GB2148666A - Private automatic branch exchange - Google Patents

Abstract

A private automatic branch exchange apparatus comprises a power supply and ring generator circuit, ten extension line circuits, and two public exchange line circuits, a switching matrix, a tone generator, and a control circuit to control the operation of the other circuits. The apparatus includes means for hunting to direct incoming calls from the extension line to another until a non-engaged extension is found or until a line is answered. The apparatus also comprises a dual tone multifrequency receiver and sender. The receiver comprises a filter (IC53), a decoder (IC52) and a fast detector (IC54a, IC54b and IC55a). The fast detector comprises a pair of voltage comparators (IC54a and IC54b) fed through potential dividers (R4, R5, R10 and R11) from the filter, and a logic AND gate (IC55a) fed by the voltage comparators through rectifiers (D1, D2), which in turn is scanned by the control circuit. <IMAGE>

Description

SPECIFICATION Private automatic branch exchange The present invention relates to a private automatic branch exchange apparatus.

According to the invention there is provided a private automatic branch exchange apparatus com- prising a power supply and ring generating circuit, at least one extension line circuit, at least one public exchange line circuit, a switching matrix, a tone generator, and a control circuit to control the operation of the other circuits.

In one embodiment of the invention the apparatus comprises a means for hunting, so that incoming calls huntfrom one extension line to the next, until a non-engaged line is available.

In another embodiment of the invention the hunting means permits hunting from one extension line to the next when a line is not being answered.

In a further embodiment of the invention means are provided to alert a user of an engaged extension line that an incoming call istrying to get through.

In a still further embodiment ofthe invention the apparatus comprises a dual tone multifrequency receiver,the receiver comprising a filter, a decoder and a fast detector.

Preferably the fast detector comprises a pair of voltage comparators fed through potential dividers from the filter, and a logic NANDgate fed by the voltage comparators through a rectifier, which in turn is scanned by the control circuit.

Advantageously the apparatus comprises a dual tone multifrequencysender.

In one embodiment of the invention means to receive up to ten extension line circuits are provided.

Advantageously means to receive up to two public exchange lines are provided.

Preferablythe ring generator comprises a means for generating a ring voltage from an AC supply, the generating means comprising meansforturning on and offthe AC supply atvarious timesthroughout each cycle ofthe AC supply.

In a further embodiment of the invention the developed voltage decays across a capacitive circuit, on the AC supply reaching its peak value afterthe optocoupled triac is turned off, and the AC supply comprises two components 180 out of phase, and the optocoupled triacs areturned on and off by the control circuit.

The invention will be more clearly understood from the following description of a preferred embodiment thereof, given by way of example only with reference to the accompanying drawings in which: Fig. lisa circuit diagram of a power supply and ringinggeneratorofa private automatic branch exchange (PABX) according to the invention.

Fig. 2 isa control circuit diagram ofthe PABX of Fig.

1, Fig.3isa circuit diagram of a switching matrixtone generator of the PABX of Fig. 1, Fig. 4 is a circuit diagram of an extension line of the PABX of Fig. 1, Fig. 5 is a circuit diagram of an exchange line circuit of the PABX of 1, Fig. 6 is a circuit diagram of a switch matrix cross point arrangement of the PABX of Fig. 1, Fig. 7 a to f is a graphical representation of outputs ofthe ring voltage of the PABX of Fig. 1, Fig. 8 is a circuit diagram of a dual tone multifrequency receiver and sender of the PABX of Fig. 1, Fig. 9 is a graphical representation of the scanning system ofthe dual tone multifrequency receiver of Fig 8,and Fig. 10 a to f is a graphical representation of outputs of the ring voltage of a PABX apparatus according to another embodiment ofthe invention.

Referring to the drawings there is illustrated circuit diagrams of a private automatic branch exchange (PABX) according to the invention. The PABX has facilities for receiving up totwo public exchange lines and ten internal extension lines. It comprises the following circuits which will each be described in detail under separate headings.

