IES922705A2 - Improvements in and relating to a private automatic¹branch exchange (PABX) telephone system. - Google Patents

Abstract

The invention provides a private automatic branch exchange (PABX) telephone system of modular construction having a minimal configuration of connected telephone exchange 10 lines and corresponding extension lines. A number of intermediate configurations are provided up to a maximum configuration where all available expansion room is occupied. The PABX is controlled by a microprocessor based controller and is adaptable to interface with all 15 national telephone networks and to conform to telephone system parameters by programming of the controller only.

Description

IMPROVEMENTS IN AND RELATING TO A PRIVATE HEMATIC BRANCH EXCHANGE (PABX) TELEPHONE SYSTEM The present invention relates to a private automatic branch 5 exchange (PABX) telephone system.

PABX systems must conform to a number of rigorous standards for line parameters and signal clarity. High quality transmission is always sought but usually involves high cost and extreme system complexity. If a PABX system is to be used in a number of different countries, it must conform to the system parameter requirements of the national telephone network in question. This usually involves adjustment either at factory level or at national distribution points. Additionally, compensation circuitry is often required to overcome differences in national network and PABX system parameters.

It is an object of the present invention to seek to alleviate the above disadvantages.

The present invention provides a private automatic branch exchange (PABX) telephone system of modular construction comprising at least a power supply unit (PSU), a backplane wiring printed circuit board (PCB), a line card having a plurality of telephone line connections provided thereon, an extension card having a plurality of connections for extension telephones, a distribution board, a controller and a user interface console unit, wherein the PABX may be provided in a minimal configuration or may be provided in a plurality of intermediate configurations up to a maximum configuration and is adaptable to interface with all national telephone networks and to conform to telephone ___system parameters by programming of the controller only.

OPEN TO PUBLIC INSPECTION UNDER SECTION 28 AND RULE 23 1.7......of - 2 IE 922705 In a minimal configuration, the PABX comprises a PSU, a backplane wiring PCB, a line card, an extension card, a distribution board to interface the line and extension cards, a controller and a user interface console unit, wherein the PABX is expandable by the addition of at least one extension card and associated distribution board and optionally by the addition of a plurality of extension cards, one or more line cards and associated distribution boards whereby the controller is programmable to ensure signalling between exchange lines and extension telephones and between two or more extension telephones is secure.

Preferably, the line cards are four-line cards adapted to receive four telephone exchange lines, and the extension cards are adapted to support the interconnection of eight extension lines with four exchange lines, that is, the lines receive by one line card, the interconnections of said extension lines to said exchange lines being facilitated by a distribution card whereby the number of distribution cards required is equal to the number of line cards utilized in a given PABX configuration.

Advantageously, a driver circuit is provided between the telephone system and an extension telephone which circuit provides a first and second feed circuit said circuits allowing the interface of standard or system telephones and providing self protecting circuitry to prevent damage by relatively high voltage ringing signals.

Additionally, a buffer circuit is provided between an incoming exchange line and the telephone system which buffer circuit provides output signals to its associated line card for the determination of exchange line status and - 3 IE 922705 to instruct a crosspoint switch matrix on the line card for placing external calls.

Preferably, the backplane wiring PCB is provided with a 5 plurality of inserting buffer amplifiers for analogue circuit paths whereby such paths are run parallel to each other providing equal and opposite signals for reducing cross-talk between adjacent paths.

The invention will now be described more particularly with reference to the accompanying drawings, which show by way of example only, one embodiment of modular private automatic branch exchange (PABX) telephone system according to the invention. In the drawings: Figure 1a is a block diagram of a modular PABX system; Figures 1b and 1c are schematic diagrams of a power supply unit and a multiple voltage feed circuit for the power supply unit respectively; Figures 2a and 2b are schematic diagrams of a backplane wiring printed circuit board (PCB); Figures 3a and 3b are schematic diagrams of a line card having four lines and associated line circuitry; Figures 4a and 4b are schematic diagrams of an extension card having eight extension ports; Figure 5 is a schematic diagram of a distribution card; Figures 6a and 6b are schematic diagrams of a controller card; - 4 ΙΕ 9227 Figure 6c is a schematic diagrams of a controller distribution card.

Figures 7a and 7b are schematic diagrams of circuitry associated with a user interface console unit; Figures 8a, 8b and 8c are schematic diagrams of an extension phone driver hybrid circuit housed in an integrated circuit chip and having first and second circuits therein, respectively; and Figures 9a, 9b and 9c are schematic diagrams of a PSTN line buffer hybrid circuit housed in an integrated circuit chip and having a first and second circuit housed therein.

