the telephone subscriber line through a series capacitor;

Fig. 5 is a schematic diagram of a way that a telephone system may be implemented that would simplify to that shown in Fig. 3;

Fig. 6 is a chart of voltage between the telephone subscriber line terminals, e.g., "tip" and "ring," versus time in seconds after a line break in a prior art monitor system;

Fig. 7 is a diagram of a first latch circuit that is used in the monitor of Fig. 2 which can be replaced with a second thyristor type latch; either unipolar silicon-controlled rectifier (SCR) or bipolar triac device;

Fig. 8 is a chart of the voltage between the telephone subscriber line terminals, e.g., "tip" and "ring," versus time in seconds after a line break when using embodiments of the present invention;

Fig. 9 is a schematic diagram of a telephone system with an AC ringer voltage being applied on top of the 48V DC battery voltage; Fig. 10 is a chart of the ringer current that flowed through the telephone subscriber line terminals versus time, during ringing, in an experiment;

Fig. 11 is a schematic diagram of a telephone system with an AC ringer voltage being applied on top of the 48V DC battery voltage, but is otherwise similar to that illustrated in Fig. 2.; and

Fig. 12 is a chart of ringer current that flowed through the telephone subscriber line terminals versus time, during ringing, in embodiments of the present invention, such as the telephone system of Fig. 11.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 represents a telephone system 100 in which a central office 102 has a subscriber local loop 104 that connects to a subscriber phone 106. A subscriber local loop 108 connects to another subscriber phone 110. A monitor 112, which is an embodiment of the present invention, is used to monitor the condition of the local loop 108.

Fig. 2 illustrates a telephone-line-connection monitoring device embodiment of the present invention, referred to herein by the reference numeral 200. The central office and battery of a telephone subscriber line connection are represented by a battery 212. Faults that can occur in the lines are represented by a pair of switches 214 and 216. The line pair being monitored are labeled

"line terminal 1" and "line terminal 2". A set of four diodes 218-221 is wired in a full-wave rectifier configuration and will produce a DC voltage across a series connected pair of resistors 222 and 224. A local battery 226 has another pair of series resistors 228 and 230 connected across it. A voltage comparator is connected to produce a digital output that is true if the voltage at a point 232 drops to less than a voltage of a point 234. Therefore, the comparator 236 will give a fault indication if the DC line voltage between line terminal-1 and line terminal-2 drops below some threshold DC value.

A set of four diodes 240-243 are wired in a full-wave bridge configuration to form an AC switch that may be controlled by a DC circuit connected across a pair of control terminals 244 and 246. The AC loads, such as a ringer and DC blocking capacitor, are controlled by connecting a pair of terminals 248 and 250 in series. A unipolar disconnecting module 252 is placed across terminals 244 and 246 and will conduct DC current when a ringing voltage is detected that flows through a large capacitor 254 and a ringer 256. Otherwise, the unipolar disconnecting module 252 will present large impedance and allow terminals 244 and 246 to float. This will effectively open-circuit the connection between terminals 248 and 250 and thereby disconnect the capacitor 254 and prevent its appearance across line terminal-1 and line terminal-2. The comparator 236 will thus be allowed to see any faults that develop on the subscriber line connection very quickly.

The circuit of unipolar disconnecting module 252 is shown to include a pair of transistors 258 and 260, a pair of resistors 262 and 264, and a capacitor 266. In the idle state, the line voltage changes very slowly, e.g., less than 1 V/second. In the ringing state, the line voltage changes relatively quickly because of the 15-20 Hertz AC ring voltage, e.g., more than 20V/second. Capacitor (C2) 266 and resistor (R8) 262 form a classic RC-network, the voltage drop over (R8) 262 being proportional to the voltage-by-time derivative. When the input goes high quick enough, the (C2) 266 will be charged and the transistor (Q2) 260 will be biased on. A voltage will drop across resistor (R7) 264 and thus turn on transistor (Q1) 258. This will then latch on transistor (Q2) 260 until no more AC ring current passes through the DC blocking ring capacitor (C1) 254. While conducting, (Q1 ) 258 discharges (C2) 266 thus preparing it to the next ring voltage cycle.

As a consequence, the ringer 256 and DC blocking ring capacitor (C1 ) 254 are effectively only connected when an AC ringing voltage is present and being rectified by the four diodes 240-243.

