GB2325113A - Line voltage monitoring; determining telephone line status - Google Patents

Abstract

A monitoring circuit 1 for determining if a voltage exists on a line, particularly a telephone line 5, has a capacitor Cx which is charged from the line 5 and which discharges completely each time a trigger pulse from a microprocessor IC6 of a PABX turns on a discharge transistor TR1. Provided the capacitor Cx has charged to at least a certain voltage, the discharge current passing through transistor TR1 and an LED D2 of a photocoupler IC2 will be sufficient to turn on a transistor TR2. Turn on of transistor TR2 is detected by a comparator IC4 giving a high output which is input to microprocessor IC6. The microprocessor IC6 gradually decreases the time period between discharge pulses so that eventually there will be insufficient time between pulses for the capacitor Cx to charge to a level high enough to turn on transistor TR2 at the next discharge. The microprocessor notes the value of the time period at which the transistor TR2 first fails to turn on at the succeeding discharge pulse, the length of this period being dependent on the voltage on the line 5. Alternatively, the period between discharge pulses may be gradually increased, the value of the period at which the transistor TR2 first turns on then being noted. The microprocessor IC6 then uses an algorithm to determine line voltage from the noted time period. It can thus be determine whether the telephone line 5 is disconnected, idle or seized. Calibration of the monitoring circuit 1 is effected by carrying out the above procedure while the actual line voltage is determined by connecting an analogue to digital port 16 of microprocessor IC6 to the same line 5. In normal use with a PBX coupled to two exchange lines 5, the port 16 is connected to a seized one of the lines and the monitoring circuit 1 is connected to the other line.

Description

"A line voltage monitoring circuit" The present invention relates to a monitoring circuit for monitoring line voltage on a line, and in particular, though not limited to a monitoring circuit for monitoring line voltage on one or more exchange lines of a public telephone network. The invention also relates to a private branch exchange (PBX) comprising the monitoring circuit.

In order to determine the status of a telephone line, and in particular, an exchange line of a public telephone network, it is necessary to determine the voltage on the exchange line.

Typically, there are three states of an exchange line, namely, a disconnected state, an idle state and a seized state. In the disconnected state the exchange line has been disconnected from the telephone network, and in which case, the voltage on the line is typically zero volts. In the idle state the exchange line is connected to the telephone network but is idle, in other words, the telephone is on-hook. Typically, in the idle state a DC voltage in the range of 40V to 57V appears on the line. In the seized state the exchange line is connected to the telephone network, and the telephone is off-hook. In the seized state, a DC voltage appears on the line which is in the range of 4.8V to 24.5V. When an exchange line or exchange lines are connected to a PBX, it is important to be able to determine the status of the respective exchange lines. There are various known circuits for determining the status of an exchange line, however, such known circuits in general, suffer from a number of disadvantages.

Public utilities and other companies which control the operation of public telephone networks and supply exchange lines, impose relatively stringent conditions on circuitry which can be connected to an exchange line. In particular, in general, it is a condition that the current drawn from the exchange line should be relatively small, and typically, should not exceed 120 microamps.

This imposes significant restrictions on the type of monitoring circuits which can be used for determining the status of an exchange line.

There is therefore a need for a monitoring circuit for monitoring the status of an exchange line which complies with the conditions imposed by telephone network providers, which effectively and efficiently determines the status of the exchange line.

The present invention is directed towards providing such a monitoring circuit, and the invention is also directed towards providing a monitoring circuit for monitoring the line voltage on any line.

According to the invention there is provided a monitoring circuit for determining if a voltage exists on a line, the monitoring circuit comprising a charge circuit for connecting to the line, the charge circuit being chargeable by the line voltage, a signal generating means for generating a plurality of trigger signals at time intervals, a discharge means for discharging the charge circuit1 the discharge means being responsive to each trigger signal for discharging the charge circuit, a control means for altering the time interval between the trigger signals for altering the charge to which the charge circuit may be charged1 and a monitoring means for monitoring the charge on the charge circuit as the time intervals are varied for determining the line voltage.

