GB2237702A - Ringing generator circuit - Google Patents

Abstract

A private automatic branch exchange (PABX) telephone system comprises: a power supply, an extension feed, an exchange circuit, a crosspoint switching matrix, a relay switch matrix, a ringing generator circuit, a dual tone multi-frequency (DTMF) receiver circuit, a call progress decoder (CPD), a microcomputer and a microcomputer control circuit. The ringing generator circuit includes a triac shunted across said circuit, the triac being operable to clamp any high inductive loads produced by a ringing bell when disconnected, thereby preventing high voltages being applied to the microcomputer. The DTMF receiver circuit includes means for polling "X" DTMF receivers between "X + Y" extensions. The polling means includes means for determining if a DTMF receiver is being utilised for manual dialling, and means, which on determination of the use of manual dialling, multiplex two extensions with said DTMF receiver. Also the polling means include means for interrogating the DTMF receivers in use for determining if an automatic dialling sequence is approaching a conclusion and means for reserving such a receiver for a particular extension.

Description

IMPROVEMENTS IN OR RELATING TO A PRIVATE AUTOMATIC BRANCH EXCHANGE (PABX) TELEPHONE SYSTEM The present invention relates to a private automatic branch exchange (PABX) telephone system.

The invention provides a private automatic branch exchange (PABX) telephone system comprising: a power supply, an extension feed, an exchange circuit, a crosspoint switching matrix, a relay switch matrix, a ringing generator circuit, a dual tone multi-frequency (DTMF) receiver circuit, a call progress decoder (CPD), a microcomputer and a microcomputer control circuit, characterized in that the ringing generator circuit includes a triac shunted across said circuit, the triac being operable to clamp any high inductive loads produced by a ringing bell when disconnected, thereby preventing high voltages being applied to the microcomputer.

Advantageously, the DTMF receiver circuit includes means for polling "X" DTMF receivers between "X + extensions, said polling means including means for determining if a DTMF receiver is being utilised for manual dialling, and means, which on determination of the use of manual dialling, for multiplexing two extensions with said DTMF receiver.

Preferably, the means for polling the DTMF receivers includes means for interrogating the DTMF receivers in use for determining if an automatic dialling sequence is approaching a conclusion and means for reserving such a receiver for a particular extension.

Conveniently, the extension feed circuits, which are operational when the PABX system is in an internal mode (i.e. on an internal call, programming or when idle), are part of a driver hybrid circuit which ensures, when the extension phone is on open circuit, that the voltage across the telephone is at the voltage required for an external exchange, and when the phone is active, the voltage is determined by its D C characteristics at a low current value resulting in the preservation of the necessary impedance to the telephone and providing low call path insertion loss.

Advantageously, the Call Progress Decoder (CPD) circuit comprises a separate low power circuit for each line which accepts AC signals from a telephone line, filters the signals and amplifies them before presenting said signals to the microcomputer control circuit for analysis, and the mircocomputer includes means, using an algorithm, for examining the input signals to determine the level and cadence of the tones present on the telephone line.

Preferably, at least one exchange line is connected to the PABX on two terminals of connectors, the or each line being switchable by relays depending on the conditions prevailing on each line, said relays providing four possible line terminations, namely: direct line to the relay switch matrix where the extension is on line, Idle/Off Line or in the case of a power failure; line is terminated in the "HOLD" circuit where the extension puts the exchange line on hold to make an enquiry or unattended transfer; line is presented with a resistance during decadic dialing make; and line is presented with a capacitance as a spark quench during decadic dialing break, and wherein in each case the exchange line status is monitored by the microcomputer which uses these signals to determine the line condition and associated signalling.

Advantageously, the exchange circuit contains the hold circuit within a buffer hybrid circuit, said hold circuit providing the correct "HOLD" DC and AC characteristics and injecting appropriate tone signals into the exchange line such as a cadenced tone to the public switched telephone network (PSTN), said hold circuit utilising the tone generator and under microcomputer control enabling and disabling the tone using a crosspoint switch.