Fig. 3 shows a telephone system 300 in a prior art configuration in which a ringer 302 is always connected to the telephone subscriber line through a series capacitor 304. This situation causes any DC voltages on the subscriber line to slew very slowly when there has been a break in the line. So a comparator 306 will be delayed in issuing a line disconnect indication 308. Such delay could be even several minutes.

Fig. 4 shows a telephone system 400 in an embodiment of the present invention in which a ringer 402 is not always connected to the telephone subscriber line through a series capacitor 404. A disconnect module 406 normally keeps the ringer circuit open. This puts a high impedance shunt across the input to a voltage comparator 408. This situation causes any DC voltages on the subscriber line to slew as fast as the stray capacitances will allow when there has been a break in the line. So the comparator 408 will not be delayed in issuing a line disconnect indication 410. Such response can be as quick as a few hundreds milliseconds.

Fig. 5 is a schematic diagram of a way that a telephone system 500 may be implemented that would simplify that shown in Fig. 3. The ringer impedance is represented by a 10KΩ resistor. The voltage from the central office is represented by a battery.

Fig. 6 is a chart 600 of voltage between the telephone subscriber line terminals, e.g., 'lip" and "ring," versus time in seconds after a line break. The starting voltage is assumed to be about fifty volts, and in this example it takes about ninety seconds for the sense voltage at the comparator 306 to drop low enough for a fault condition to be recognized.

Fig. 7 illustrates a latch circuit 700 that is used in the monitor 200 of Fig. 2 which can be replaced with a unipolar silicon-controlled rectifier (SCR) type latch 720. A bipolar thyristor (triac) could also be used, and this would make the diodes 240- 243 unnecessary. The triac is an AC device that could be placed directly in series with the ringer impedance 256, for example.

Fig. 8 is a chart 800 of voltage between the telephone subscriber line terminals, e.g., "tip" and "ring," versus time in seconds after a line break when using embodiments of the present invention. Notice that the time scale of chart 800 is 3.0 seconds versus 120 seconds for chart 600. The starting voltage in chart 800 is also assumed to be about fifty volts, but in this example it takes a little over two hundred milliseconds for the sense voltage, e.g., at the comparator 408, to drop low enough for a fault condition to be recognized. The difference in the detection speed can be critical for certain users.

Fig. 9 is a schematic diagram of a telephone system 900 with an AC ringer voltage being applied on top of the 48V DC battery voltage. Otherwise, the system 900 is similar to that illustrated in Fig. 5. The ringer voltage can be 30V peak-to-peak (P-P) at fifteen Hertz, or even 70V P-P at twenty Hertz. Fig. 10 is a chart 1000 of the ringer current that flowed through the telephone subscriber line terminals versus time, during ringing, in an experiment. The experimental circuit was the prior art circuit of telephone system 900 in Fig. 9.

Fig. 11 is a schematic diagram of a telephone system 1100 with an AC ringer voltage being applied on top of the 48V DC battery voltage. Otherwise, the system 1100 is similar to that illustrated in Fig. 2. The ringer voltage can be 30V peak-to-peak (P-P) at fifteen Hertz, or even 70V P-P at twenty Hertz.

Fig. 12 is a chart 1200 of ringer current that flowed through the telephone subscriber line terminals versus time, during ringing, in embodiments of the present invention, such as the telephone system 1100 of Fig. 11. The results of experiments showed that the addition of a disconnect module 406 (Fig. 4), for example, modifies the ringing current only slightly from that seen in chart 1000 (Fig. 10).

Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the Claims included below.

Claims

1. A telephone-line-connection monitoring device, comprising: a normally disconnected ringer circuit that self connects its load impedance to a telephone subscriber line only in the presence of an alternating current ringing signal; and a voltage monitor connected across telephone subscriber line that indicates a line fault when a voltage below a predetermined threshold is measured.

2. The telephone-line-connection monitoring device of claim 1 , wherein: the normally disconnected ringer circuit includes a normally open altemating current (AC) switch in series with a ringer having high direct (DC) current impedance and low AC impedance.

3. The telephone-line-connection monitoring device of claim 2, wherein: said normally open altemating current (AC) switch includes a set of four rectifier diodes in a bridge configuration and provides for control of the A C impedance at a pair of AC connections to said bridge with a DC load connected to a pair of DC connections to said bridge.

4. The telephone-line-connection monitoring device of claim 3, wherein: said DC load includes a latch that triggers on when an AC ringing signal appears on said telephone subscriber line.