Referring to the drawings and initially to Figure 1a, a private automatic branch exchange (PABX) telephone system which may be assembled using a minimum of modules, is 2θ shown. Among the first of these modules are a power supply unit (PSU) 10 shown in Figures 1b and 1c and a backplane wiring printed circuit board (PCB) 20 shown schematically in Figures 2a and 2b, respectively.

The telephone system is built from the backplane PCB 20 and PSU 10 using the following modules; (a) a line and card 30 as detailed in Figures 3a and 3b; (b) an extension card 40 as detailed in Figure 4; (c) a distribution card 50 as detailed in Figure 5; (d) a controller card 60 as detailed in Figures 6a and 6b; - 5 IE 922705 and (c) a controller distribution card 65 as detailed in Figure 6c.

As shown in the block diagram of Figure 1a, the PABX system may be expanded by adding modules or cards such as for example additional line 30 and extension 40 cards and associated distribution cards 50. Additionally, expansion may be provided by an option card 100 which will be described in more detail hereinafter.

A PSU 10 comprises a voltage selector circuit (Figure 1c) and a PSU board (Figure 1b). Mains live and neutral are connected via connector CN3 to the voltage selector circuit. Six primary coil connections and taps from the transformer are provided via connector CN1. A set of links are used to select which tap on the transformer is connected to mains neutral, thus selecting the input voltage. Fuses FS1, FS2 with varistor VDR1 provide against overvoltage and power related fault conditions.

The output transformer secondary is provided on the PSU board via connector CN1 and is full-wave rectified by diodes D6-D9 and smoothed by capacitor C3. The resulting DC output of 8-10 Volts is regulated by IC1 to provide a 5 volt rail.

Resistors R11, R12 are used across the bridge rectifier comprising diodes D6-D9 to provide a 4.5 Volt peak to peak mains interrupt signal to microprocessors on each card in the system.

A further transformer output from connector CN1 is - 6 IE 922705 rectified by diodes D2-D5, smoothed by capacitor C2 and regulated by a discrete series-pass regulator circuit consisting of transistors TS1-TS3, Zener diodes ZD1, ZD2 and associated resistors and capacitors, to provide the +24 Volt DC rail. Transistor TS1 is mounted off-board due to the physical size of the heatsink required.

A yet further transformer secondary is half-wave rectified by diode D1, smoothed by capacitor CT and regulated by voltage regulation IC3 to provide the +58 Volt DC rail.

A.C. secondary provides the Ringing Signal, (70V AC) which is fed out via varisor VDR1 and shunt resistor R13.

Crowbar protection is provided on each of the three DC rails by taking the raw unregulated supplies to a common point via diodes D11, D12, and resistor R19 and shunting these to ground using thyristor SCR1. The gate of thyristor SCR1 is triggered when the output voltage of one of the regulators rises high enough to break down any of Zener diodes ZD3, ZD4, ZD5.

Referring now to Figures 2a and 2b a backplane wiring PCB is shown. For convenience and clarity the board has been separated into Figures 2a and 2b. The backplane board 20 comprises a series of connectors CN1-CN10 adapted to each receive a card.

The PCB 20 can accept a total of ten cards one in each of the slots provided by the connectors CN1-CN10. These cards are arranged as follows: CN1 CN2 CN3 Controller Card Option Card Line Card - 7 IE 922705 CN4 : Extension Card CN5 : Line Card CN6 : Extension Card CN7 : Line Card CN8 : Extension Card CN9 : Line Card CN10 : Extension Card The provision of cards in connections CN5-CN10 is optional, depending of the PABX configuration required.

The PSU 10 is connected via a connector CN21. The supply rails are split up in to analog, digital, and relay drive supplies and grounds to remove the possibility of interaction, and to reduce power related noise.

The supply rails are connected to cards situated in the connectors CN2-CN10 by connectors CN12-C20, along with earth, digital signals from the controller card, such as the three wire high speed serial bus (for communication between the controller and other cards in the system), 2Mhz digital clock and card reset signals. In addition to these, three resistors set the slot address which is read by each of the extension and line cards on powerup to determine where in the numbering plan they appear.