Preferably, an intercom switch matrix is provided comprising two crosspoint circuits for accepting a plurality of extension line AC signals and two crosspoint circuits which accept AC signals via series resistors, said matrix having at least two lines as independent intercom speech paths, one line for injecting the dial tone to the extensions and the remaining line or lines being used to connect the extensions to one of a plurality of DTMF receivers, said matrix being managed by microcomputer control of the individual crosspoint switches enabling the microcomputer to select and strobe each crosspoint individually.

Conveniently, the DTMF receiver circuit comprises three separate DTMF receivers which monitor for and decode the DTMF signalling on each active extension and line, and are selectable by the microcomputer which further interfaces with the DTMF receivers, insofar as that when a valid DTMF signal is detected for the required duration, the microcomputer is signaled and reads the data via a binary to decimal decoder, the data being considered valid until the DTMF receivers are released.

Advantageously, the PABX system uses unbalanced internal voltages but presents balanced voltages at each exchange line.

Preferably, the PABX system has a hold circuit which interfaces with an exchange line via a full wave bridge rectifier, wherein a zenor diode and a decoupling capacitor clamp the voltage presented to the transistor circuit so that the transistor present in the circuit acts as a constant current sink, whereby the DC characteristics of this circuit are defined at low input voltages before the zenor diode conducts by a single resistor and the AC impedance is scaled by an external resistor.

The invention will hereinafter be more particularly described with reference to the accompanying drawings, which show by way of example only, one embodiment of a PABX according to the invention, with particular emphasis being given to the novel aspects of the invention. In the drawings: Figure 1 (i.e. Figs. IA, 1B and 1C combined) is a schematic diagram of a portion of the circuit of the PABX according to the invention; Figure 2 (i.e. Figs. 2A, 2B and 2C combined) is a schematic diagram of a further section of the circuit showing connections for eight internal phones and three external telephone lines; Figure 3 (i.e. Figs. 3A, 3B and 3C combined) is a schematic diagram of a further section of the circuit in respect of a module for the third telephone line; Figure 4 (i.e.Figs. 4A, 4B and 4C combined) is a schematic diagram of a driver hybrid circuit of the PABX according to the invention; and Figure 5 (i.e. Figs. 5A, 5B and 5C combined) is a schematic diagram of the buffer hybrid circuit of the PABX according to the invention.

Referring to the drawings, the PABX according to the invention will now be generally described with particular reference being given subsequently to the novel aspects of the invention.

The PABX of the invention is a factory modular PABX system with a maximum capacity of three exchange lines and eight extensions. The exchange ports are modular in steps of one from one to three lines while the extensions are modular in steps of two from two to eight ports. The system is housed in a ABS plastics case and is controlled by an advanced microcomputer. Up to three simultaneous calls on the external lines and a maximum of four internal calls are possible i.e. the system is non blocking. All calls are private. The system accepts standard two wire phones, both dual tone multi-frequency (DTMF) and decadic and standard two wire exchange lines again, both DTMF and decadic. Power is supplied via a two core mains cable, with an integral two pin plug. The system has an easy to use numbering plan and incorporates all the advanced features normally found on much larger systems.

POWER SUPPLY All the required internal DC power supplies (6V, 24V and 48V DC) and ringing signals are generated by an integral linear power supply. The supply circuit accepts the 220V AC, 50Hz mains input and transforms these down to provide two low voltage outputs of 23 Vrms and 63 Vrms.

The two AC supplies are rectified and smoothed to provide 30V DC and 80V DC unregulated supplies. The 30V DC supply is first regulated to provide 24V DC and then further regulated to provide 5V DC. The 80V DC supply is regulated by a third regulator to provide 48V DC. The 63V AC low voltage output is also used as a supply for the ringing generator circuit.

EXTENSION FEED The extension feed circuit is realised by a constant current source in the A leg contained in the driver hybrid circuit and signalling detection resistors in the B leg.

There are eight such circuits in the PABX of the invention, one for each extension.

The purpose of this circuit is to power the phone during internal calls, detect all internal signalling conditions and present a path for voice and DTMF signalling to the crosspoint switch matrix. The B leg resistors detect the current in the extension loop and convert this to a proportional voltage centred about 2.5V.