In one embodiment of the invention the discharge means comprises a discharge circuit connected to the charge circuit. Preferably, a first switch means is located in the discharge circuit, the first switch means being responsive to the trigger signals for closing the discharge circuit for discharging the charge circuit in response to each trigger signal. Advantageously, the first switch means is provided by a transistor of a first optocoupler, a light emitting diode of the first optocoupler being responsive to the trigger signal.

In another embodiment of the invention the monitoring means comprises an output means for providing an output signal in response to the charge to which the charge circuit is charged between trigger signals. Preferably, the output means is responsive to the charge to which the charge circuit is charged exceeding a predetermined charge.

In another embodiment of the invention the output means comprises a voltage comparator having an output pin on which the output signal is provided, and a pair of input pins for receiving respectively a reference voltage and an applied voltage, the applied voltage being responsive to the charge in the charge circuit, the output signal on the output pin changing from one state of a high and low state to the other of the two states, each time the value of the voltage on the applied voltage pin crosses the value of the voltage on the reference voltage pin.

Preferably, the applied voltage is applied to the applied voltage pin of the voltage comparator in response to the level of current flowing in the discharge means during discharge of the charge circuit.

In a further embodiment of the invention a second switch means controls the applied voltage which is applied to the applied voltage pin of the voltage comparator, the second switch means being responsive to the level of current flowing in the discharge means exceeding a predetermined value.

In one embodiment of the invention the second switch means comprises a transistor of a second optocoupler, a light emitting diode of the second optocoupler being located in the discharge means so that current flowing in the discharge means flows through the light emitting diode of the second optocoupler for switching the transistor of the second optocoupler.

In a further embodiment of the invention the signal generating means comprises a pulse generator for generating a plurality of trigger pulses for acting as the respective trigger signals.

Preferably, the control means comprises a timing means for timing the intervals between the respective trigger signals.

Advantageously, the intervals between the trigger signals timed by the timing means are of decreasing duration.

In one embodiment of the invention the charge circuit comprises a charge capacitor. Preferably, the charge circuit comprises a means for controlling the rate at which the charge capacitor is charged. Advantageously, the means for controlling the charge rate of the charge capacitor comprises at least one resistor in the charge circuit in series with the charge capacitor.

Preferably, a means for controlling the rate at which the charge capacitor is discharged is provided. Advantageously, the means for controlling the discharge rate of the charge capacitor comprises at least one resistor in the discharge means in series with the charge capacitor.

In one embodiment of the invention the monitoring circuit is adapted to distinguish between at least two different voltage levels on the line.

In another embodiment of the invention the monitoring circuit is adapted to distinguish between respective voltage ranges about at least two different voltage levels on the line.

In a further embodiment of the invention the monitoring circuit is adapted to determine a zero voltage state on the line.

In a still further embodiment of the invention the monitoring circuit is adapted for monitoring the status of a telephone line.

In a further embodiment of the invention the monitoring circuit is adapted for monitoring the status of an exchange line of a public telephone network.

In another embodiment of the invention the circuit comprises a storing means for storing an algorithm for determining line voltage from the time interval between the trigger signals at which the output signal from the output means fails to change from one state to the other state in response to a trigger signal, and a computing means for computing the line voltage using the algorithm from the time interval between the trigger pulses at which the output signal from the output means failed to change from one state to the other.

In a further embodiment of the invention a calibrating means is provided for calibrating the monitoring circuit, the calibrating means comprising an analogue means for reading the line voltage, and a means for computing a relationship factor for substituting into the algorithm stored in the storing means by substituting into the algorithm the read line voltage and the time interval between the trigger signals at which the output signal from the output means failed to change from one state to the other in response to a trigger signal.

In one embodiment of the invention the value of the time interval entered into the algorithm for determining the relationship factor is the. time interval between the trigger signals at which the output signal from the output means fails to change from one state to the other in response to a trigger signal as the time interval between the trigger signals is being decreased.

Additionally, the invention provides a PBX comprising the monitoring circuit according to the invention for determining the status of an exchange line to which the PBX is connected.

In one embodiment of the invention the PBX is adapted for receiving at least two exchange lines, and a means is provided for switching the monitoring circuit sequentially onto the exchange lines for determining the status of the respective exchange lines.

