IES65389B2 - A communicating circuit - Google Patents

Abstract

A communicating circuit (1) communicates a system phone (2) to a PBX (3) and comprises wires (5) and (6) for signalling between the system phone (2) and PBX (3). A transmitting circuit (10) in the system phone (2) applies a Manchester encoded digit signal on the wires (5) and (6) which is detected by a differential amplifier circuit (14) in a receiving circuit (12) in the PBX (3). On detection of a signal on the wires (5) and (6) the differential amplifier circuit (14) outputs a signal corresponding to the signal on the wires (5) and (6), which is, in turn, fed to a comparator circuit (15) where it is converted to a HCMOS logic signal suitable for feeding to a buffer of a microprocessor of the PBX. The differential amplifier circuit (14) detects a change in voltage between the wires (5) and (6) which remains constant irrespective of whether induced spurious signals appear on the wires (5) and (6).

Description

A communicating circuit The present invention relates to a communicating ► circuit for communicating a system phone with a private branch exchange (PBX), and in particular, for facilitating signalling between the system phone and the PBX.

In general, system phones which comprise a number of facilities over and above those which are normally found in a standard telephone require four wires for communicating with a PBX. Two wires are required for speech and ringing signals, and the other two wires, namely, signalling wires are required for signalling between the system phone and the PBX, typically, to facilitate signalling between the keyboard on a system phone, and the PBX, and to facilitate signalling from the PBX to the system phone for illuminating light emitting diodes on the system phene.

Signalling from the keyboard to a PBX, in general, is carried out using Manchester encoded digit signals.

Such signals are typically of amplitude of r, approximately 150 millivolts which are biased on a 46 volt DC supply provided on the two signalling wires. * Typically, the Manchester encoded digit signal is applied differentially to the two signalling wires by 365389 the system phone. The signal is collected from one of the two signalling wires and fed through appropriate analysing circuitry. In general, provided there are no adverse external influences on the circuitry, such analysing techniques are adequate. However, in practice, where an extension line has to be run over a reasonable distance from a PBX to a system phone, the extension line is vulnerable to signals being induced in the line by external sources, such as, for example, other electrical or electronic apparatus. Such induced signals can cause spikes and other imperfections, which distort the signals being carried on the extension line, thereby rendering such signals unrecognisable.

There is therefore a need for a communicating circuit for communicating a system phone with a PBX which is unaffected by spikes and the like.

The present invention is directed towards providing such a communicating circuit.

According to the invention there is provided a communicating circuit for communicating a system phone with a PBX, the communicating circuit comprising two wires of a telephone extension line for connecting the system phone with the PBX and for carrying a Manchester encoded digit signal between the system phone and the PBX, a differential amplifier connected to the two wires at one end thereof for detecting a variation in the voltage across the two wires caused by a Manchester encoded digit signal applied differentially to the two wires at the other end thereof and for outputting a signal corresponding to the detected Manchester encoded digit signal, a converting means for converting the output of the differential amplifier into a usable signal.

By virtue of the fact that the differential amplifier is connected to the two wires of the telephone extension line, the two wires form the inputs of the differential amplifier, thus, the differential amplifier detects a change in voltage on one wire relative to the other. Accordingly, spikes or other induced signals in the two wires of the extension line will not affect identification of the Manchester encoded digit signal. Since the two wires of the extension line run side by side if a signal is induced in one line from an external source, a similar signal is induced in the other line. Thus, by feeding the signal from the two wires to a differential amplifier the affect of the induced signal is eliminated.

Preferably, the differential amplifier comprises a means for attenuating the gain at high frequencies for filtering out high frequency signals so that the differential amplifier is immune to high frequency signals .

In one aspect of the invention the converting means converts the signal from the differential amplifier which corresponds to the Manchester encoded digit signal to a HCMOS compatible level for delivery into a buffer. Ideally, the signal from the conveying means is suitable for a 74 HC244HCMOS buffer.

In one aspect of the invention a DC voltage is maintained across the two wires.

Preferably, the differential amplifier is coupled to the two wires of the telephone extension line by a DC blocking means. Ideally, the DC blocking means comprises a pair of DC blocking capacitors.

In one aspect of the invention the converting means comprises a comparator.

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 block representation of a communicating circuit according to the invention, and Fig. 2 is a circuit diagram of part of the communicating circuit of Fig. 1.

Referring to the drawings there is illustrated a communicating circuit according to the invention indicated generally by the reference numeral 1 for communicating a system phone 2 with a PBX 3 for facilitating signalling between the system phone and the PBX, typically, signalling between a keyboard (not shown) of the system phone 2 and the PBX 3. The communicating circuit 1 comprises a two wire signalling system comprising two signalling wires 5 and 6 which connect the system phone 2 to the PBX 3. The wires 5 and 6 are two of a four wire extension line (not shown) between the system phone 2 and the PBX 3, the other two wires being used for speech. A -46 volt DC supply is provided on the wires 5 and 6, the wire 5 being connected to a 0 volts terminal of the -46 volts supply through a biasing resistor Rl in the PBX, and the wire 6 being connected to a -46 volts terminal of the supply through a biasing resistor R2. The value of the resistors Rl and R2 are selected to provide a voltage differential on the wires 5 and 6 of approximately 33 volts .

A transmitting circuit 8 under the control of a microprocessor (not shown) in the PBX applies signals for transmission to the system phone 2 on the wire 5. Such signals may be Manchester encoded digit signals which would typically be of amplitude of approximately 150 millivolts, and thus, biased on the -46 volt DC supply. Such signals may be for controlling light emitting diodes on the system phone 2 and for dealing with other signalling functions of the system phone 2.

