GB2056092A

GB2056092A - Line signal monitor

Info

Publication number: GB2056092A
Application number: GB7927421A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1979-08-07
Filing date: 1979-08-07
Publication date: 1981-03-11
Also published as: GB2056092B

Abstract

In telephone systems it is often desirable to be able to monitor the conditions of the wires of the subscriber's lines served. One example of where this is needed is in a call data logger. In the arrangement described herein, for each wire to be monitored, of which each line has four, there is a high impedance detector. This detector has an operational amplifier OA to the input of which the wire to be monitored is connected as a high resistance potential divider R1-R2. The amplifier feeds a sample and hold circuit 3, to which is connected a storage capacitor 4. During a sample period, defined by a pulsed input, the output of the amplifier OA is sampled and stored on the capacitor 4. During the hold period the capacitor 4 is isolated from the amplifier OA and connected to a multiplexor plus analogue-digital connector 2, the latter carries a succession of digital outputs to be produced, one per wire to be monitored. Thus the wires to be monitored are sampled simultaneously and the results of the sampling read out successively. <IMAGE>

Description

SPECIFICATION Line signal monitor This invention relates to an electrical circuit for monitoring the electrical condition of a wire, one example of which is a wire of a telephone subscriber's line.

It is often necessary to be able to monitor the electrical condition of a wire, with a view to the determination of the voltage present on the wire.

Such monitoring is needed, among other cases, in remote control and telemetering systems, as well as in telecommunication systems. In all cases it is desirable for this monitoring to be effected without it having any significant effect on the condition of the wire being monitored.

An object of the invention is to provide a simple and economical circuit for providing the above function.

According to the invention there is provided an electrical circuit for monitoring the electrical condition of a wire, which includes a buffer to the input of which is connected the wire to be monitored the buffer having a high input impedance, and a sample and hold circuit to which the output of the buffer is connected, wherein the sample and hold circuit is controlled by a pulsed input waveform having a first and a second input condition, wherein during the first condition of the waveform the output of the buffer, which is an electrical condition corresponding to the voltage on the wire, is sampled, and the result of the sampling placed in a temporary storage device, and wherein the input waveform is in its second condition the condition stored in the temporary storage device is maintained and can be read from the monitoring circuit.

An embodiment of the invention will now be described with reference to the accompanying highly schematic, drawing.

The invention is described in its application to the monitoring of the electrical conditions of the various wires of a telephone subscriber's line, and for each of the lines to be monitored there is provided a printed circuit card indicated schematically at 1 in the drawing. This card carries five identical monitoring circuits, one of which is shown inside the dashed line box 1 representing the printed circuit card. Five such circuits are provided, one for each of the four line wires and one for a grounded test input. The four line wires are the + and - speech wires, the usual P wire and a wire M for metering purposes.

At the right hand side the card 1 is connected to inputs to a unit 2 which has a multiplex input feeding an analogue to digital converter. This supplies an eight-bit digital output over the leads D1 to D8 to a micro-processor, which in the present case forms part of a call data recording equipment. This circuit 2 is also under the control of the microprocessor, as indicated by the inputs thereto marked "Control from ,toP". Note that if the data handling conditions permit, more than one card such as 1 can feed a unit such as 2.

The circuits on the card, one per wire to be monitored, are as already mentioned identical, so only one is shown. At its input the wire to be monitored is connected to the upper end of a potential divider formed by resistors R1 and R2, which in the circuit shown are of 2 megohms and 82 kilohms respectively, so that the input to the circuit has a high impedance. The input voltage can be measured in the range +40 volts to -88 volts, being the voltage present on the wire to be monitored. The voltage Vb at the lower end is 3.5 volts in the example shown, this voltage being so chosen that the voltage present at the centre point of R1 and R2 lies in the range of 0 volts to +5 volts.

The mid-point of the two resistors is connected to the non-inverting input of an operational amplifier OA, which serves as a buffer. This input is connected to ground via a 6.2 volt zener diode ZD, to protect the buffer against excessively high voltages on its input. The output of the buffer OA is connected back to its own inverting input, and also to a sample and hold circuit 3. This latter has a second input which is pulsed to control the operation of the circuit, the pulse waveform being shown in an inset to the drawing. The pulse has hold pulses of 1.5 ms duration, separated by sample periods of 4.3 ms each.

During operation, while the second input to the circuit 3 is in the Sample condition, the output of the buffer OA is connected via gating means of known type to the capacitor 4, which therefore charges to a level dependent on the voltage to be monitored. During the sampling period the capacitor 4 also acts as a high frequency noise suppressor. During the hold period, the capacitor 4 is isolated from the buffer OA, and connected to the input of the circuit 2 so that the latter responds to produce the digital output on the leads D1 to D8. The output from the circuit 3 to the circuit 2 is grounded via a catching diode D1 and connected to +5 volts via another catching diode D2, so that the voltage of this output is constrained to lie in the range of approximately O to +5 volts.