In another embodiment of the invention the calibrating means calibrates the monitoring circuit from a voltage on any one of the exchange lines.

The invention will be more clearly understood from the following description of a preferred embodiment thereof which is given by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram of a monitoring circuit according to the invention for monitoring the status of a plurality of exchange lines of a public telephone network, and Fig. 2 illustrates a graph (not to scale) of trigger pulses used in the monitoring circuit.

Referring to the drawings there is illustrated a monitoring circuit according to the invention which is indicated generally by the reference numeral 1 for determining the status of a plurality of exchange lines by determining the value of the line voltage on the exchange line. The monitoring circuit 1 is suitable for a PBX (not shown) having capacity for two exchange lines 5 of a public telephone network. The monitoring circuit 1 comprises a pair of terminals 2 and 3 for connecting to the A and B wires of the exchange lines 5. A switching circuit indicated in block representation by the broken line block 6 switches the terminals 2 and 3 of the monitoring circuit 1 between the A and B wires of the two exchange lines 5 under the control of a control means, namely, a microprocessor IC6 of the PBX. The switching circuit 6 also switches an analogue to digital port 16 of the microprocessor IC6 to one of the exchange lines 5, when that exchange line 5 has been seized. In this event it is not possible to connect the analogue to digital port 16 of the microprocessor IC6 to the other exchange line 5 for monitoring the line voltage thereon, since the voltage on the two exchange lines may be referenced from different grounds. While the analogue to digital port 16 of the microprocessor IC6 is switched to the seized exchange line 5, the monitoring circuit 1 is switched to the other exchange line 5 for monitoring the voltage on that exchange line 5.

A charge circuit 8 is connected to the terminals 2 and 3 through a bridge rectifier 9. The bridge rectifier 9 ensures that irrespective of how the terminals 2 and 3 are connected to the A and B wires of each of the exchange lines 5, current flow in the charge circuit 8 will always be in the same direction. A charge capacitor Cx is located in the charge circuit 8 and is charged through resistors R2, R3 and R4 by the DC voltage on the exchange line 5 to which the monitoring circuit 1 is connected. The resistors R2 and R3 are of appropriate value to limit the current drawn from the exchange lines 5 by the monitoring circuit 1 to a level which complies with the requirements of the public telephone network provider. The resistors R2, R3 and R4 control the rate at which the charge capacitor Cx is charged. In this embodiment of the invention the capacitor Cx is of 470 nanofarads, and the resistors R2 and R3 are each of 2.2 Mohms, and the resistor R4 is of 470 ohms.

A discharge means, namely, a discharge circuit 10 discharges the charge capacitor Cx through the resistor R4 and a resistor R5.

The resistors R4 and R5 control the rate at which the charge capacitor Cx is discharged. The resistor R5 is of 47 ohms. A first switch means, namely, a first transistor TR1 of a first optocoupler IC1 closes the discharge circuit 10 for discharging the charge capacitor Cx. The first transistor TR1 is responsive to a light emitting diode D1 of the first optocoupler IC1 for closing the discharge circuit 10 in response to respective trigger signals, namely, trigger pulses which are applied to a line 11 by a signal generating means, namely, a signal generator of the microprocessor IC6. The trigger pulses are of 5 milliseconds duration and are applied to the line 11 at intervals which are timed by the microprocessor IC6, see Fig. 2. The time interval between the pulses is varied by the microprocessor IC6 and is reduced from a time interval of 315 milliseconds until the value of the line voltage has been determined as will be described below. The duration of each trigger pulse of 5 milliseconds is sufficient for discharging the charge capacitor Cx completely while each trigger pulse is applied, irrespective of the value of the line voltage up to the typical maximum value of 57V. A supply voltage Vs is connected to ground through the light emitting diode D1 of the first optocoupler IC1 and through a resistor R9 and a transistor TR3. The trigger pulses from the signal generator IC3 are applied to the base of the transistor TR3 for switching on the transistor TR3 while each trigger pulse remains on the base of the transistor TR3. The transistor TR3 while each trigger pulse remains on its base, conducts the voltage Vs to ground through the light emitting diode D1 of the first optocoupler IC1, thereby switching on the transistor TR1 of the first optocoupler IC1. A biasing resistor R6 between the base and the emitter of the first transistor TR1 holds the first transistor TR1 off when there is no current flowing in the light of emitting diode D1.

