IES60843B2 - Modular Private automatic branch exchange (PABX) telephone system - Google Patents

Abstract

A private automatic branch exchange (PABX) telephone systems of modular construction having a motherboard comprising a power supply unit (psu), a line circuit having a plurality of telephone line connections provided thereon, an extension circuit having a plurality of connections for extension telephones, a controller and a connection means adapted to receive a daughterboard. The above comprises a minimal configuration which may be expanded with the addition of one or more daughterboards. Each daughterboard has a plurality of line circuits and extension circuits. The PABX is adaptable to interface with all national telephone networks and to conform to telephone system parameters by programming of the controller only. The controller is also programmable to ensure signalling between exchange lines and extension telephones and between two or more extension telephones is secure.

Description

The present invention relates to a private automatic branch exchange (PABX) telephone system.

PABX systems must conform to a number of rigourous 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. Modular PABX systems provide sufficient flexibility to be adapted for connection to different networks. Unfortunately, these modular configurations are complex and often very expensive.

It is an object of the present invention to seek to alleviate the above disadvantages and to reduce the cost associated with modularity by optimising the modular configuration.

The present invention provides a private automatic branch exchange (PABX) telephone system of modular construction having a motherboard comprising a power supply unit (PSU), a line circuit having a plurality of telephone line connections provided thereon, an extension circuit having a plurality of connections for extension telephones, a controller and a connection means adapted to receive a daughterboard, wherein the PABX may be provided in a 860843 - 2 minimal configuration or with the addition of one or more daughter boards in 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.

Conveniently, the PABX may form a minimal configuration which comprises the motherboard and a user interface console unit, wherein the PABX is expandable by the addition of a daughterboard whereby the controller is programmable to ensure signalling between exchange lines and extension telephones and between two or more extension telephones is secure.

The daughterboard comprises line circuits having a plurality of telephone line connections provided thereon and extension circuits having a plurality of connections for extension telephones.

Preferably, the motherboard comprises two line circuits and six extension circuits and the daughterboard comprises two line circuits and six extension circuits.

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 circuit for the determination of exchange line status and to instruct a crosspoint switch matrix on the line circuit for placing external calls.

Conveniently, each extension circuit incorporates a distribution circuit to interface and interconnect the line 5 and extension circuits.

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 1 is a schematic diagram of a power supply unit; Figure 2 is a schematic diagram of a motherboard controller circuit; Figures 3a and 3b are schematic diagrams of an extension circuit having six extension ports and full distribution 20 circuitry, and Figure 3c is a schematic diagram of an extension phone driver hybrid circuit housed in an integrated circuit chip and having first and second circuits therein; and Figures 4a and 4b are schematic diagrams of a line circuit having two lines and associated line circuitry.

Referring to the drawings, a private automatic branch exchange (PABX) telephone system is shown with a minimum 30 configuration of two line ports and six extension ports on a motherboard comprising a power supply unit (PSU) as shown in Figure 1, a controller as shown in Figure 2, extension port circuitry as shown in Figures 3a to 3c and line circuitry as shown in Figure 4a and 4b. An extra module is 35 available to expand the minimum configuration into a system having four line ports and twelve extension ports. The additional module is provided on a daughterboard which comprises extension port circuitry similar to that shown in Figures 3a to 3c and line circuitry similar to that shown in Figures 4a and 4b.

The basic system architecture is based around three microprocessors, configured as a master and two slaves, all communicating via a three wire serial bus. When a daughterboard module is connected the system architecture is reconfigured about four microprocessors.

One of the slave processors controls all the functions required by PSTN line interfaces, i.e. monitoring for incoming ringing etc., and communicates all of this information back to the master processor.

The other slave processor carries out a similar function for six extension ports. The master processor co-ordinates all system functions by analysing information from the slaves and instructing them to carry out certain tasks.

Distribution circuits comprise a switching matrix which is based around two 8x12 crosspoint switches and a 4x4 switch. These switches enable any extension to be connected to any other extension or line. The distribution circuits are formed integrally with the extension circuits.