All the analog connections such as the twenty-four path horizontals H1-H24, paging, dialtone DT1, alternative dial tone DT2, Music on hold (MUS), and dialtone off functions are assigned to pins A1-A29 of connectors CN2-CN10. All of these horizontals except dialtone off are fed into inverting buffer amplifiers IC1-IC7, and their associated passive components and connected to the pin opposite each on CN2-CN20, that is pin A1 is inverted and fed to pin BI . - 8 IE 922705 This provides an equal and opposite signal for each horizontal on the backplane to reduce cross-talk between adjacent horizontals. On the PCB, each pair of tracks cross over one another to reduce capacitance between pairs, further reducing crosstalk.

Each line and extension card brings out three Dual Tone Multi-Frequency (DTMF) receiver inputs to the backplane, and these are connected so that one receiver chip is connected to each of the twenty-four horizontals on a full sixteen line, thirty-two extension system (four line cards, four extension cards and three DTMF receivers).

In a further embodiment (not shown) a smaller version of the backplane PCB is provided reducing apparatus cost and space. This embodiment contains connectors for four extension or line cards. The board is otherwise identical to the normal ten connector version.

Smoothing circuits provide voltage supply rails for internal power requirements.

For simplicity all busses are shown as relatively thick lines in the drawings.

Referring now to Figures 3a and 3b which show a line card 30 which is used to interface four PSTN telephone lines to the PABX, system. It is managed by a single chip microprocessor IC1, which monitors the lines for incoming ringing, sending the information back to the controller card 60 and when instructed, seizes lines using a hybrid IC hold circuit, to make and receive calls. These hybrid circuits are described in detail hereinafter with reference - 9 IE 922705 to Figures 9a to 9c. The hold circuit consists of an active current sink with a high AC impedance. The path signal from each of the four line hybrids is transformer coupled (T1-T4) to the four four for four Crosspoint Switches (IC52-IC55) used to route calls via the backplane 20 to the extension cards 40 in order to setup external calls .

Integrated circuit IC60 is a DTMF tone generator, switched 10 via integrated circuit IC51 , another four by four crosspoint IC, used for the re-generation of the internal signalling information from system telephones onto the lines, as well as stored number dialling, Decadic to MF conversion, etc. Also switched via integrated circuit IC51, is the music signal from the controller card 60 to calls on hold.

Included on each of the four line hybrids is a buffer amplifier, the output of which, on pin 10 is the line voltage scaled down to be fed in to the microprocessor analog ports to check line status and perform incoming ringing detection.

In addition, there are two other outputs on the hybrid circuits, one (Pin 9) is a tone signal from the line, squared off, for tone frequency detect, and another (Pin 11) detects the same tone signal peak for level measurement.

Decadic dialling is performed by turning Opto-Coupler OP11, 0P21, 0P31, or OP41 on (reducing the hold circuit impedance) and turning Opto-Coupler OP12, OP22, OP32 or OP42 and the hold circuit current sink on and off.

IE 922705 - 10 Relays RL11, RL21, RL31 and RL41 are used to switch between putting the hold circuit on-line or connecting the idle impedance across the line.

Relays RL12, RL22, RL32 and RL42 are used for earth signalling. When fired, they shunt the A+B legs (see details later) through separate resistors to Network Earch.

The eight opto-couplers used are derived from pins PB0-PB7 10 on the microprocessor IC1. The eight relays are driven from IC's 57 & 58 8 way Darlington driver (transistor TS5, resistor R70 and diode D1 give the option of using a seven way driver chip and an extra discrete driver instead). Integrated circuit IC57 also drives four LED's used to indicate line status.

Referring now to Figures 4a and 4b, an extension card 40 having a capacity to handle eight telephone extension circuits is shown. The extension card 40 is provided with connectors which are matingly engagable with connectors of the first backplane wiring portion 20.

The eight extension module card 40 is designed to interface to eight extension telephones, which maybe one of three types, either ordinary loop-disconnect (Decadic), DTMF types, or telephone types of the present invention. The card 40 is managed by a single chip microprocessor IC1 which monitors the extensions, passing the information (signalling/extension status, etc.) to the controller card 60 over the backplane serial bus, via connector CN3 and carrying out instructions sent back by the controller card 60 to ring telephones, route path connections to the backplane 20, and send dialling and other tones to extension telephones via connector CN1 of Figure 4b. - 11 IE 922705 As detailed below, normal two wire telephones receive a 25mA DC feed on their A+B wires, while telephones of the present system receive an additional 7mA C+D wire feed, which is used to drive the microprocessor, LED's and LCM display in the telephone as shown in Figures 7a and 7b. Signalling to and from system telephones is also performed over this pair.