A monitoring means comprising an output means, which is provided by a voltage comparator, namely, an op-amp IC4 is responsive to the current flowing in the discharge circuit 10, and provides a high output signal on an output pin 14 of the op-amp IC4 when the level to which the charge capacitor Cx is charged between discharges exceeds a predetermined level. The charge capacitor Cx is charged by the line voltage between the time the first transistor TR1 opens the discharge circuit 10 and the time the next trigger pulse is applied to the line 11 for closing the first transistor TR1 which closes the discharge circuit 10, in other words, between the end of each trigger pulse and the beginning of the next trigger pulse. Accordingly, the longer the time interval between trigger pulses for a given line voltage the greater will be the charge on the charge capacitor Cx, subject to its saturation level. Accordingly, the value of the line voltage can be determined from the value of the time interval between the trigger pulses at which the output signal on the output pin 14 of the op-amp IC4 fails to go high as the time interval between the trigger pulses is being reduced.

A reference voltage, which is derived from the supply voltage Vs through a potential divider comprising resistors R11 and R12 of equal value is applied to a reference input pin, namely, pin 12 of the op-amp IC4. Accordingly, the reference voltage applied to the pin 12 is equal to half the supply voltage Vs. The supply voltage Vs is applied to an applied voltage input pin, namely, input pin 13 of the op-amp IC4 through a resistor R8, which holds the input pin 13 above the reference voltage on the pin 12, and this in turn causes a low output signal to appear on the output pin 14.

A second switch means, namely, a second transistor TR2 of a second optocoupler IC2 controls the voltage applied to the input pin 13 of the op-amp IC4 in response to the current flowing in the discharge circuit 10. Current flowing in the discharge circuit 10 flows through a light emitting diode D2 of the second optocoupler IC2, and on the current in the discharge circuit 10, which is proportional to the charge on the charge capacitor Cx, exceeding a predetermined level, the second transistor TR2 is switched on.

The second transistor TR2 holds the input pin 13 of the op-amp IC4 at approximately ground voltage for so long as the second transistor TR2 is switched on. During periods when the second transistor TR2 is switched off, the voltage applied to the input pin 13 of the op-amp IC4 is substantially equal to the supply voltage Vs. Accordingly, depending on the conducting state of the second transistor TR2, the voltage applied to the input pin 13 of the op-amp IC4 is either approximately equal to the supply voltage Vs or ground. This, thus, causes the applied voltage on the input pin 13 of the op-amp IC4 to cross over the reference voltage on the reference pin 12 each time the second transistor TR2 changes state. When the applied voltage on the input pin 13 falls below the reference voltage on the reference pin 12, the output signal on the output pin 14 of the op-amp IC4 goes high indicating that the second transistor TR2 is conducting. The second transistor TR2 conducts for so long as the current flowing through the light emitting diode D2 of the second optocoupler IC2 is above a predetermined value. Since the current flowing through the light emitting diode D2 of the second optocoupler IC2 is a function of the charge on the charge capacitor Cx, the output signal on the output pin 14 of the op-amp IC4 during each trigger pulse is thus responsive to the level to which the charge capacitor Cx had been charged prior to that trigger pulse. The level to which the charge capacitor Cx is charged is a function of the line voltage and the time interval between the trigger pulses. Accordingly, by knowing the time interval between two trigger pulses at which the output signal on the output pin 14 of the op-amp IC4 fails to go high for the first time as the time interval is being reduced, a determination of the line voltage can be made.

A resistor R7 connected between the base of the second transistor TR2 and ground holds the transistor TR2 switched off while the current flowing through the light emitting diode D2 is below the predetermined value for switching on the transistor TR2. A resistor R14 which connects the output pin 14 of the op-amp IC4 to the supply voltage Vs and a resistor R13 which connects the output pin 14 to the reference pin 12 provide for hysteresis in the circuit.