The power supply circuit shown in Figure 1 provides regulated +5 volt, +24 volt, and +58 volt rails and a 60 volt sine wave ringing signal at the mains frequency (50 Hertz). The +5 volt DC supply is used by the digital control logic and analog circuits. The +24 volt DC supply is used to feed the extension ports and drive relays. The +58 volt DC rail is used to feed the C&D (signalling) wires to the system telephones. The 60 volt AC supply is used to generate ringing signals on the extension circuits.

The mains input is connected via a 3 amp, 3 core mains cable. The mains live L and neutral N is connected via a connector JP1 to the motherboard. A fuse FS1 together with varistors VR1, VR1A provide against overvoltage and power related fault conditions. Capacitors C700-C702 are provided as electromagnetic compatibility (EMC) suppressors.

A mains transformer TR1 comprises two secondary windings the first of which is full-wave rectified by diodes D11-D14 and smoothed by a capacitor C83. The resulting DC output is regulated by a first voltage regulator REG1 to provide the +24 volt rail. Resistors R20, R21 condition the output voltage of the voltage regulator REG1 to 24V. This 24 volt rail is further regulated by second voltage regulator REG2 to provide the +5 volt rail. A network comprising two resistors R22,R30 and a capacitor C87 provide a 4 volt peak-to-peak AC signal at 50Hz which is used as a timed interrupt signal.

The second secondary winding of the transformer TR1 provides the 60 volt AC ring signal. This voltage output is half-wave rectified by a diode D6, smoothed by a capacitor C81 and regulated by a third voltage regulator REG3 to provide the 60 volt rail. Resistors R45,R19 condition the output voltage of the voltage regulators REG3. Diodes, D7,D8, D9 provide protection for the voltage regulators. A connector HD1 is used to supply all power rails to the two line/six extension daughterboard.

The controller as shown in Figure 2 is sued to supervise the operation of line and extension slave processors as well as generating dialtone, 1 2MHz microprocessor clock signals, reset signals and providing a doorphone interface, central bell interface, internal music-on-hold and a printer port. The controller circuit comprises a 78C10 microprocessor integrated circuit (IC) with associated ICs providing 32 Kbytes of RAM IC3, 64 Kbytes of EPROM IC2 and some glue logic circuitry for address decoding, etc. One 12MHz clock signal is used by all of the slave processors in the system. This clock signal is generated by the internal clock oscillator on the microprocessor IC1, controlled by a crystal X1. Stability is aided by capacitors C1,C2. The clock signal is then buffered by buffer amplifiers IC6 and fed on to all other processors. A latch circuit IC4 is used to multiplex a data bus (data lines D0-D7) with the lower 8-bits of the address bus (address lines A0-A7), allowing a 16-bit port (PDH-7,ΡΈ0-Ί) on the microprocessor IC1 to provide a 24-bit (address and data) path to ROM and RAM memory circuits IC2,IC3.

The contents of the RAM memory IC3 (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 an analogue line ANO via a potential divider comprising two resistors R6,R7 and a capacitor C4 and when the voltage starts to drop, i.e. during brownout, or when the power is disconnected, the chip select line and the +5 volt DC rail to the RAM circuit IC3 are disabled by a signal from a line PC6 on the microprocessor IC1, under software control, via transistors TS1-TS3 and resistors R12-16 and thus the battery back-up function is enabled.

A watchdog circuit IC31 is strobed via a line PC3 on the microprocessor IC1 on a regular basis to ensure that if the software malfunctions (due to mains glitch, etc.), the reset line /RESET will go low and the system will reset.

On power-up the microprocessor IC1 is reset. The microprocessor IC1 then proceeds to determine how many 5 slave processors are in the system by resetting each one individually by putting a 3-bit code on signal lines PB0-PB2. When reset, a slave processor will return an acknowledgement signal. qq The microprocessor IC1, under ROM-based program control, communicates with the other processors in the system via a high speed serial bus (lines PC0-PC2) to monitor the status of the system and, when required, instructs the extension and line processors to implement the various system facilities.

When two or more extensions and a line are in a conference call, a conference bridge circuit is connected into the path for impedance matching. The bridge circuit comprises 20 an Op-Amp IC7b, with its associated resistors and capacitors. The combination of positive and negative feedback on the Op-Amp IC7b results in a negative impedance looking into a series capacitor C8 on the conference horizontal path H21 .