A hybrid IC is used to provide both A+B and C+D pair DC feed to each extension. This is described in detail hereinafter with reference to Figures 8a to 8c.

Each of the eight hybrid circuits consists of two DC constant current sources (A+C legs) and 2 current sinks (B+D legs), to give two balanced feed circuits. An alternative B leg connection is provided for ring detection. In addition, tone detect, path, and C+D wire data transmit and receive connections are provided.

The microprocessor IC1 is a 78C18 device, with internal read only memory (ROM) (32 KByte, Mask Programmed) and random access memory (RAM) (1 KByte). Under Program Control, it scans the extensions via its analog ports AN0-AN7, which are used to detect extension hook status, ringing and loop disconnect signalling.

Integrated circuits IC4, IC5 are twelve by eight Crosspoint switch IC1s used to route extension path signals on to the backplane PCB 20.

Additional IC's IC2, IC3 are four by four crosspoint IC's used to route dial tone and paging signals to the extensions via series resistors. These are needed as - 12 IE 922705 connecting a telephone to a zero Ohm dialtone source would negate DTMF signals from said telephone.

The ringing signal is split to ring two groups of 5 extensions (1-4, and 5-8) this is done to reduce both the heat dissipation in the series resistors (R14+R15, & R20+R21) and reduce the amount of ringing current available to any one extension. Triacs TY1, TY2 are used to shunt the ringing signal to ground while the relays are switching the extension telephones between the DC feed circuit and the ringing source. This prevents voltage spikes from inductive bells in certain types of telephone from getting into the system, and prolongs the life of the relay contacts. Capacitors C10+R11 & C11+R17 are used for snubbing across the triacs.

Three DTMF receivers are provided on the extension card. These are not used directly on the extension card, but are connected to the backplane horizontals, to be used globally as required. When an extension goes off hook, or whenever a DTMF receiver is required, it is assigned the next available receiver in a circular fashion.

Microprocessor ports PB0-PB7 are used as a data/address bus, to access the crosspoint switches, DTMF receivers, and via IC6, a latch used of input/output (I/O) expansion, to drive the eight ringing relays via IC7, a Darlington Drive IC.

A distribution card 50, shown in Figure 5, is designed to handle, a capacity of four telephone lines and eight extension telephones. The distribution card is connected by connector blocks to one line card 30 and one extension card 40. For example, if the PABX system required demands - 13 IE 922705 eight incoming telephone lines and twenty four extension telephones, three distribution cards are necessary. The first two distribution cards will be connected to four telephone lines and eight extension telephones each and the third distribution card is connected to the remaining eight extension telephones.

The distribution card 50 brings out four lines and eight extension from their respective cards to terminals. This provides connection to exchange line and extension telephone wiring, as well as providing overvoltage and Radio Frequency Interference (RFI) protection. Power fail capability for the exchange lines is also provided.

The A+B wires of each exchange line are brought in from CN1, fed to the line connector terminals CN3, CN4 which are four way, grouped as two A+B pairs on each.

Longitudinal overvoltage protection is provided by a varistor from each leg (wire) to network earth.

The C+D wires of all eight extensions CN5-DN12 and the A+B wires of the upper four extensions CN9-CN12 are brought in from the extension card via connector CN2 and connected directly to eight, four way connector terminals (the A,B,C & D terminals for each extension) CN5-CN12.

The lower four extension A+B terminals CN5-CN8 are held switched internal to their respective extension card ports CN3-CN4 by four double pole relays, RL1-RL4.

On power fail, they are connected out to the lines, extension 1 A+B to line 1, etc.

These relays are powered from the line card 30 via CN1, - 14 IE 922705 pins A15 and A16, from the +24 Volt DC rail.

Longitudinal overvoltage protection on each extension terminal is provided by Varistors VR11-12, VR15-16, VR19-20, VR23-34, VR27-28, VR31-32, VR35-36 and VR39-40.

The varistors used to provide longitudinal protection on extensions 1-4, (VR9-10, VR13-14, VR17-18 and VR20-21,) are fitted on the extension feed side of the relays, to prevent them from being switched out to line.

A small ceramic capacitor is connected between each leg of all the line and extension terminals and network earth to provide RFI protection.