1. A power supply and ring generator.

2. An extension circuit.

3. An exchange line circuit.

4. Acontrol circuit.

5. Aswitching matrix and tone generator.

6. A dual tone multifrequency receiver.

7. A dual tone multifrequency sender.

PowerSupplyand Rlng Generator The power supply circuit generates the following voltages: (a) -48 Volt DC supplyto power relays RL1 ? and RL12 in the exchange line circuit (Fig. 5), and the extension lines (Fig.4).

(b) -43 Volt DC supplyto power relays RL1 to RL10 in the extension line circuits (Fig. 4).

(c) +5 Volt DC supply to power most of the integrated circuits in the systems (Figs. 2,3,4,5).

(d) + 12 Volt DC supply to power the remaining integrated circuits and relays. The +5 Volt DC supply and the + 12 Volt DC supply also provide the source voltagesforthetransistors and the reference voltages forthe voltage dividers used in the systems.

(e) 5 Volt pulse, 100Hz as an interruptforthe control circuit (Fig.2).

(f) 50 Volt, 25Hz which provides the ringing signal for the system.

AtransformerT1 fed from the mains at 220 V provides three supply voltages, namely a 16 V, 45 V and 50 V AC supplies. Dealing firstly with the 16 V supply, a circuit comprising a rectifier BR3 and regulator REG 1 provides the regulated 12 V DC supply. A smoothing capacitor C56 in the circuit smoothes the full-wave rectified output of BR3. The common line is maintained at a fixed potential 1.2 V below earth by the voltage drop across diodes D38 and D39, which are biased on by a trickle current flowing through a resistor R1 38. The level of output voltage from the regulator REG 1 is determined by a potential divider comprising resistor R1 39 and R1 40.

The +5 V output is provided by a regulator REG 2 through a potential divider formed by resistors R141 and R142.The regulator REG 2 is supplied bythe 12V supply from the regulator REG 1.

Diodes D35 and D36 produce a full-wave rectified 100 Hz signal which is fed th rough a potential divider comprising resistor R135 and R136to the input of an inverter IC25. A resistor R137 holds the interrupt line at 5 V. When the output ofthe inverter goes low, a negative going pulse on the interrupt line is produced.

The duration ofthe pulse is determined bythetime constantofthe resistorR137 and a capacitorC57.

When the output ofthe inverter goes high again, the voltage ofthe interrupt line is tied to the 5 V line by a diode D37.

Dealing now with the 45 V AC supply, a full-wave rectifier BR4 is fed with the 45 Vsupply, and in turn feeds the input of a regulator REG 3, which provides the stable -48 V DC.

The 43 V DC supply is derived from the -48 Vsupply througha5VZenerDiodeZD31.

Two 50 VACwindings on thetransformer provides the AC component ofthe ringing voltage. The ringing voltage is developed across capacitors C51 and C52 by turning the optocoupled triacs IC26 and IC27 on at various points in the cycle ofthe 60 he supply to produce a voltage with a fundamental frequency of 20 Hz. Thetraics are switched on by a control signal from the microprocessors on output ports OUT23 and OUT 24. This is described in detail with reference to Fig. 1.

The waveforms and timing ofthe control signals are illustrated in Fig. 7.

When the positive-going control pulse is applied to the base oftransistorTRl 8 the triac lC26 conducts and the voltage across the capacitors C51 and C52follows the waveform of V1 until it reaches its negative peak value. Atthis point the currentthrough the triac IC26 changes direction and the triac switches off. Both triacs remain in the non-conducting statefora short time, and the voltage across capacitor C51 and C52 decays at a range which depends on the current drawn by the load. When the positive going control signal is applied to the base of transistorTR17, triac IC27 is turned on and the voltage across the capacitors C51 and C52 follows the waveform of V2 until it reaches the negative peak, see Fig. 7.At this point the triac switches off and the voltage decays slowly until the triac IC26 is again turned on and the ringing voltage follows the waveform of V1 until it reaches its positive peak value. At this pointthe triac switches offand the voltage again decays. Triac IC27 then turns on for a short time until V2 reaches its positive peak and then turns offagainto complete the cycle. Capacitors C53 and C54 are provided for suppression of transient voltages produced when the triacs turn off.