A dial tone on the system is generated by an internal timer on the microprocessor which outputs a timer signal (along line PC4) as a 438Hz square wave. A third-order low-pass filter built around an Op-Amp IC7a converts this square 30 wave to a clean sine wave which is fed out to the cross-point switches in the system. When silence is required on an extension or line port, it is connected to a dial tone off circuit, which is a 2.5 volt DC reference voltage provided by a reference circuit comprising resistors R43,R44, and a capacitor C17. When the paging facility is invoked, the audio signal is routed via a horizontal path H27 to a buffer amplifier built around an Op-Amp IC7c, and on to a horizontal path PGE, to which all the system telephones to be paged are connected. The music-on-hold signal music is generated by a melody circuit IC8, which is buffered by a Op-Amp IC7d and fed on to cross-point switches associated with the lines. A relay RL1 is used as a door opening switch and a further relay RL2 is used to control an external bell unit. Both are normally open switches. A printer port output is also provide via an opto-isolator 0P1 and associated components. The opto-isolator 0P1 is required to provide a 1.5MW isolation barrier between the system and the outside world.

Analogue ports (lines AN1-AN4) on the microprocessor IC1 are used to monitor for 50Hz metering pulses.

Referring now to Figures 3a to 3c, the six extension circuit is designed to interface into six telephones which may be one of three types, either ordinary loop-disconnect (Decadic), DTMF types, or system telephones of the type disclosed in the present invention. The circuit is managed by a single chip processor IC9 which monitors the extensions, passing the information (signalling/extension status, etc.) to the controller over the serial bus, and carries out instructions sent back by the controller to ring telephones, route path connections and send dialling and other tones to extension telephones.

Normal two wire telephones receive a 25mA DC feed on their A+B wires, while system telephones receive an additional 7mA C+D wire feed, which is used to drive the processor, LEDs and a LCM display in the telephone. Signalling to and from system telephones is also performed over this pair. Each of the six extensions driver circuits shown in Figure 3c consists of two DC constant current sources (A+C legs) and two current sinks (B+D legs) to give two balanced feed circuits. An alternative leg connection B' is provided for ring detection. In addition, tone detect, path and C+D wire data transmit and receive connections are provided. Under program control the slave processor IC9 scans the extensions via its analog ports AN2-AN7 which are used to detect extension hook status, ringing and loop disconnect signalling.

A 12x8 crosspoint switch circuit IC12 is used to route extension path signals to other extensions/lines. Two 4x4 crosspoint circuits ICl0,ICl1 are used to route dial tone and paging signals to the extensions via series resistors.

The series resistors are needed as connecting a telephone to a zero Ohm dialtone source would kill DTMF signals from it. The ringing signal is split to ring two groups of extensions (extensions 1-4 and extensions 5-6). This reduces both the heat dissipation in the series resistors R67,R68,R73,R74 and 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 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 and resistor pairs C63,R72,C63,R66 are used for snubbing across the triacs. Relays RL1 00,RL200,RL300,RL400,RL500, RL600 are used to route ringing voltage out to their respective extensions.

Three DTMF receivers ICl3,IC14,IC15 are provided in the extension circuit, as shown in Figure 3a, are connected to horizontal paths H1,H3,H9 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.

The processor IC9 uses a number of ports or lines PB0-PB7 as a data/address bus to access the crosspoint switches, DTMF receivers and also for I/O expansion via a latch IC6 to drive the six ringing relays via a Darling driver circuit IC17.

The six extension path connections A,B,C,D are brought to the outside world via extension connectors CN2-CN7. Varistors VR100,101,200,201 , etc. provide protection on the extension ports against high voltage spikes.

Capacitors C102-C107......C602-607 in the extension drive circuits provide radio frequency suppression.

The line circuit, as shown in Figures 4a and 4b, is used to interface two PSTN telephone lines to the system. Like the extension section it is managed by a single chip processor IC18 which monitors the lines for incoming ringing, sends information back to the controller circuit and, when instructed, seizes lines using a hybrid IC hold” circuit HYx to make and receive calls.