The test input and its monitor circuit serve to fix a reference level for the equipment fed by the leads D1 to D8 and also to test that that equipment is functioning correctly. In addition it allows the following equipment which in the present case includes a microprocessor to compensate for inaccuracies due to component tolerances, ageing and temperature effects since it provides a reference reading.

It will be seen that with the above arrangement, it is possible for a number of input wires to be sampled at the same time, while the results of the sampling are dealt with successively. An advantage of this simultaneous sampling of the wires of a telephone line is that in the subsequent analysis of the line voltages by the processor, the effects of longitudinal disturbances on the line can be eliminated by simple subtraction techniques.

The arrangement described herein provides to the following equipment, which in this case includes a microprocessor, the maximum amount of information on the potentials of the wire, and thus allows the microprocessor to use its intelligence to connect these potentials into telephoning signals. This overcomes the disadvantage of previous hand-wired arrangements which could only perform limited processing on the information first before presenting it to the microprocessor or equivalent with the telephoning signals.

Claims

1. An electrical circuit for monitoring the electrical condition of a wire, which includes a buffer to the input of which is connected the wire to be monitored, the buffer having a high input impedance, and a sample and hold circuit to which the output of the buffer is connected, wherein the sample and hold circuit is controlled by a pulsed input waveform having a first and a second input condition, wherein during the first condition of the waveform the output of the buffer, which is an electrical condition corresponding to the voltage on the wire, is sampled, and the result of the sampling placed in a temporary storage device, and wherein when the input waveform is in its second condition the condition stored in the temporary storage device is maintained and can be read from the monitoring circuit.

2. A circuit as claimed in claim 1, wherein the connection from the wire to be monitored is connected to the end of a high resistance potential divider, and wherein the buffer is an operational amplifier and the input to the buffer is connected to a tapping on the potential divider.

3. A circuit as claimed in claim 1 or 2, and wherein the temporary storage device is a capacitor.

4. An electrical circuit for monitoring the electrical condition of a number of wires, such as the wires of one or more telephone subscriber's lines, which includes a number of individual monitoring circuits each of which includes a buffer to which one of the wires to be monitored is connected, the buffers all having high input impedances, wherein each said buffer has its output connected to a sample and hold circuit controlled by a pulsed input waveform having a first condition and a second condition, the pulsed inputs to all of said sample and hold circuits being in phase, wherein when during the first condition of a said input waveform, the outputs of said buffers, each of which is an electrical condition corresponding to the condition of one of said wires, is sampled, and the result of the sampling placed in a temporary storage device, wherein when the input waveform is in its second condition the temporary storage devices are each isolated from the respective buffer and their outputs are applied to a multiplexor, and wherein the multiplexor connects the said outputs singly and successively to an analogue to digital converter, whose output is a succession of pulse code combinations each representative of the electrical condition of one of the wires to be monitored.

5. A circuit as claimed in claim 4, wherein the connection from each said wire is connected to an end of a high resistance potential divider, and wherein the input to each said buffer is connected to a tapping on the potential divider, each said buffer being an operational amplifier.

6. A circuit as claimed in claim 4 or 5, and wherein each said temporary storage device is a capacitor.

7. An electrical circuit for monitoring the condition of a number of wires, such as the wires of one or more telephone subscriber's lines, substantially as described with reference to the accompanying drawing.

New claims or amendments to claims filed on 31 Jan. 1980.

New or amended claims:- 8, 9, 10.

8. An electrical circuit unit for monitoring the electrical conditions of the wires of a telephone subscriber's line, which includes a number of individual monitoring circuits one for each wire of the line to be monitored, said monitoring circuits all being mounted in the same equipment unit, wherein each said monitoring circuit includes a buffer to which the respective one of the wires to be monitored is connected, the buffers all being high input impedance devices, wherein each said buffer has its output connected to a sample and hold circuit controlled by a pulsed input waveform having a first condition and a second condition, the pulsed inputs to all of the sample and hold circuits all being in phase, wherein during the first condition of the said pulsed input waveform the outputs of said buffers, each of which is an electrical condition corresponding to the voltage on one of said wires, is sampled and the result of the sampling placed in a temporary storage device, wherein the pu!sed input waveform is in its second condition the temporary storage devices are each isolated from the respective buffer and their outputs are applied to a multiplexor, wherein the multiplexor connects the outputs singly and successively to an analogue-to-digit converter, whose output is a succession of pulse code combinations each representative of the electrical condition of one of the wires of the said line to bes monitored, and wherein the said pulse code combinations are applied to processing means which executes tests thereon to assess the current state of the subscriber's line being monitored.

9. A circuit as claimed in claim 8, and wherein the equipment unit includes an additional one of said monitoring circuits whose input is connected to a reference potential, such as ground, the output code combination due to the additional monitoring circuit being used as a reference by the said processing means.

1 0. A circuit as claimed in claim 8 or 9, wherein the connection from each said wire is connected to the end of a high resistance potential divider forming part of that wire's one of said monitoring circuits, wherein the input of each said buffer is connected to a tapping on the respective one of said potential dividers, wherein each said buffer is an operational amplifier, and wherein each said temporary storage device is a capacitor.