The output signal on the output pin 14 of the op-amp IC4 is read by the microprocessor IC6. An algorithm is stored in the microprocessor IC6 for determining the line voltage from the time interval between the trigger pulses at which the output signal on the output pin 14 of the op-amp IC4 fails to go high for the first time during a trigger pulse as the time interval between trigger pulses is being reduced. Before the algorithm stored in the microprocessor IC6 can be used for determining the line voltage the monitoring circuit 1 must be initially calibrated. The calibration of the monitoring circuit 1 is described in detail below.

To determine the status of either one of the exchange lines 5 the terminals 2 and 3 of the monitoring circuit 1 are switched to the A and B wires of that exchange line 5 by the switching circuit 6 under the control of the microprocessor IC6. Initially the trigger pulses are applied to the line 11 by the signal generator 3 at the time intervals of 315 milliseconds, see Fig. 2. If the line voltage on the exchange line is zero the output signal on the output pin 14 of the op-amp IC4 remains low. This is because the charge capacitor Cx remains uncharged, and accordingly, when the first transistor TR1 of the first optocoupler IC1 is switched on by the trigger pulse to close the discharge circuit 10 no current flows in the discharge circuit 10. Thus, the second transistor TR2 of the second optocoupler IC2 remains switched off, and the voltage applied to the input pin 13 of the op-amp IC4 is at Vs.

Accordingly, a low output signal on the output pin 14 of the opamp IC4 during a trigger pulse on the line 11, and the time interval between the trigger pulses is 315 milliseconds indicates that the status of the exchange line being tested is disconnected from the public telephone network. The time interval of 315 milliseconds between each trigger pulse is chosen to be more than sufficient to allow the charge capacitor Cx to charge to a sufficient level for causing the output signal on the output pin 14 of the op-amp IC4 to go high even when the line voltage on the exchange line 5 being tested is as low as 4V.

At a time interval of 315 milliseconds between trigger pulses, should the output signal on the output pin 14 of the op-amp IC4 go high after each trigger pulse, this indicates that there is a voltage on the exchange line 5 being tested. Under the control of the microprocessor IC6 the time interval between the trigger pulses is gradually reduced until the output signal on the output pin 14 of the op-amp 4 fails to go high during a trigger pulse.

The time of the interval between the trigger pulses at which this occurs is stored and substituted into the algorithm for determining the line voltage from the length of the time interval, and the line voltage is computed. If the computed line voltage lies in the range 4.8V to 24.5V the status of the exchange line 5 being tested is determined as being connected to the public telephone network and in the seized state. Should the voltage determined from the algorithm lie in the range 40V to 57V, then the status of the exchange line 5 being tested is determined as being connected to the public telephone network but in the idle state.

The calibration of the monitoring circuit is carried out by a calibration means, which determines a relationship factor to be substituted into the algorithm so that the line voltage can be determined by the algorithm from the time interval at which the output signal on the output pin 14 of the op-amp IC4 failed to go high during a trigger pulse. The calibration is carried out as follows. The monitoring circuit 1 and the analogue to digital port 16 of the microprocessor IC6 are switched to one of the exchange lines 5 on which there is a voltage by the switching circuit 6. Commencing with a time interval of 315 milliseconds between trigger pulses, the trigger pulses are applied to the line 11 and are gradually reduced. The microprocessor IC6 reads the output signal on the output pin 14 of the op-amp IC4, and also reads the line voltage appearing on the analogue to digital port 16. The microprocessor IC6 stores the line voltage appearing on the analogue to digital port 16 and the time interval between trigger pulses at which the output signal on the output pin 14 fails1 for the first time to go high during a trigger pulse. The stored analogue value of the line voltage and the stored time interval are then substituted into the algorithm, and the relationship factor between the line voltage and the time interval is computed. The relationship factor is then substituted into the algorithm which can then be subsequently used for determining line voltage from the relevant time interval between the trigger pulses. When the monitoring circuit 1 has been calibrated once from either of the exchange lines 5 no further calibration is required.

The microprocessor IC6 comprises appropriate computer software for carrying out the calibrating function initially, and for controlling the monitoring circuit. Such software will be readily apparent to those skilled in the art.