Extension Line Circuit Referring now to Fig. 4 an extension line circuitfor the PABX is illustrated. For simplicity, all components designations will be those associated with extension 1, which in use is extension 21. Each leg of the line is fed from the48V supplythrough a 740 ohm resistance R71 and a winding of an inductance L1. Resistor R61 and capacitor Cl 1 provide proper impedance conditions on the line during speech transmission.

Ringing voltage is provided on the extension by operating relay RL1.The relay is operated byturning on a transistorTR1 from the output port OUT 9 ofthe control circuit,which is described below.

The operation ofthe off-hook detection circuit depends on the conditions on the extension line, that is, DC feed or ringing voltage. With DC conditions on the line the off-hook detection is done by a potential divider comprising resistors R1 and R1 1. The voltage atthe mid-pointofthe divider is held high in the on-hook state giving a logic "1" on the input port IN1 of the control circuit, Fig. 2. When the telephone is taken off-hook a low-resistance loop is connected across the a and b legs. The voltage on the a leg will therefore drop and pull the mid-pointofthevoltage divider low giving a logic "0" onthe in-put port IN1 of the control circuit.

When ringing voltage is applied tithe lire the off-hook detection is done by a voltage dividerwhich comprises resistors R21, R3T and RE1. When the telephone is on-hookthe voltage atthejunction of resistors R21 and R31 is held at +5 V. The AC componentofthe ringing current flows through resistor R51 but because of the long time constant of the resistorlcapacitor network R21, R31 and Cl (~100 ms), the voltage at the junction ofthe resistors R21 and R31 does not vary to any significant extent.

When the telephone goes off-hook the DC compo nent of the ringing also flows through the resistor R51, thus pulling thevoltage level at the junction of resistors R21 and R31 low and changing the logic level on input port IN1 from "1 " to "O".

Capacitors C21 and C31 isolate the switching matrix described below, from the signalling voltages used on the line. Zener diodes ZD1 and ZD1 1 protectthe solid state switching arrays from voltage surges which may occur during connection and disconnection ofthe ringing voltage or during decadic pulsing from the extension.

Exchange Line Circuit Referring nowto Fig. Sthe exchange line is illustrated.

Relays RLl 1 and RLl 2 are continuously operated while power is connected. If the power supply fails or is disconnected, relays RL11 and RL12 release and connect the exchange lines directly to extensions 1 and 2which in this case is extensions 21 and 22, The component designations used in the description below are those associated with exchange 1ine 1.

Incoming Call Dealing firstly with incoming callus, wheThe.public exchange sends ringing on the exchange lines activates the ringing detector which comprisesoptocoupler IC28 and its associatedcircuitry. Ringing current flows via capacitor C4T, resistance R161, zener diodes ZD27 and ZD29 to the aptocoupleFThe rippie component of the voltage onthe basof tlXe optocou- pled transistor is removed by capacitorC43so that the transistor conducts continuously during each burst of ringing currentthus putting a low on input port IN 11.

To answer an incoming call relays RL13 and RL15 are operated.

This connects the exchange line through to the transmission bridge. The Darlington Pair transistors TR19 andTR21 act as a current sinkforthe DC current flowing in the line. When the DC line voltage is connected to the sinkthe voltage across capacitor C45 builds up and turns on TR19 and TR21 which then carry most ofthe DC current. The voltage on the base of is stabilised by the capacitor C45 which acts as a low impedance path forAC signals and by transistorTR23 which acts as a feedback element.

This gives the current sink a high impedance to AC signals in the line. The rectifier bridge BR1 ensures that the polarity of the voltage atthe current sink is correct regardless ofthe polarity on the exchange line.

A gas dischargetube GDT 1 is connected across the line for protection against atmospherically induced voltage surges. Additional protection is provided by resistors R123 and R125 and varistors VR1 and3.

The fast-acting Zener DiodesZD21 andZD23 are placed at the inputto the switching matrix to protect the solid state arrays and ZD25 similarly protects the elements in the current sink.

Outgoing Call To seize an exchange line relays RL13 and RL15 are operated allowing line current to flow through the currentsink. Loop-disconnect signalling pulses can nowbetransmittedtoline. Before each pulsetrain commences relay RL15 is released to connect a short circuit between the a and b legs. Pulses are sent to line by alternately releasing and operating RL13. Spark quenching for the contact of relay RL13 is provided by capacitor C41, resistor R161, zener diodes ZD27, ZD30 and the optocoupler IC28. At the end of a pulse train relay RL15 is operated again to re-establish speech conditions.