This is presented to one of the microcomputer A/D ports, to enable the controller to determine the status of the extension during ringing, during calls and during dialing.

EXCHANGE CIRCUIT Each of the three incoming lines is terminated in an exchange circuit. The purpose in an exchange is to present the exchange with a correct termination during each phase of line operation. The circuit comprises a buffer hybrid circuit (Fig. 5), two relays and associated components.

There are four possible terminations for the line realised by the four possible relay combinations. These four conditions are: 1. Idle and On Line (both the same) 2. Hold 3. Decadic Dialing Make 4. Decadic Dialing Break The discrete components associated with the two relays realise the AC and DC termination impedances while the buffer hybrid circuit provides the hold circuit and four other buffer circuits necessary for the microcontroller to control and monitor the line. These are: 1. Line Voltage Detection 2. Dial Tone Detection 3. DTMF Detection 4. Tone/Speech Injection CROSSPOINT SWITCH MATRIX This switch matrix, (see Fig. 1) under microcomputer control, makes all the internal call paths as well as presenting the DTMF signals from both line and extensions to one of three DTMF receivers. A third function is to switch the tone generator to one of three lines.The matrix comprises five crosspoint switch integrated circuits (IC's) each containing sixteen switches arranged in a 4 x 4 matrix. The first four crosspoint switches are arranged in two banks of two, so that there are eight vertical and eight horizontal switches in a matrix. The eight verticals are each assigned to an individual extension. The first three horizontals are assigned to the three DTMF receivers so that a receiver can be assigned to an extension. The next horizontal is assigned to a dial tone generator so that dial tone can be injected to the extensions. The remaining four horizontals are used as internal call paths so that each of the four extensions can talk to the other four extensions.The first four horizontals also connect to the four horizontals on the fifth crosspoint switch which enables the system to monitor the DTMF signalling on the three lines and to inject tones into the lines when required.

RELAY SWITCH MATRIX This switch matrix comprises thirty two relays arranged in a matrix of four horizontal and eight verticals. The three line circuits connect to three of the horizontals while the fourth is connected to the ringing generator. The eight verticals connect the eight extension circuits to the eight pairs of connector terminals. In this way the four relays associated with each extension can present the phone with one of five possible paths.

1. Connection to line 1 2. Connection to line 2 3. Connection to line 3 4. Connection to ringing extension 5. Connection to extension circuit The relays are controlled via six relay driver IC's each containing seven drivers. These are in turn controlled by six shift register IC's each containing eight outputs. The drive circuit is controlled by the microcomputer serial bus which controls data, clock and output strobe.

RINGING GENERATOR CIRCUIT The ringing generator circuit accepts its 63Vrms, 50Hz input voltage from the power supply circuit and feeds this via a resistor to the ringing generator bus in the relay switch matrix.

The relays in the relay matrix present this signal to the individual extensions. However the relays do not generate the ringing cadence. This is performed by a thyristor circuit in the ringing generator which shorts and opens the output signal to the required cadence.

DTMF RECEIVER CIRCUIT This circuit comprises three DTMF Receiver Integrated circuits which are individually selected by the microcomputer via the crosspoint switch matrix to detect the presence of DTMF signalling on extensions and exchange lines. The circuits detect and decode valid DTMF signals and then inform the microcontroller that a new digit has been detected. The microcomputer then selects the required receiver and reads the data via a four bit bus.

DIAL TONE GENERATOR The tones generated within the PABX system of the invention are derived from an internal timer within the microcomputer. This is fed to an output port and filtered by a second order low pass filter.

The resulting sinusoidal tone is cadenced as required by the associated horizontal on the crosspoint switch matrix to provide the tone signals such as buzy, alert tone, ring tone, etc..

CALL PROGRESS DECODER The CPD (Call Progress Decoder) circuit resides in the buffer hybrid (Fig. 5). It accepts tones from the exchange line processes them and presents the resulting conditioned signal to the microcontroller for analysis.