A controller module 60, shown in Figures 6a and 6b, has a microprocessor and associated logic to manipulate the distribution of lines and extension telephone as required. The microprocessor is programmed for the particular requirements which are demanded for a predetermined environment. For example, the microprocessor is provided with telephone system parameters for the telephone system to which it is to be connected. Consequently, telephone line interfaces are reduced in complexity or may in some cases be dispensed with. Additionally, compensation components may be made redundant.

The controller card 60 is used to supervise the operation of line 30 and extension 40 cards and the option card 100 in the system, as well as generating two dialtone sources, 12Mhz Microprocessor Clock, Reset signals, and providing a Doorphone Interface, Music on Hold, 2 channel RS232C Transmit and Receive, and a Real Time Clock function. - 15 IE 922705 The control circuit comprises a 78C10 Microprocessor chip IC1 with 32 Kbytes of RAM IC3, 32 or 64 Kbytes of Electrically Programmable Read Only Memory (Eprom) IC2 and some glue logic for address, decoding, etc.

One 1 2Mhz clock signal is used by all of the cards in the system. This is generated by the internal clock oscillator IC1, controlled by crystal X1. Stability is aided by capacitors C1, C2. This system is then fed to the backplane PCB 20 via connector CN1, Pin A10.

A latch chip IC4, is used to multiplex the data bus D0-D7 with the lower 8 bits of the address bus, allowing a 16-bit port on the microprocessor to provide a 24-bit (Address+Data) path or ROM and ROM memory, and the Real Time Clock Chip (RTC) IC16.

The contents of the RAM memory (system programming, etc.) are retained on power-fail by a Ni-Cad battery BT1. Only when the system is powered up for the first time will the RAM be cleared.

The Microprocessor IC1 monitors the +24V rail on port ANO via the potential divider R6, R7 and C4, and when the voltage starts to drop, i.e. during brownout, or when the power is disconnected, the chip select and +5 Volt DC Rail to the RAM and RTC Chip is disabled by pin Pc6 on the microprocessor under software control, via TS1-TS3 + R12-16, and thus the battery backup function is enabled.

A watchdog circuit comprising integrated circuits IC6a, IC6b, and associated discrete components, is strobed via pin PC3 on the microprocessor on a regular basis to ensure that if the software malfunctions, (due to mains glitch, - 16 IE 922705 etc.) the output of IC6c will go low, and the system will reset.

On power-up the processor chip IC1 is reset by potential 5 divider R6, R7 and C4, until the +24 Volt rail has settled. then the microprocessor proceeds to determine how many cards are in the system by resetting each one individually by putting a 4-bit code on pin PB0-PB3, which is decoded by chip IC9 to generate the reset signal for each card on the backplane PEB 20. When reset, a card, if present will return an acknowledgement signal.

The microprocessor under ROM-based program control, communicates with the micro on other cards in the system via a high speed serial bus PC0-PC21 to monitor the status of the system and when required, instructs the extension 40 and line 30 cards to implement the various system facilities .

A crosspoint IC IC8 is used to route music signals to the backplane 20 and Conference Calls to the conference bridge circuits.

When two or more extensions and a line are in a conference call, one of two Conference Bridge Circuits are connected in to the path for impedance matching. The Bridge Circuits comprise Op-Amps IC11a and IC11b, with their associated resistors and capacitors. The combination of positive and negative feedback on the Op-Amps results in a negative impedance looking into capacitors C8 to C12.

Normal dial tone of the system is generated by an internal timer on processor chip IC1 fed via pin PC4, as a 428Hz square wave. A third-order low-pass filter around chip - 17 IE 922705 ICIOa converts this to a clean sine-wave, which is fed out to the backplane 20 via connector CN1, Pin A31.

Alternative dial tone is generated by a 400Hz Oscillator 5 based around chip IC14a. The output of this is fed through a third-order low-pass filter around chip IC14b, into an active mixer around IC14c, along with normal dial tone.

The output of this circuit is a 438 Hertz tone modulated with a 37 Hertz beat.

When silence is required on an extension or line port, it is connected to a dial tone off circuit, which is a 2.5 Vdc reference, buffered by chip IClOb, and fed onto the backplane PCB 20 via connector CN1, Pin B32.

When the paging facility is invoked, the audio signal is routed via horizontal A27 on the backplane 20 to a buffer amplifier around IC10c, and on to horizontal A28, to which all the system telephones to be pages are connected.

Two sources of music on hold are possible on the system.

The first is generated internally by a melody IC IC12, buffered by Op-Amp IC10d, and fed in to the crosspoint chip, at Pin 14.