When speech conditions have been set up on either an incoming or an outgoing call the exchange line may be put on 'hold' by releasing relay RL13 while holding relay RL15 operated. This connects a 600 ohm resistive termination across the exchange line.

When relay RL13 is re-operated speech conditions are re-established. When the exchange line is in the 'hold' state protection against over-voltage is provided by the positive temperature co-efficient resistorVR5.

When RL13 and RL15 are released the call is cleared and the exchange line returnstothe 'idle' state.

A summary ofthe Circuit conditions for each line state are give in Table 1 below: Table I - Input/Output Ports Line State IN 11 OUT21 (RL13) OUT22 (RL15) Idle High Low (Rel.l Low (Rel.) Seized High High (Op.) High (Op.) Loop-dis. Make High High (Op.) Low (Rel.) Break High Low (Rel.) Low (Rel.) Speech High High (Op.) High (Op.) Clear High Low (Rel.) Low (Rel.) Ringing Low Low (Rel.) Low (Rel.) Answer High High (Op.) High (Op.) Hold High Low (Rel.) High (Op.) Control Circuit Fig. 2 illustratesthe control circuit. The control circuit comprises four sections: i) Central Processing Unit (CPU) IC1 and associated circuitry, ii) Programme Store, lC2, iii) Data Store, lC3, iv) InputlOutputSection (1/0).

The CPU IC1 is an RCA 1802 COSMAC microprocessor. It is a single chip CMOS 8-bit registerorientated microprocessor. A detailed guide to the architecture and programming technique is contained in the 'User Manual MPM-201 C'.

The Programme Store IC2 is an 8K 2764 EPROM, and the Data Store IC3 is a 2K RAM.

The 1/0 section contains address decoding and latching circuits for the 16 Input and 24 Output ports.

At power-up the CLEAR line of microprocessor is held lowfora period determined bythe time constant of resistor R145 and capacitor C60. This ensures that the programme does not run until all parts ofthe circuit are fully powered-up. When the CLEAR line goes high program execution begins from the first location in the Program Store. A4-pole dual-in line switch SW1 selects various programme options.

The speed of operation of the processor is determined bythefrequency of a crystal CR1 (3 MHz).

The microprocessor has eight address lines. These eight lines supply a 1 6-bit address in the form oftwo consecutive 8-bit bytes. The higher order address byte appears on the address lines first followed by the lower order byte. The timing pulseTPA is used to strobe the higher order address byte into an address latch consisting of integrated circuits IC4and IC5.

The allocation of addresses in the system is as follows: Programme Store 0000 - 1FFF Spare 2000 - 7FFF Data Store 8000 - 87FF Spare 8800 - FFFF The input and output ports are divided into five groups of eight. Each group is connected to an octal latch circuit IC8 - IC12. The CPU can interact with any one ofthe groups by sending out the identity ofthe grouponthe N lines, NO, N1 and N2.The identity is converted from parallel binaryformatto one-out-offive in the decoder chip IC6. When a group of ports is selected in this way information can betransferred between the eight ports and the eight lines of the processor data bus. In the case ofthe input ports the direction ofthe information flow is from port to data bus. In the case ofthe output ports the information flows in the opposite direction.

The processor can determine the state of various elements in the PABX circuitvia the input ports, for example, extension off-hook, incoming call on exchange line etc., and can in turn send out signals via the output ports in response.

A detailed table of the functions ofthe input and output ports is given atthe end ofthis description.

Switching Matrix Fig. 6 illustrates the switching matrix schematical ly. The matrix comprisestwo types of switches, ,namely4 x 4 x 2 arrays which are lC17, lC18 and IC19, and 2 x 2 x 2 arrays which are IC20, IC21, IC22, IC23 and IC24. ICs 17 to 19 are used for setting up connections between two extensions orforconnect ing a tone to an extension. A call between two extensions must use one ofthetwo internal paths.

Two sets of crosspoints are operated therefore to set up such a connection. Resistor group RG8 - RG12 are connected in series with the extensions.