The hardware circuit interfaces with the exchange line, buffers, the input, provides a third order high-pass filter, amplifies the tones and peak detects the resultant AC signal. The microcomputer then examines this DC level to determine the level and cadence of the tone on the line.

MICROCONTROLLER CIRCUIT The control circuit for the PABX of the invention comprises a 78010 microcomputer and associated memory and interface circuits which realise an advanced, powerful program controller. The microcomputer is configured to interface with up to 32K bytes of Program EProm memory, and 8K bytes of RAM memory and a 256 byte EEProm memory.

The software program in machine code within the EProm controls the operation of the PABX system. The 8K bytes of RAM and the 256 bytes of internal RAM within the microcomputer are used for temporary storage of all program parameters. The non-volatile EEProm stores a copy of the user programmed parameters. The microcontroller via the analog ports and some of it's input ports scans the extension and exchange lines on a cyclic basis, processes the data and controls the call paths and port conditions via the output ports which control the operation of the crosspoint switch and relay matrices.

PRINTER PORT The printer port provides a two wire (signal ground and transmit) RS232 serial interface to the user printer.

Using this port the microcontroller sends basic call management information on the external calls being made by individual extensions to the printer. The printer port becomes active after each call is terminated.

AUXILIARY PORT A six bit auxiliary port is provided within the PABX system of the invention for future upgrade. This six bit port is directly under microcomputer control and will enable such features such as fax select and integral modem to be realised.

Having described the system, some of the particularly novel aspects of the invention will now be described in further detail.

1. RINGING SIGNAL GENERATION CIRCUIT This aspect of the invention is shown in the area generally designated 1 in Figure 1. The ringing signal generator accepts a 50Hz 63Vrms signal from transformer secondary SW2. Control of the ringing signal is achieved by the switching of triac TY1. Output port Q1 (Pin 4) of the serial latch (IC 19) switches the triac TY1 via resistors R23 and R25 and transistor TS1. With TS1 turned on, the triac TY1 is turned on and the ringing voltage is dropped across resistor R22. With TS1 turned off, triac TY1 is turned off and the ringing signal is presented to the ringing generator bus in the relay switch matrix (see Section 6).

Capacitor C7 and resistor R24 act as a snubber across the triac. Resistor R36 provides the triac gate current with a path when switching off.

The problem normally associated with ringing generator is the high inductive load produced by a ringing bell when it is disconnected. This high inductive load, which may produce a voltage kick of 300+ V must be clamped in order to prevent the microcomputer from crashing. The triac TY1 is shunted across the ringing generator circuit.

It does not need to be opto-isolated. When the triac TY1 is activated, the excess voltage is dissipated within approximately 40 milliseconds. After the triac TY1 is activated, the appropriate ringing relays RL5 to RL20 of whichever of the extensions E1-E8 is ringing may be operated without any risk of arcing between the contacts.

2. DRIVER HYBRID CIRCUIT The driver hybrid circuit of the invention shown in Figure 4 comprises two identical feed circuits which supply the necessary DC and AC currents and voltages to the extension telephone. These circuits are used in the PABX of the invention, when the extension is internal (i.e. on an internal call, programming or when idle). The circuit accepts the 24V DC and 48V DC supplies and drives a 25mA DC current into each extension. As a result the open circuit (on hook) voltage across the phone is 48V DC while the active (off hook) voltage is determined by the DC characteristics of the phone at 25mA. The circuit also provides an AC impedance transformation from the 600 Ohm phone to the 10K Ohm switch matrix.As a result small capacitors and low power resistors can be used to couple the phone to the switch matrix, while preserving the necessary impedance to the phone and providing low call path insertion loss.

The schematic for the circuit, which is Figure 4, shows two identical driver circuits.

The common pins to the two drivers are the following: 1. Pin 1 +5 V ii. Pin 12 +24V iii. Pin 8 +48V iv. Pin 5 0 V The extension drive pins are 11 and 6 while the switch matrix input/output pins are 4 and 3 respectively for each circuit.

As both circuits are identical only one will be described in detail.