The second music source is provided by an external Tape Recorder of CD player with a 600 Ohm Line level Audio Output, which is connected, via the controller distribution card of Figure 6c and buffer Op-Amp IC14d into the crosspoint chip.

A transformer T1 provides an isolation barrier and diodes D12-D15 provide protection. - 18 »E 922705 A doorphone interface is provided on the controller card.

The two wire audio signal from the doorphone unit is connected via the controller distribution card 65 (Figure 6c) to pins A8 and B8 of connector CN2 on the controller card 60. This is transformer coupled through a two-tofour wire converter, the output of which is connected to the doorphone horizontal on the backplane PCB 20 via Pin A26 on connector CN1.

Relays RL1 and R12 provide switching to drive a door opener and to control a doorphone unit amplifier (N/O contacts).

Relay RL3 is used to control an external bell unit.

These three relay outputs are brought out to the controller distribution card 65 as pairs of normally open contacts.

Two RS232C transmit and two receive channels are provided on the system. Opto-Coupler based inverting amplifiers 0P1-4 and their associated resistors and diodes are used, as there is an isolation requirement in this area. The isolated side received power from the printer Ready to Send (RTS) signal.

Pins PA0-PA4 on the microprocessor IC1 are used as a bus to control the crosspoint switch as well as a latch chip IC13 used for i/o expansion, to select the RTC chip and, in conjunction with the darlington driver chip IC7, drive the three relays, two RS232C transmit ports, as well as two status LED's on the Control Card. - 19 IE 922705 In a further embodiment (not shown) a smaller version of the controller card 60 is available. This is used in sub-equipped, lower cost system configurations. A controller card of this type does not have the Real Time Clock, full RS232C communication ports, external music on hold input, external bell or the doorphone port contained on the full card and as a consequence does not require a controller distribution card. A transmit-only printer port which is connected via the backplane 20 may be provided.

A controller distribution module card 65 is shown in Figure 6c. The controller distribution card brings out the RS232C serial ports, External Bell, External Music, Doorphone, Doorphone amplifier control, and door opening connections to installer accessible terminals CN2-CN4, CN9 and in addition provides overvoltage protection on each of these. Longitudinal overvoltage varistor protection VR1-5 is provided on each of the RS232 lines, with series carbon resistors R1-R5 providing current limiting. The external music and doorphone ports at connector CN4 are provided with both differential VR6-7 and longitudinal VR11-14 Varistor overvoltage protection. Three relay outputs for External bell, Door opener, and doorphone amplifier control, are given differential protection by varistors VR9, VR10 and VR18, respectively.

Referring now to Figures 7a and 7b a user interface console is shown. This console forms what is know as the system telephone as all incoming lines are directed from this telephone and user functions are selectable from this telephone. The telephone may be described by reference to three distinct sub-sections. - 20 IE 922705 The first is analog section which handles all the functions required to interface a voice path in the main Central Control Unit (CCU) to either the handset or loudspeaker and microphone. This includes detecting ringing voltage on the line and providing an audible indication to the user.

The second section is the DC - DC convertor. This takes a constant current supply at 58V from the extension port CN1 (Figure 7a) and converts it to a 5V DC supply as required by the third section.

The third section is the digital section. This section controls all signalling required between the telephone and the CCU in order to control the busy lamp field, the keys and the Liquid Crystal Module (LCM). The telephone may be configured in either of two ways i.e. as a full system telephone, or as a standard system telephone (a full system telephone stripped down to remove the extension busy lamp field and keypad).

The A+B wires (pins 1+2) from the extension interface are connected to the telephone via connector CN1. Capacitor C43 and Resistor R48 provide the off hook idle impedance required to limit the ring current from the CCU. Chip U4 and its associated components detect when ringing voltage appears on the A+B pair and provide a signal to the piezo-electric ringer BZ1. The tone and volume of this signal are adjustable via potentiometers VR2, VR3. Zener diodes D9, D10 prevent the idle impedance from affecting normal speech level signals on the A+B pair.

The A leg is connected to hookswitch SW1 and relay RL1, either of which can put the telephone in the off hook condition. Both A+B legs are routed through a diode - 21 IE 922705 bridge D1-D4 to cater for polarity reversal. Varistor RV1 and diode D5 provide over voltage protection.