In conjunction with the parallel RC circuits consist ing of resistor R157 and capacitor C66 (Internal Path 1) or resistor R1 58 and capacitor C64 (Internal Path 2) theyform an attenuation pad on extension to extension calls.

The line marked 'Tone' in Fig. 6 is connected to a single frequency generator (425 Hz). The different tones required in the system (Dial Tone, Busy Tone, RingbackTone etc.) are derived by applying different cadences to this signal. The control circuit can connect a given tone to an extension by operating and releasing the crosspoints connecting the exten sion to the 425 Hzsignal in such a manner asto produce the cadence of the required tone.

The 425 Hzfrequency is derived by applying the output of a square-wave generatorthrough a low pass filter. IC25b and IC25eform an astable multivib ratorwhich oscillates at 425Hz. The output of this is applied via the buffer IC30a and a voltage divider to the active low-passfilterformed by IC30b, C62, C63 and R1 56. IC30e acts as a buffer between thefilter output and the switching arrays.

IC20 - IC24 are used for connecting extensions to the exchange lines. Such a connection requires operation of a single set of crosspoints only.

The crosspoint arrays operate from a 12 Volt power supply compared with a 5 Volt supplyforthe Control Circuit. IC13 and IC14 act as voltage level shifters at the interface. The Control data for operation and release of crosspoints is sent from output ports OUT 1 - OUT8 and the Qoutputfrom the microprocessor.

The data is in parallel binaryformat. OUT 1 - OUT4 are decoded by binary to decimal decoders IC15 and IC16 to select the appropriate crosspoint IC.

Dual Tone Multifrequency Fig. 8 illustrates the dual tone multifrequency (DTMF) receiver circuit.

The DTMF Receiver comprises three elements, namely, a filter, a decoderandafastdetector.

Thefilter provided by integrated circuit IC53 separates the incoming DTMF signal into its high group and low-group components. The resulting pair of sine-waves FHTare limited to produce square waves having the same frequencies as the individual inputtones. The limited high group and low group tones appear at the FH and FL outputs respectively.

The decoder provided by the integrated circuit IC52 accepts the high-group and low-group square wave signals from the Filter and provides a zri-state 4-bit binary output. When both high-group and low-group signals have been simultaneously detected aflag EST goes high.

The final validity check requires the input DTMF signal to be present uninterrupted by drop-out or excessive distortion (which would result in EST going low again) fora minimum time before being considered valid. Capacitor C2 (Fig. 8) is charged via resistors R2 and R3 in series from ESTwhen a DTMF tone pair is detected. After a period determined bythe time constant of capacitor C2, and resistors R2 and R3, the voltage on ST exceeds its inputthreshold which sets an internal flag indicating the detected signal is valid. A4-bit binary code word corresponding to the received character is stored in the output latch and the STD output goes high to indicate a valid character has been received.

The fast detector provided by voltage comparators IC54a and IC54b and a NAND gate IC55a and their associated circuitry gives a rapid indication of the presence of a high-group and a low-group compo nent. The FHT and FLT outputs of the Filter are connected to voltage comparators IC54b and IC54a respectively. The square-wave outputs of the comparators are rectified and applied to the inputs of the NAND gate IC55a which gives a logic low at its output IN14.

The DTMF receiver is connected to a horizontal of the solid state switching matrix, see Fig. 6, when an extension goes off-hook the appropriate cross-point is operated to connect the receiver to the extension.

Signalling from the extension may be either decadic or DTMF. If first digit received is decadicthe receiver is disconnected.

The handling of DTMFdigits depends on the number of extensions which are in a 'DTMF Signalling State' at any given time.

If there is only one extension in this state the receiver is connected to the extension as soon as the off-hook condition is detected and remains connected until all digits have been received or the inter-digittime supervision expires.

If more than one extension is in a 'DTMFSignalling State' the extension number is placed in a queue. If the MF receiver is free it is connected to the extension atthetop of the queue. In the next primary interval, see Fig. 9 the Fast Detectorwill indicate the presence (or absence) of a high-group and low-group tone.

If expected signal is absentthen the receiver is disconnected, the extension number is returned to the end of the queue and the receiver is connected to the extension atthetop ofthe queue.