The driver comprises transistors TS1, TS2 and associated components. TS1 is biased by resistors R3, R4 and R5 to provide a 0.5mA constant current. This current is injected into the extension via resistor R6 and forward biased protection diode D4. As a result a constant voltage appears across resistor R6.

This constant voltage is dropped across the base emitter junction of transistor TS2 and resistor R7 which results in a constant current of 25mA being injected via diode D4 into the extension.

Resistor R8 feeds a low current 48V feed to the extension. When the phone is on hook the resistor pulls the port voltage at pin 11 to 48V and diode D4 is reverse biased. Diode D3 prevents transistor TS2 from being reverse biased by leakage currents.

The AC operation of this driver circuit is as follows. Signals injected at pin 4 of the hybrid are presented to the base of transistor TS2 via decoupling capacitor C1. A midrail 2.5V bias is generated by resistors R1 to R2 which are connected to the 5V DC and zero Volt rail respectively. Schottky diodes D1 and D2 prevent transients from exceeding the 5 Volt range by more than a diode voltage drop.

The signal presented to the base of transistor TS2 is injected into the extension by the emitter follower action of this transistor.

In a similar way AC signals generated at the phone are presented to the junction of resistors R6 and R7. The AC signals are not attenuated by resistor R6 and appear at the base of transistor TS2 due to the high AC impedance operation of the two constant current sources. This signal is then presented to the switch matrix via decoupling capacitor C7.

3. CALL PROGRESS DECODER The Call Progress Decoder circuit which is illustrated in Figure 5 accepts AC signals from the line and having filtered and amplified these signals, presents them to the microcomputer for analysis. There are three separate low power circuits, one for each line.

Each circuit comprises three Op-amps and associated components which realise a low-pass filter buffer amplifier, two cascaded hard limiters, and a peak detector. The low-pass filter buffer comprises Op-Amp 1/4 IC1 and associated resistors R12-R15 and capacitors C2, C3. These components form a first order low pass filter which accept the balanced line input, filters it and presents it to the hard limiting amplifiers as a single ended signal referenced to 2.5 Volts.

This signal is then presented to two cascaded amplifiers/low pass filters comprising Op-Amp 1/4 IC2 and associated capactor C4 and resistor R16, R17, R18 and an identical second stage comprising Op-Amp 1/4 IC2 and associated components.

These two amplifier stages accept the single ended signal, amplify it and present the resultant signal to the micro-computer analogue input AN6 (IC1 pin 40) via a peak detector comprising diode D1, capacitor C6 and buffer resistors R24 and R25.

The microcomputer, using an algorithm, then examines this input to determine the level and cadence of the tones present on the line.

4. EXCHANGE CIRCUIT The three exchange lines are connected to the PABX of the invention on two terminals of connectors CN1 and CN2.

These connectors are located in the access area. As all three circuits are identical, only exchange line 1 will be described in detail.

For line 1 the A leg is connected to pin 3 while the B leg is connected to pin 4 of connector CN1. NETWORK EARTH is connected at screw terminal connector CN1 pins 1 and 2. This earth references the system ground. The exchange line is then presented to the exchange termination circuit comprising relay RL1, RL2 and associated passive components capacitor C1 resistors R1, R3, R5 zeners ZD1, ZD2 and hybrid circuit HY1.

The two relays are controlled by the serial latch ports Q1 and Q2 (pins 4 and 5 of IC 18 via relay driver IC26 pins 1 and 2 input, pins 16 and 15 output). The two relays provide the exchange circuit with four possible terminations.

With both relays RL1 and RL2 released, the exchange line is presented straight through to the relay switch matrix. This is the case during power failure or when an extension in On Line or Idle/Off Line. When the exchange line is in this condition, the termination is provided by capacitor C1, resistor R1, zeners ZD1 and ZD2 and varistor VR1. These components present the exchange line with the correct Off Line idle impedance for both ringing and low voltage AC signals.

With relay RL1 activated and relay RL2 released, the exchange line is terminated in the "HOLD" circuit. This is the case when an extension puts the exchange line on hold to make an enquiry or unattended transfer. The hold circuit termination will be discussed in more detail at the end of this section.