The advantages of the monitoring circuit according to the invention are many. When the monitoring circuit is used in a PBX for determining the status of one or more of the exchange lines to which the PBX is connected, the monitoring circuit provides for monitoring the status of one or more of the exchange lines, while the analogue to digital port of the microprocessor is switched to another of the exchange lines. This is a significant advantage, since the microprocessor can only monitor the line voltage on one exchange line at one time. Typically, the analogue to digital port of the microprocessor is connected to a seized exchange line.

Another advantage of the monitoring circuit according to the invention is achieved by providing the simple calibration means.

By providing for calibration of the monitoring circuit while the monitoring circuit is in use in a PBX or other circuitry, off-theshelf optocouplers and other off-the-shelf components can be used in the monitoring circuit. Without providing the facility for calibrating the monitoring circuit, each monitoring circuit would have to be calibrated on the production line, and this would be a relatively costly exercise, or alternatively, optocouplers capable of operating within very tight tolerances would have to be selected for the first and second optocouplers of the monitoring circuit.

While the monitoring circuit has been described for monitoring the line voltage on a line for determining the status of an exchange line, it will be readily apparent to those skilled in the art that the monitoring circuit could be provided for monitoring line voltage on any line, and while the monitoring circuit has been described for monitoring three states of an exchange line, in other words, zero volts, a first voltage level within a predetermined range, and a second voltage level within a second predetermined range, it will be readily apparent that the monitoring circuit may be provided for monitoring only two states, or indeed, many states of a line. It will also be appreciated that the monitoring circuit could be used in a PBX adapted for receiving more than two exchange lines.

It will also be appreciated that the line voltage could be determined by reading the output signal on the output of the opamp while the time interval between the trigger pulses is being increased. In which case, the time interval between two pulses at which the output signal from the op-amp went high for the first time would be recorded, and the line voltage would be computed by the algorithm from that time interval. However, the advantage of commencing with a long time interval between trigger pulses and reducing the time interval is that a zero volt condition on a line can be initially detected, and on such detection being made, there is no need to proceed with the test by reducing the time interval between trigger pulses.

The invention is not limited to the embodiment hereinbefore described which may be varied in construction and detail.

Claims (31)

1. A monitoring circuit for determining if a voltage exists on a line, the monitoring circuit comprising a charge circuit for connecting to the line, the charge circuit being chargeable by the line voltage, a signal generating means for generating a plurality of trigger signals at time intervals, a discharge means for discharging the charge circuit, the discharge means being responsive to each trigger signal for discharging the charge circuit, a control means for altering the time interval between the trigger signals for altering the charge to which the charge circuit may be charged, and a monitoring means for monitoring the charge on the charge circuit as the time intervals are varied for determining the line voltage.

2. A monitoring circuit as claimed in Claim 1 in which the discharge means comprises a discharge circuit connected to the charge circuit.

3. A monitoring circuit as claimed in Claim 2 in which a first switch means is located in the discharge circuit, the first switch means being responsive to the trigger signals for closing the discharge circuit for discharging the charge circuit in response to each trigger signal.

4. A monitoring circuit as claimed in Claim 3 in which the first switch means is provided by a transistor of a first optocoupler, a light emitting diode of the first optocoupler being responsive to the trigger signal.

5. A monitoring circuit as claimed in any preceding claim in which the monitoring means comprises an output means for providing an output signal in response to the charge to which the charge circuit is charged between trigger signals.

6. A monitoring circuit as claimed in Claim 5 in which the output means is responsive to the charge to which the charge circuit is charged exceeding a predetermined charge.

7. A monitoring circuit as claimed in Claim 5 or 6 in which the output means comprises a voltage comparator having an output pin on which the output signal is provided, and a pair of input pins for receiving respectively a reference voltage and an applied voltage, the applied voltage being responsive to the charge in the charge circuit, the output signal on the output pin changing from one state of a high and low state to the other of the two states, each time the value of the voltage on the applied voltage pin crosses the value of the voltage on the reference voltage pin.

8. A monitoring circuit as claimed in Claim 7 in which the applied voltage is applied to the applied voltage pin of the voltage comparator in response to the level of current flowing in the discharge means during discharge of the charge circuit.