If the expected signal is present then the receiver remains connected for a furthertwo primary intervals to allow validation ofthe signal. When the signal is received the receiver is disconnected,the extension number is returned to the end ofthe queue and the receiver is connected to the extension atthe top ofthe queue.

When all digits have been received orwhen the inter-digittime supervision expires the extension is removed from the queue.

Dual Tone MultifrequencySender Fig. 8 also illustrated the DTMF slender. DTMF signals are generated by a CMOS integrated tone dialler IC58 primarily intended to interface directly to a standard telephone keypad and generates all dual tone multifrequency pairs required in tone dialling systems. When theappropriatesignals areappliedto the control inputs R1 - R4 and C1 - C4, see Fig. 8, the tone pair is applied via the emitterfollower circuitTR1 to the drain input ofthe switching FET's TR2 - TR5.

The DTMF signal may be applied to either exchange line circuit by operating two of the FET's.

An octal latch IC56 provides 6 additional output ports OUT 25 - OUT 30. The information to control the digit sent by IC58 is generated using OUT 25 - OUT 28.

This is converted, by IC57 which is a dual 1 -out-of-4 decoder, to the correct form for application to the inputsoftheDTMFgenerator.Thecoding of OUT25 to OUT 28 is shown in Table 3.

When the control data is set up the tone sending is initiated by setting OUT 29 low which also operates a pair of FET's to transmit the tone to line. The pair of FET'swhich operates is determined bythestate of OUT 30.

DTMFSending The DTMF generator is used only on calls where last number redial or the abbreviated dialling has been invoked. On all other extensions to exchange line calls the MF signalling comes directly from the extension. When the system requires generation of an MFsignal, the control data forthe digit is set up on output ports OUT25-OUT28 according to the arrangementin Table 3. Simultaneously OUT30 is set toselectFETpairforconnection ofthesendertothe required exchange line. In the next primary interval OUT 29 goes low to initiate sending ofthe DTMF signal andtoturn on the FET pair.

Approximately 70 milliseconds later OUT 29 goes high again to terminate the signal and to turn offthe FET's. A minimum pause time of 70 milliseconds is allowed between digits.

Table 2 - Input/Output Ports Input Port Function 1 Extension 1 Line Condition (On-Hook = 1; Off-Hook = 0) 2 Extension 4 Line Condition 3 Extension 2 Line Condition 4 Extension 3 Line Condition 5 Extension 9 Line Condition 6 Extension 10 Line Condition 7 Extension 8 Line Condition 8 Extension 7 Line Condition 9 Extension 6 Line Condition 10 Extension 5 Line Condition 11 Exchange Line 1 Ringing Detector (Ringing on = 0;Ringing off = 1) 12 DTMF Decoder 13 Exchange Line 2 Ringing Detector 14 DTMF Fast Detector 15 Spare 16 Spare Output Port Function 1 Operation of Switching Matrix 2 Operation of Switching Matrix 3 Operation of Switching Matrix 4 Operation of Switching Matrix 5 Operation of Switching Matrix 6 Operation of Switching Matrix 7 Operation of Switching Matrix 8 Operation of Switching Matrix 9 Extension 1 Ringing Relay (0 = Ringing On; 1 = Ringing Off) 10 Extension 4 Ringing Relay 11 Extension 2 Ringing Relay 12 Extension 3 Ringing Relay 13 Extension 9 Ringing Relay 14 Extension 10 Ringing Relay 15 Extension 8 Ringing Relay 16 Extension 7 Ringing Relay 17 Extension 6 Ringing Relay 18 Extension 5 Ringing Relay 19 Exchange Line 2 - Relay RL16 (1 = Operated;; 0 = Released) 20 Exchange Line 2 - Relay RL14 21 Exchange Line 1 - Relay RL13 22 Exchange Line 1 - Relay RL15 23 Ringing Voltage Generator (1 = Triac On; 0 = Triac Off) 24 Ringing Voltage Generator 25 Control Data for DTMF Sender (See Table 3) 26 Control Data for DTMF Sender (See Table 3) 27 Control Data for DTMF Sender (See Table 3) 28 Control Data for DTMF Sender (See Table 3) 29 Operation of DTMF Sender and FET pair 30 Selection of FET pair for connection of DTMF sender to Exchange Line.