With relays RL1 and RL2 activated, this condition presents the exchange line with resistor R5 and is used during a decadic dialing make.

With relay RL1 released and RL2 activated, this presents the exchange with capacitor C1 as a spark quench during a decadic dialing break. To dial a digit, the system must first sieze the exchange line and then short and open the line at a defined rate to generate loop disconnect dialing.

The network, comprising capacitor C1 varistor VR1 and associated resistors and zeners, provides the dialing relay with spark quench snubbing. The exchange line status is monitored by the analog port AN7 of the microcomputer (Pin 41, IC1) via a buffer contained within the hybrid circuit HY1 (Pins 2 and 3 input, pin 4 output).

The buffer accepts the balanced line AC and DC voltage and provides the microcomputer with a proportional single ended voltage centered about a 2.5 Volt reference.

The microcomputer then uses this input by applying software algorithms to determine the line condition and signalling.

i.e. Off Hook On Hook Line present or not Frequency and level of incoming ringing.

The output of this buffer is also used by the call progress decoder as discussed in the previous section.

The final sub-section within the exchange circuit is the hold circuit. This comprises Bridge BR1 and associated resistors, diodes and active components within the buffer hybrid HY1. The function of this circuit is to provide the correct "Hold" DC and AC characteristics and to enable the system to inject tone out to the exchange line. Diode Bridge BR1 rectifies the exchange line DC voltage to provide the correct polarity for the semiconductor circuit.

A cadenced tone is injected by the hold circuit into the public switched telephone network (PSTN) to indicate the hold condition. To inject a tone into the exchange line, the system utilises the tone generator and under microcomputer control enables and disables the tone using Cross Point Switch 1010 (Pin 13 input, Pin 11 output). If the switch is disabled then the output of the tone generator is disconnected from the hold circuit. If, however, the switch is enabled then the output of the tone generator is presented to the hold circuit via pin 6 of the hybrid.

5. INTERCOM SWITCH MATRIX The Intercom Switch Matrix comprises two crosspoint integrated circuits IC7, IC9 which accept the eight extensions' AC signals and two crosspoint integrated circuits IC6, IC8 which accept this AC signal via series resistors RG7, RG8.

Four horizontals are utilized as independent intercom speech paths while a horizontal is used to inject dial tone into the extensions. The remaining three horizontals are used to connect the extensions to one of three DTMF receivers.

The microcomputer controls the individual crosspoint switches via digital outputs PB 0, 2, 4, 6 and PC7 (pins 9, 11, 13, 15 and 24 of IC1). These are presented to the address inputs A, B, C and D (pins 3 to 6 of each IC) and the data input (pins 2 of each IC). The strobe input (pin 7 of each IC) is controlled by the microcomputer via the binary to decimal decoder IC14 which enables the microcomputer to select and strobe each crosspoint IC individually.

6. SERIAL DATA CONTROL OF RELAYS To change their states, the microcomputer outputs digital signals on it's output ports which, via the various interface circuits, activates or releases relays and crosspoint switches, generates tones and sends serial character data. This in turn controls call paths and signalling and generates tone for the extension or exchange lines.

The relays are driven via six serial latches (IC17 -IC28) which drive six Darlington driver IC's namely IC22 -IC38 via three control ports.

The open collector outputs from each Darlington are protected from flyback, when releasing the inductive relay drive coil, by an integral clamping diode. Microcomputer ports PCO, PC1 and PC2 are used to control the serial latches. PCO provides the data for each latch while PC2 clocks the data from register to register. When all 48 bits of data (i.e. 6 x 8 bits) have been output, then PC1 strobes the new data to the output. Resistor R7 and capacitor C22 provide a power on reset for the serial latches so that the relays will remain off until the initial setup is completed.

7. DTMF RECEIVER CIRCUIT The PABX of the invention incorporates three separate DTMF receiver integrated circuits which monitor for and decode the DTMF signalling on each active extension and line. It interfaces to the extensions via two crosspoint switch IC's namely IC7 and IC9 which in turn interface via the driver hybrid circuits.