The heart of the telephone comprises the following 5 integrated circuits (IC's): - UI Telephone transmission - U2 Handsfree controller - U3 Loudspeaker amplifier The telephone may be in any of four states as follows: a) Idle With the handset in it cradle and the relay in the reset condition, the A leg is disconnected from U1 and hence all IC’s are powered down. b) Handset Mode With the handset lifted from the cradle transmission IC and the relay RL1 in the reset condition, the A leg is connected to transmission IC U1 which consequently powers up and sets and the DC conditions across the line. Any audio signals appearing on the line are routed to the handset earpiece. The second pole of the hook switch disables the chip select on the handsfree IC U2 and hence routes the handset microphone to the transmit input on transmission IC U1. The second pole of the relay RL1 disables the loudspeaker LS1. The nett effect of this is that the handset earpiece and microphone are active, and the external microphone and loudspeaker are disabled. c) Monitor Mode - 22 IE 922705 With the handset lifted from the cradle and the relay in the set condition, the A leg is connected as before.

The handset microphone and earpiece are also active as before. However, in this state, the second pole of the relay enables the loudspeaker amplifier IC U3 and any signals appearing at the handset earpiece are amplified and supplied to the loudspeaker via handsfree IC U2. d) Handsfree Mode With the handset placed in the cradle and the relay in the set A condition, the A leg is connected via the relay.

The second pole of the hookswitch enables the handsfree IC, U2 and the second pole of the relay activities the loudspeaker. In this mode, handsfree IC U2 disables the handset microphone and activates the external microphone, thus providing full hands free operation.

The C+D pair (Pins 3+4) from the CCU supply a balanced 7mA current source with modulated digital data to the telephone via connector CN1. Over voltage protection is provided by varistor RV2. Diodes D12-D16 form a bridge which protects against polatity reversal.

Referring now to Figure 7b, all keys, Leds, LCM etc are controlled by a 4-bit microprocessor IC1 (NEC 74008) which is powered by the DC - DC converter. Resistor R100 and capacitor C100 provide a reset for the processor IC1. A crystal X1 and capacitors C101, C102 provide the necessary 4Mhz clock to the processor.

The keypad is arranged as a 10 x 8 matrix which is continuously polled by the processor IC1. The hook switch IE 922705 is also wired into this matrix so that the processor IC1 can determine when the handset is placed in the cradle.

The Leds are also arranged in a 10 x 8 matrix, the current 5 to each column being supplied via transistors Q100 - Q107.

The current from each of the first 8 rows is sunk by transistors Q108-Q109. This enables the LED matrix to be multiplexed in such a fashion as to ensure that the current capabilities of the DC - DC converter are not exceeded.

Figures 8a to 8c show an integrated circuit for an extension telephone driver hybrid circuit of which Figure 8a is a suggested chip construction comprising a first circuit of Figure 8b and a second circuit of Figure 8c.

The driver hybrid circuit two balanced DC feed circuits to supply the necessary power to the telephone, processor as well as providing outputs to allow the extension card microprocessor to monitor the telephone connected, and an AC path connection to allow the injection of tones, and inter-extension and extension to line paths.

The circuit of Figure 8b accepts the +24 Volt power supply provide a 25mA A+B leg feed for standard two wire telephones, and the analog portion of system telephones.

The circuit comprises an A leg leg (Pin 1 of Figure 9b) DC constant current source, around transistors Q1 , Q2 and chip IC1a. The B current sink circuit (from Pin 11) based around chip IC1b and transistors Q3 acts to keep the output at pins 2 and 3 balanced. When the ringing signal is switched to the extension port via an external relay, the extension B leg is switched from Pin 3 to Pin 4 (to - 24 IE 922705 protect components Q3, IC1 from the ring voltage) so that resistor R11 always has the extension current flowing through it.

Diodes D3-D6 protect the feed circuit from voltage spikes, and diodes D1, D2 protect the Crosspoint switching circuits connected externally to pin 1 from spikes.

The voltage developed across resistor R11 is brought to a 10 potential divider R16, R17 to the +5 Volt Rail. The midpoint of this is fed out to the analog port of the microcomputer as an analog voltage representation of the extension current.

The C+D (Pins 12 + 13) wire circuit of Figure 8a is similar to the A+B Pins (1 + 11) legs, in that is has a DC constant current feed of 7 mA (Cleg, Pin 5) based around transistor Q4, and a current sink based around components IC2A, Q5, appearing on pin 6, acting with the c leg to keep the output balanced.