Table 3 - OUT 25 - OUT 28: Coding of Data Output Ports Digit 28272625 R4 R3 R2 R1 C4 C3 C2 C1 1 LLLL HHHL HHHL 2 LLLH HHHL HHLH 3 LLKL HHHL HLHH 4 LHLL HHLH HHHL 5 LHLH HHLH HHLH 6 LHHL HHLH HLHH 7 HLLL HLHH HHHL 8 HLLH HLHH HHLH 9 H L H L H LH H H LH H 0 HHLH LHHH HHLH * HHLL LHHH HHHL &num; HHHL LHHH HLHH A LLHH HHHL LHHH B LHHH HHLH LHHH C HLHH HLHH LHHH D HHHH LHHH LHHH The useofthe PABXwill now be described.

Making Calls Making an Internal Call 1. Liftthe handset and waitfor internal dial tone.

2. Dial number of required extension '20' - '29'.

Making an External Call When making an external call the exchange line to be used may be selected by the system or bythe user.

Also the previous external number called may be redialled automatically.

System Selection Exchange Line 1. Liftthe handset and waitfor internal dial tone.

2. Dial '9' and wait for public exchange dial tone.

3. Dial required number(18 digits maximum).

The system selects the Exchange Line as follows: If only 1 line isfree, thatfree line is selected.

If both lines arefree,that line which was not selected forthe East external call is selected.

User Selection Exchange Line 1. Liftthe handset and waitfor internal dial tone.

2. Dial '31 'to select exchange line No. 1 or '32' to select exchange line No.2. Waitfor public exchange dial tone.

3. Dial required number (18 digits maximum).

Repeat Last Number Each extension retains in its memorythe last number dialled by that extension on an external call.

To re-diaWthis number automatically, lift the handset, waitfor internal dial tone, dial '5'. The system will automatically select and seize an exchange line as in system selection of exchange line above, and after a 2.5 second pause, will send outthe digits ofthe last number dialled on an external call.

The2.5 second pause isto ensure that digitsending does not commence until it is highly probable that public exchange dial tone has been received.

Receiving Calls To answer a call at an extension which is ringing, just liftthe handset and speak. Different ringing candences aregiven at the called extension for calls originating internally and externally (See Page 38).

Handling oflncoming External Calls 1. If Night Service has not been invoked (See Page 37) all incoming exchange line calls are presented initiallyto Extension 21 if it is free. The extension answers by lifting the handset.

2. If Extension 21 isengaged orfailsto answer within 20 seconds, the call is presented to Extension 22 if it is free.

3. If both Extensions 21 and 22 are engaged, an alerting tone is given to whichever one is not engaged on an external call.

If both extensions are busy on internal calls, the alerttone is given first of all to Extension 21. If Extension 21 then hangs up within 20 seconds, the call rings at this Extension and is answered by lifting the handset. If Extension 21 does not hang up within 20 seconds, the alert tone is transferred to Extension 22.

4. If after a further 20 seconds, Extension 22 has not gone on-hook, the call is presented to Extension 23. If a central bell has been fitted at Extension 23 the call rings at the central bell until answered. If a central bell is not fitted, the call rings for 20 seconds at each free extension in turn, starting at Extension 23 and continuing to Extension 20 before returning to Extension 21 or 22 where it will remain until answered or until the caller hangs up.

5. If Night Service has been invoked (See Page 37), incoming exchange line calls ring Extension 23 only, or a central bell if fitted at this position. There is no hunting as described in 1-4 above.

Universal Answer Any extension mayansweran incoming exchange line call which is ringing at another extension or at a central bell by lifting the handset of his own extension and dialling '8'.

Call Hold/Enquiryffransfer/Conference - External Calls The following assumes that speech connection has already been established between an extension and an exchange line.

The connection may have been set up by the extension dialling an external numberorbythe extension answering an incoming external call.

1. Putoutside line on "Hold" by dialling '1 '.

2. Internal dial tone is now heard.

3. Dial extension number required. If busy, revert to outside line by dialling '1'. If not busy, called extension rings. On answer, confer in private (enquiry).