5. The telephone-line-connection monitoring device of claim 4, wherein: said latch includes a pair of transistors in which a first transistor has a base input that is connected to resistor-capacitor differentiator that senses the slew rates of voltages on said telephone subscriber line.

6. The telephone-line-connection monitoring device of claim 5, wherein: said first transistor is connected with a collector load resistor, and to a base input of a said transistor, such that when said first transistor is biased on said second transistor will also be biased on by a voltage drop across said collector load resistor.

7. The telephone-line-connection monitoring device of claim 6, wherein: said second transistor is connected to said first transistor such that when the second transistor is biased-on it will draw a voltage drop across said resistor of said resistor-capacitor differentiator that will bias-on said first transistor and thus cause a latching condition between said first and second transistors enough to load said DC connections of said bridge enough that said AC connections of said bridge become conductive to AC ringing currents.

8. The telephone-line-connection monitoring device of claim 1 , wherein: said voltage monitor includes a set of four rectifier diodes in a bridge configuration.

9. The telephone-line-connection monitoring device of claim 8, wherein: said voltage monitor includes a first voltage divider across said telephone subscriber line through said set of four rectifier diodes in a bridge configuration.

10. The telephone-line-connection monitoring device of claim 9, wherein: said voltage monitor includes a second voltage divider across a reference battery, and a voltage comparator to which each of said first and second voltage dividers are input.

11. The telephone-line-connection monitoring device of claim 10, wherein: said voltage monitor includes an output of said voltage comparator to provide a digital indication of fault/no-fault when a tap-voltage from said first voltage divider differs from a tap-voltage from said second voltage divider.

12. A telephone-line-connection monitoring device, comprising: a direct current (DC) voltage monitoring circuit for connection to a telephone line pair and providing for a measurement of a battery voltage across a telephone subscriber line; a threshold comparator connected to the DC voltage monitoring circuit and providing a fault/no-fault output signal that depends on whether said measurement of said battery voltage exceeds a predetermined value; a ringer circuit including a DC blocking capacitor in series with a bell ringer; an altemating current (AC) switch in series with the ringer circuit and across said telephone subscriber line; and a ring detector connected to the AC switch and providing for an open- circuiting of the ringer circuit except when an AC ringing signal appears across said telephone subscriber line from a central office (CO); wherein, said DC blocking capacitor is prevented from interfering with said measurement of said battery voltage.

13. The telephone-line-connection monitoring device of claim 12, wherein: the DC voltage monitoring circuit includes a full-wave bridge that allows a measure of the absolute value of the subscriber line voltage to be measured without regard to polarity of connection.

14. The telephone-line-connection monitoring device of claim 12, wherein: the AC switch circuit includes a full-wave bridge that allows a unipolar D C circuit connected across a pair of DC output terminals to control the series impedance across a pair of AC input terminals.

15. The telephone-line-connection monitoring device of claim 14, wherein: the ring detector includes a latch connected across said DC output terminals that is triggered-on when said AC ringing signal appears across said telephone subscriber line from said CO, and wherein said latch drops out when said AC ringing signal ceases.

16. A method of monitoring a telephone subscriber line, comprising the steps of: open-circuiting a ringer circuit that includes a DC blocking capacitor in series with a bell ringer across a telephone subscriber line to-be-monitored; measuring a central-office battery voltage across said telephone subscriber line; and comparing a measurement obtained in the step of measuring to a predetermined value, and outputting a fault/no-fault output signal that depends on whether said measurement of said battery voltage exceeds a predetermined value; wherein, said DC blocking capacitor is prevented from interfering with said measurement of said battery voltage.

17. The method of claim 16, wherein the step of open-circuiting includes the steps of: detecting any central-office ringing signals on said telephone subscriber line and closing said ringer circuit for as long as an alternating current (AC) ringing signal is present.

18. The method of claim 17, wherein: the step of detecting said central-office ringing signals on said telephone subscriber line includes discriminating between signals that slew at approximately one volt per second and twenty volts per second.

19. The method of claim 16, wherein: the step of comparing uses a first voltage divider across said telephone subscriber line and a second voltage divider across a reference battery, and a voltage comparator to which each of said first and second voltage dividers are input.

20. The method of claim 16, wherein: the step of comparing uses an output of voltage comparator to provide a digital indication of fault/no-fault when a tap-voltage from said first voltage divider differs from a tap-voltage from said second voltage divider.