Pin 12 provides a digital input from the microprocessor to transmit data to a system telephone. The information is modulated onto the current feed by shifting the bias around transistor Q4,s base via resistors R18, R20, R21.

Diodes D7-D10 protect the Feed Circuit from voltage spikes.

Finally, with reference to Figures 9a to 9c a line buffer hybrid circuit provides an interface between exchange lines and the PABX system. An integrated circuit housed in a suggested chip construction of Figure 9a comprises an impedance matching 2, 4, 2 wire converter circuit of Figure 9b (IC1A, B, C, D and associated components) which in - 25 IE 922705 conjunction with the line hold circuit of Figure 9c (transistors Q1-Q3 and associated components) advantageously provides a very low loss transmission circuit for speech.

This results in an almost ideal lossless speech transmission circuit from extension to exchange which results in very cost effective speech switching circuit as it requires no compensation circuitry. 0 The circuit is powered from the +24 Volt rail and provides outputs to the line card microprocessor to enable it to determine line status, and a path connection to the crosspoint switch matrix on the card, in order to place external calls .

The buffer hybrid will always be in either of two states, offline or online. When the buffer is offline, its only function is to send a scaled down version of the line voltage (from pin 10 on the hybrid chips), to the line card microprocessor. This is the output of integrated circuit IC2a, which with resistors R30-R35, form a differential buffer amplifier. When the hybrid is online, the current sink circuit appearing at pins 3 and 4 is connected across the line (through bridge rectifier external to the hybrid, to maintain correct polarity).

The hold circuit comprises transistors Q1 , Q2 and Q3, and their associated discrete components. The three transistors with resistors R27-R29 form a three stage Darlington circuit, which when biased through resistors R24, R25 and diode D4 act as a current sink, with a defined AC impedance, assisted by capacitors C4, C5. - 26 IE 922705 Diodes D5 provides a measure of overvoltage protection.

The AC path signal is externally transformer coupled from the hold circuit to Pin 12 of the hybrid, into a two-to-four wire converter circuit, which compensates for an internal drop of 6dB going out from the PABX to the PSTN line, and a 6dB gain in the other direction, on the path connection at Pin 13.

Diodes DI , D2 protect the line card crosspoint switch matrix from voltage spikes, while resistors R1, R2 bias the path signal to 2.5 volts.

Pins 15 and 16 allow the connection of a 12KHz notch filter to remove metering pulses if required.

Integrated Circuit IC2b and its associated discrete components are used to amplify tons signals on the line and feed them squared off on Pin 9, for frequency measurement, and peak detected on pin 11 for level measurement, by the Line Card Microprocessor.

Resistors R22 and R23 provide a mid rail reference for each of the Op-Amp circuits.

An option card 100 may also be provided to select operation of the telephone system between, for example, standard mode, answer machine mode, facsimile mode and printer mode.

It will of course be understood that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the appended claims.

Claims (5)

CLAIMS :

1. A private automatic branch exchange (PABX) telephone system of modular construction comprising at least a power supply unit (PSU), a backplane wiring printed circuit board (PCB), a line card having a plurality of telephone line connections provided thereon, an extension card having a plurality of connections for extension telephones, a distribution board, a controller and a user interface console unit, wherein the PABX may be provided in a minimal 1 θ configuration or may be provided in a plurality of intermediate configurations up to a maximum configuration and is adaptable to interface with all national telephone networks and to conform to telephone system parameters by programming of the controller only. 1 5

2. A PABX telephone system as claimed in claim 1 having a minimal configuration which comprises a PSU, a backplane wiring PCB, a line card, an extension card, a distribution board to interface the line and extension cards, a controller and a user interface console unit, wherein the PABX is expandable by the addition of at least one extension card and associated distribution board and optionally by the addition of a plurality of extension cards, one or more line cards and associated distribution boards whereby the controller is programmable to ensure signalling between exchange lines and extension telephones and between two or more extension telephones is secure.

3. A PABX telephone system as claimed in any one of the preceding claims wherein a driver circuit is provided between the telephone system and an extension telephone which circuit provides a first and second feed circuit said circuits allowing the interface of standard or system - 28 IE 922705 telephones and providing self protecting circuitry to prevent damage by relatively high voltage ringing signals

4. A PABX telephone system as claimed in any one of the 5 preceding claims wherein the backplane wiring PCB is provided with a plurality of inserting buffer amplifiers for analogue circuit paths whereby such paths are run parallel to each other providing equal and opposite signals for reducing cross-talk between adjacent paths. 1 0

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