9. A monitoring circuit as claimed in Claim 7 or 8 in which a second switch means controls the applied voltage which is applied to the applied voltage pin of the voltage comparator, the second switch means being responsive to the level of current flowing in the discharge means exceeding a predetermined value.

10. A monitoring circuit as claimed in Claim 9 in which the second switch means comprises a transistor of a second optocoupler, a light emitting diode of the second optocoupler being located in the discharge means so that current flowing in the discharge means flows through the light emitting diode of the second optocoupler for switching the transistor of the second optocoupler.

11. A monitoring circuit as claimed in any preceding claim in which the signal generating means comprises a pulse generator for generating a plurality of trigger pulses for acting as the respective trigger signals.

12. A monitoring circuit as claimed in any preceding claim in which the control means comprises a timing means for timing the intervals between the respective trigger signals.

13. A monitoring circuit as claimed in Claim 12 in which the intervals between the trigger signals timed by the timing means are of decreasing duration.

14. A monitoring circuit as claimed in any preceding claim in which the charge circuit comprises a charge capacitor.

15. A monitoring circuit as claimed in Claim 14 in which the charge circuit comprises a means for controlling the rate at which the charge capacitor is charged.

16. A monitoring circuit as claimed in Claim 15 in which the means for controlling the charge rate of the charge capacitor comprises at least one resistor in the charge circuit in series with the charge capacitor.

17. A monitoring circuit as claimed in any of Claims 14 to 16 in which a means for controlling the rate at which the charge capacitor is discharged is provided.

18. A monitoring circuit as claimed in Claim 17 in which the means for controlling the discharge rate of the charge capacitor comprises at least one resistor in the discharge means in series with the charge capacitor.

19. A monitoring circuit as claimed in any preceding claim in which the monitoring circuit is adapted to distinguish between at least two different voltage levels on the line.

20. A monitoring circuit as claimed in Claim 19 in which the monitoring circuit is adapted to distinguish between respective voltage ranges about at least two different voltage levels on the line.

21. A monitoring circuit as claimed in Claim 19 or 20 in which the monitoring circuit is adapted to determine a zero voltage state on the line.

22. A monitoring circuit as claimed in any preceding claim in which the monitoring circuit is adapted for monitoring the status of a telephone line.

23. A monitoring circuit as claimed in any preceding claim in which the monitoring circuit is adapted for monitoring the status of an exchange line of a public telephone network.

24. A monitoring circuit as claimed in any preceding claim in which the circuit comprises a storing means for storing an algorithm for determining line voltage from the time interval between the trigger signals at which the output signal from the output means fails to change from one state to the other state in response to a trigger signal, and a computing means for computing the line voltage using the algorithm from the time interval between the trigger pulses at which the output signal from the output. means failed to change from one state to the other.

25. A monitoring circuit as claimed in Claim 24 in which a calibrating means is provided for calibrating the monitoring circuit, the calibrating means comprising an analogue means for reading the line voltage, and a means for computing a relationship factor for substituting into the algorithm stored in the storing means by substituting into the algorithm the read line voltage and the time interval between the trigger signals at which the output signal from the output means failed to change from one state to the other in response to a trigger signal.

26. A monitoring circuit as claimed in Claim 25 in which the value of the time interval entered into the algorithm for determining the relationship factor is the time interval between the trigger signals at which the output signal from the output means fails to change from one state to the other in response to a trigger signal as the time interval between the trigger signals is being decreased.

27. A monitoring circuit for monitoring line voltage on a line1 the monitoring circuit being substantially as described herein with reference to and as illustrated in the accompanying drawings.

28. A PBX comprising the monitoring circuit as claimed in any preceding claim for determining the status of an exchange line to which the PBX is connected.

29. A PBX as claimed in Claim 28 in which the PBX is adapted for receiving at least two exchange lines, and a means is provided for switching the monitoring circuit sequentially onto the exchange lines for determining the status of the respective exchange lines.

30. A PBX as claimed in Claims 28 or 29 in which the calibrating means calibrates the monitoring circuit from a voltage on any one of the exchange lines.

31. A PBX substantially as described herein with reference to and as illustrated in the accompanying drawings.