The three resistors comprising RG6 present the signals appearing on the three DTMF receiver integrated circuits namely IC11, IC12 and IC13. Capacitors C14, C15 and C16 decouple each horizontal. In a similar way the three receiver IC's are connected to the three lines via crosspoint switch IC10, the three verticals of which connect to the line via buffer hybrid circuit pins 2 and 3 input and pin 6 output.

Each of the DTMF receiver circuits is identical so only that associated with IC11 will be discussed in detail. Having been selected to monitor one of the extension or exchange ports via the crosspoint, any AC signal is presented to the input amplifier of IC11 pin 2.

Resistor R19 selects the input amplifier gain while the connection from IC11 pins 1 and 4 bias the input to 2.5 Volts. Resistor R18 and associated capacitor C11 determine the maximum valid DTMF signal (set to 50ms or milliseconds). Crystal XT2 provides the required oscilation frequency while capacitors C8 and C9 aid stability.

The microcomputer interfaces with the DTMF receiver via a 4 bit tristate bus (PB0, PB2, PB4 and PB6 of IC) and on an individual steering input (STD) (Pin 15 of IC11) which is presented to the microcomputer input port (Pin 40, IC).

When a valid DTMF signal is detected for the required duration the microcomputer is signalled via STD and then selects to read the data by selecting the TDE input (Pin 10, IC11) via the binary to decimal decoder (IC14, Pin 9 output). The data is valid on the four bit bus until the microcomputer releases the DTMF receiver by deselecting the TDE input.

All three receivers share the same crystal input by feeding the output of the first receiver to the input of the next.

The polling method by which the DTMF receivers are accessed will now be described.

In a situation where all 3 DTMF receivers are in use and a further extension requests a DTMF receiver, then the polling method of the invention is activated.

The microcomputer first monitors each DTMF receiver to see if any extension is finished or is about to finish with a receiver. If so, the fourth extension will be given that DTMF receiver when it is free.

If all DTMF receivers are in use, then the microcomputer checks to see if the dialling is being done manually or automatically, e.g. speed dialling. If one of the receivers is being used for manual dialling, then that receiver is multiplexed between the two extensions with 40 microseconds of receiver time being given alternately to each extension.

8. VOLTAGE BALANCE The PABX of the invention uses unbalanced voltages internally but presents balanced voltages at each exchange line. Cross-talk is kept at a very low minimum level by ensuring that only very short lengths of internal cable are ducted together.

9. HOLD CIRCUIT The Hold Circuit of the buffer hybrid circuit (Fig. 5) comprises transistor TS1, Optocoupler IC3 and associated passive components. It interfaces to the exchange line via an external full wave bridge rectifier connected to pins 13 (positive) and pins 11 (negative).

An external resistor connected between pins 13 and 12 provides bias current via resistor R33 to the base of the transistor of the optocoupler. (1/2 IC3, pin 6). The emitter of this optocoupler, pin 4 in turn drives the emitter follower formed by transistor TS1 and resistor R35.

Zenor diode ZD1 and decoupling capacitor C8 clamp the voltage presented to the transistor circuit and results in the transistors acting as a constant current sink.

Resistor R32 defines the DC characteristics of this circuit at low input voltages before the zenor conducts.

The external resistor between pins 12 and 13 scales the AC impedance.

Claims (13)

1. A private automatic branch exchange (PABX) telephone system comprising: a power supply, an extension feed, an exchange circuit, a crosspoint switching matrix, a relay switch matrix, a ringing generator circuit, a dual tone multi-frequency (DTMF) receiver circuit, a call progress decoder (CPD), a microcomputer and a microcomputer control circuit, characterized in that the ringing generator circuit includes a triac shunted across said circuit, the triac being operable to clamp any high inductive loads produced by a ringing bell when disconnected, thereby preventing high voltages being applied to the microcomputer.

2. A PABX telephone system as claimed in Claim 1, in which the DTMF receiver circuit includes means for polling "X" DTMF receivers between "X + Y" extensions, said polling means including means for determining if a DTMF receiver is being utilised for manual dialling, and means, which on determination of the use of manual dialling, for multiplexing two extensions with said DTMF receiver.