As the system is non-metallic path based, external call, call hold, and signalling are performed by using a line buffer hybrid circuit HYx. The hold circuit consists of a active current sink with a high AC impedance. The path signal from each of the two line hybrid ICs is transformer coupled (at TRx50) to the 4x4 crosspoint switch IC23 used to route calls via the horizontal speech path matrix to the extension circuit in order to set-up external calls. Each line buffer hybrid circuit HYx provides an interface between exchange lines and the PABX system and comprises a very low loss transmission circuit for speech. The buffer circuit provides output signals to its associated line circuit for determining exchange line status and instructing a crosspoint switch matrix on the line circuit for placing external calls.

A DTMF tone generator IC21 is switched via a latch IC22 and used for the regeneration of the internal signalling information from system telephones onto the lines, as well as stored number dialling, Decadic to MF conversion, etc.

This MF transmitter is also routed to the crosspoint switches in the extension circuit to enable the sending of MF digits out on an extension port. The music signal for calls on hold is also switched via the latch IC22.

Included on each of the two line hybrids HYx is a buffer amplifier, the output (pin 8) of which is the line voltage (on pin 10) scaled down to be fed into the microprocessor's IC1 analogue ports (AN1-AN4) to check line status and perform incoming ringing detection. In addition, there are two other outputs on the hybrids, one is a tone signal on pin 9 from the line, squared off for tone frequency detect, and on pin 11 the same tone signal, peak detected, for level measurement.

Decadic dialling is performed by turning opto-couplers 0P15 and OP25 on, (reducing the hold circuit impedance) and turning opto-couplers OP150 or OP250 on and hence the hold circuit current sink on and off.

Relays RL152, RL252 are used to switch between putting the hold circuits on-line, or connecting the idle impedance across the line. Relays RL151, RL251 are used for earth signalling. When fired, they shunt the A+B legs through a resistor to Network Earth. An Op-Amp IC125 and associated resistors monitor for 50Hz metering pulses on the line A and B legs with respect to Network Earth. The resulting signals are fed to the motherboard processor IC1 for call costing. k Although the processor in the line circuit only controls two lines, it is capable of controlling an additional two line interfaces provided on a two line/six extension expansion daughterboard. All control signals and speech paths are routed from the expansion daughterboard into the 10 line circuit via connectors headers HD18, HD19.

The two line AB pairs are brought to the outside world via a connector CN1. Both lines are power failed to the first two extensions via relays RL101, RL201. Varistors VR150, 15 VR151, VR152, VR250, VR251, VR252 provide protection for the line circuits against longitudinal and transverse voltage spikes. Capacitors C150, C151, C152, C250, C251, C252 provide suppression of radio frequency signals coming in on the lines.

The two line/six extension expansion daughterboard is essentially a subset of what exists on the motherboard. The daughterboard extension circuit which controls the six extension ports is identical to the extension circuit already described and shown in Figures 3a to 3c. A connector HD1 is provided on the daughterboard for connection to a corresponding connector on the PSU circuit for all power equipment. All control signal and speech paths etc, required by the daughterboard are provided from 3Q the motherboard via a connector or header HD2. All control and speech paths required by the two line interfaces on the daughterboard are supplied via connectors or headers HD18, HD19. Both line interfaces are identical to those already described and shown in Figures 4a and 4b. All control functions are carried out by the line processor IC18 in the line circuit of the motherboard.

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 (4)

1. A private automatic branch exchange (PABX) telephone system of modular construction having a motherboard comprising a power supply unit (PSU), a line circuit having a plurality of telephone line connections provided thereon, an extension circuit having a plurality of connections for extension telephones, a controller and a connection means adapted to receive a daughterboard, wherein the PABX may be provided in a minimal configuration or with the addition of one or more daughterboards in 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.

2. A PABX telephone system as claimed in claim 1 having a minimal configuration which comprises the motherboard and a user interface console unit, wherein the PABX is expandable by the addition of a daughterboard 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 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 preceding claims wherein a buffer circuit is provided between an incoming exchange line and the telephone system which buffer circuit provides output signals to its