A receiving circuit 9 in the system phone 2 receives and analyses signals from the transmitting circuit 8, and in combination with control circuitry (not shown) in the system phone 2 operates the appropriate light emitting diodes or other functions of the system phone 2 in response to the received signals.

A transmitting circuit 10 in the system phone 2 applies signals differentially on the wires 5 and 6 for transmission to the PBX 3. Such signals, would typically be signals from the keyboard (not shown) of the system phone 2, and in general, would be Manchester encoded digit signals of amplitude of approximately 150 millivolts biased on the -46 volt DC supply. A receiving circuit 12 in the PBX 3 receives and analyses signals on the wires 5 and 6 transmitted by the transmitting circuit 10.

The receiving circuit 12 comprises a differential amplifier circuit not illustrated in Fig. 1 but illustrated in Fig. 2 and indicated generally by the reference numeral 14 which is fed by the two wires 5 and 6 for detecting a change in the voltage across the two wires 5 and 6 caused by the Manchester encoded digit signal applied to the wires 5 and 6. On detecting such a change in voltage across the wires 5 and 6 the digital amplifier circuit 14 transmits a signal which corresponds with the specific digit signal on the wires 5 and 6 to a comparator circuit, not illustrated in Fig. 1 but illustrated in Fig. 2 and indicated generally by the reference numeral 15. The comparator circuit 15 converts the signal from the differential amplifier circuit 14 into a HCMOS compatible logic level signal for delivery to a 74 HC244HCMOS buffer (not shown) associated with the microprocessor (also not shown) in the PBX 3.

A DC blocking means, namely, a pair of DC blocking capacitors Cl and C2 couple the differential amplifier circuit 14 of the receiving circuit 12 to the two wires 5 and 6 for blocking the -46 volt DC supply. Input pins 1 and 2 of an integrated circuit differential amplifier IC1 of the differential amplifier circuit 14 are fed from the wires 5 and 6, respectively, through resistors R3 and R4 and a capacitor C3 and a resistor R5. The capacitor C3 and a capacitor C4 attenuate the gain of the differential amplifier circuit 14 at high frequencies, thereby providing immunity to high frequency differential signals which are not relevant. The resistors R5 and R6 co-operate with the capacitors C3 and C4, respectively, for attenuating the gain at high frequencies of the differential amplifier circuit 14. On the differential amplifier ICI detecting a Manchester encoded digit signal on the wires 5 and 6 corresponding output signal which corresponds to the signal on the wires 5 and 6 is fed from the output pin of the differential amplifier ICI to the comparator circuit 15. The output from the pin 3 of the differential amplifier ICI is fed into the positive pin of an integrated circuit comparator IC2 of the comparator circuit 15. The comparator circuit 15 as well as comprising the comparator IC2 also comprises resistors R7, R8, R9, RIO, Rll, R12 and R13. The comparator IC2 converts the signal received on the pin 4 from the differential amplifier ICI to a HCMOS logic signal which appears on pin 6 of the comparator IC2.

The signal on pin 6 is fed through a terminal 18 to the 74 HC244HCMOS buffer (not shown) which is associated with the microprocessor (also not shown) of the PBX 3. The resistors R7 to R13 provide biasing levels for the comparator circuit 15.

Terminals 19 and 20 of the receiving circuit 12 are connected to the power reference levels within the PBX. However, since these do not form part of the invention, no further explanation will be given.

In use, a Manchester encoded digit signal applied by the transmitting circuit 10 of the system phone 2 on the wires 5 and 6 is detected by the differential amplifier IC1, and a signal corresponding to the specific Manchester encoded digit signal is provided on the output pin 3 of the amplifier IC1 which is, in turn, fed to the input pin 4 of the comparator IC2.

The comparator IC2 converts the signal to a HCMOS logic signal which appears on the pin 6 of the comparator IC2, and which is, in turn, fed through a terminal 18 to the 74 HC244HCMOS buffer (not shown) associated with the microprocessor (net shown). The signal is then processed by the microprocessor (not shown) and appropriate action is taken in the PBX under the control of the microprocessor (not shown).

By virtue of the fact that the Manchester encoded digit signal on the wires 5 and 6 is detected by the differential amplifier circuit 14 which detects a change in voltage across the wires 5 and 6, there is no danger of the differential amplifier circuit 14 failing * to detect the signal, since irrespective of whether spurious signals are induced on the wires 5 and 6, the i change in voltage across the wires 5 and 6 caused by 5 the Manchester encoded digit signal will remain constant, in other words, the Manchester encoded digit signal sits on top of what ever voltages are on the wires 5 and 6.

The invention is not limited to the embodiment 10 hereinbefore described which may be varied in construction and detail.

Claims (5)

1. A communicating circuit for communicating a system phone with a PBX, the communicating circuit comprising two wires of a telephone extension line for connecting 5 the system phone with the PBX and for carrying a Manchester encoded digit signal between the system phone and the PBX, a differential amplifier connected to the two wires at one end thereof for detecting a variation in the voltage across the two wires caused by 10 a Manchester encoded digit signal applied differentially to the two wires at the other end thereof and for outputting a signal corresponding to the detected Manchester encoded digit signal, a converting means for converting the output of the 15 differential amplifier into a usable signal.

2. A communicating circuit as claimed in Claim 1 in which the differential amplifier comprises a means for attenuating the gain at high frequencies for filtering out high frequency signals. 20

3. A communicating circuit as claimed in Claim 1 or 2 in which the converting means converts the signal from the differential amplifier which corresponds to the Manchester encoded digit signal to a HCMOS compatible level for delivery into a buffer.

4. A communicating circuit as claimed in any preceding claim in which a DC voltage is maintained across the two wires, and the differential amplifier is coupled to the two wires of the telephone extension line by a DC 5 blocking means .

5. A communicating circuit substantially as described herein with reference to and as illustrated in the accompanying drawings .