3. A PABX telephone system as claimed in Claim 2, in which the means for polling the DTMF receivers includes means for interrogating the DTMF receivers in use for determining if an automatic dialling sequence is approaching a conclusion and means for reserving such a receiver for a particular extension.

4. A PABX telephone system as claimed in any one of the preceding claims, wherein the extension feed circuits, which are operational when the PABX system is in an internal mode (i.e. on an internal call, programming or when idle), are part of a driver hybrid circuit which ensures, when the extension phone is on open circuit, that the voltage across the telephone is at the voltage required for an external exchange, and when the phone is active, the voltage is determined by its D C characteristics at a low current value resulting in the preservation of the necessary impedance to the telephone and providing low call path insertion loss.

5. A PABX telephone system as claimed in any one of the preceding claims wherein the Call Progress Decoder (CPD) circuit comprises a separate low power circuit for each line which accepts AC signals from a telephone line, filters the signals and amplifies them before presenting said signals to the microcomputer control circuit for analysis, and the mircocomputer includes means, using an algorithm, for examining the input signals to determine the level and cadence of the tones present on the telephone line.

6. A PABX telephone system as claimed in any one of the preceding claims, wherein at least one exchange line is connected to the PABX on two terminals of connectors, the or each line being switchable by relays depending on the conditions prevailing on each line, said relays providing four possible line terminations, namely: direct line to the relay switch matrix where the extension is on line, Idle/Off Line or in the case of a power failure; line is terminated in the "HOLD" circuit where the extension puts the exchange line on hold to make an enquiry or unattended transfer; line is presented with a resistance during decadic dialing make; and line is presented with a capacitance as a spark quench during decadic dialing break, and wherein in each case the exchange line status is monitored by the microcomputer which uses these signals to determine the line condition and associated signalling.

7. A PABX telephone system according to any one of the preceding claims, wherein the exchange circuit contains the hold circuit within a buffer hybrid circuit, said hold circuit providing the correct "HOLD" DC and AC characteristics and injecting appropriate tone signals into the exchange line such as a cadenced tone to the public switched telephone network (PSTN), said hold circuit utilising the tone generator and under microcomputer control enabling and disabling the tone using a crosspoint switch.

8. A PABX telephone system according to any one of the preceding claims, wherein an intercom switch matrix is provided comprising two crosspoint circuits for accepting a plurality of extension line AC signals and two crosspoint circuits which accept AC signals via series resistors, said matrix having at least two lines as independent intercom speech paths, one line for injecting the dial tone to the extensions and the remaining line or lines being used to connect the extensions to one of a plurality of DTMF receivers, said matrix being managed by microcomputer control of the individual crosspoint switches enabling the microcomputer to select and strobe each crosspoint individually.

9. A PABX telephone system according to any one of the preceding claims, wherein the DTMF receiver circuit comprises three separate DTMF receivers which monitor for and decode the DTMF signalling on each active extension and line, and are selectable by the microcomputer which further interfaces with the DTMF receivers, insofar as that when a valid DTMF signal is detected for the required duration, the microcomputer is signaled and reads the data via a binary to decimal decoder, the data being considered valid until the DTMF receivers are released.

10. A PABX telephone system according to any one of the preceding claims, wherein the PABX system uses unbalanced internal voltages but presents balanced voltages at each exchange line.

11. A PABX telephone system according to any one of the preceding claims, having a hold circuit which interfaces with an exchange line via a full wave bridge rectifier, wherein a zenor diode and a decoupling capacitor clamp the voltage presented to the transistor circuit so that the transistor present in the circuit acts as a constant current sink, whereby the DC characteristics of this circuit are defined at low input voltages before the zenor diode conducts by a single resistor and the AC impedance is scaled by an external resistor.

12. A PABX telephone system substantially as herein described with reference to and as shown in the accompanying drawings.

13. The features of the foregoing specification, or any obvious equivalent thereof, in any novel selection.