4. Revert to outside line bydialling '1'. This puts the 'enquiry' extension on hold.

5. Shuttle back and forth as required between the 'enquiry' extension and the outside line by dialling '1 ' each time.

6. Transferthe outside call to the other extension by hanging up while in speech connection with the other extension.

7. To have a conference (outside line + own extension + another extension) dial '4' after making speech contact with the other extension as described in 1-3 above. Either extension may subsequently withdraw from the conference (without affecting the connection ofthe other extension to the outside line) by hanging up.

8. All attempts to abandon an outside line which has been put on "Hold" e.. by hanging up, will result in the outside line ringing back the extension which abandoned it.

Call Hold/Enquiry/Conference - Infernal Calls The following assumesthatspeech connection has already been established between 2 extensions.

1. Dial '1' to put the 2nd extension on "Hold".

2. Internal dial tone is now heard.

3. Dial number of 3rd extension. When answered, conversation proceeds in private. If 3rd extension is busy, revertto the 2nd extension by dialling '1'.

4. Shuttle back and forth as required between 2nd extension and 3rd extension by dialling '1' each time.

5. To have a conference (own extension + 2nd extension + 3rd extension) dial '4' while in speech contact with the 3rd extension. Any ofthe 3 extensions may subsequently leave the conference (without affecting the othertwo) by replacing the handset.

Call Diversion Call Diversion allows any extension to divert all incoming calls to another 'host' extension (e.g. to a Secretary). Calls may still be made from the extension.

To divert one's calls: 1. Liftthe handset and waitfor internal dial tone.

2. Dial '41 ' followed by number of'Host' extension (e.g. to divert ones calls to Extension 25 dial '4125').

3. Acknowledgement (Special Dial Tone) is now heard to indicate acceptance of diversion.

4. Special Dial Tone (See Page 31) is subsequently heard bythe diverting extension whenever he lifts his handset as a reminderthat he has diverted incoming calls.

5. Diversion will not be accepted if an attempt is made to divert calls to an extension which has already diverted incoming calls. Busytoneisgiven if this is attempted.

6. An extension which is already acting as 'Host' for another extension cannot divert calls. Busytone is given ifthis is attempted.

7. The 'Host' extension is allowed to call the diverting extension and to transfer calls to it.

8. Diversion is cancelled by dialling '41' and then hanging up. Removal of diversion is indicated by receipt of Normal Dial Tone whenever the handset is subsequently lifted.

Night Service A "Night Service" facility is availablewhereby incoming exchange line calls ring at 1 extension only (Extension 23) or at a central bell iffitted in the position. Night Service may only be invoked or revoked from Extension 21.

1. To invoke Night Service, liftthe handset of Extension 21 ,waitfor internal dial tone, and dial '44'.

Dial tone returns as acknowledgement.

2. To revoke Night Servie, liftthe handset of Extension 21, waitfor internal dial tone and dial '45'.

Dial tone returns as acknowledgement.

TECHNICAL DATA Internal Tones Thevarious tones encountered when using the apparatus are as follows: Dial Tone: 425 he continuous Busy Tone: 425Hz - 0.38 sec. on, 0.38 sec. off RingbackTone: 425 he - 1.0 sec. on, 2.55 sec. off AlertTone: 425Hz- 0.1 sec. on, 2.50 sec. off Special Dial Tone: 425 he - 0.75 sec. on, 0.75, sec. off Ringing Cadence Different ringing cadences are applied to extension telephones depending on whether the caller is internal or external.

Internal: 25Hz - 1.0 sec. on, 2.0 sec. off External: 25 he - 0.4 sec. on, 0.2 sec. off 0.4 sec. on, 2.0 sec. off Signalling to Public Network Signalling to the Public Network is by means of loop disconnect pulsing at 10 impulses per second.

Break is 70 milliseconds, make is 30 milliseconds. The inter-digital pause is 850 milliseconds, including a pulse loop setup time of 350 milliseconds. The first digit is not sent to line until at least 2.5 seconds after the line has been looped by the apparatus.

Referring now to Fig. 10, and in particularto Fig. 10 f, there is illustrated a ring voltage wave form generated by a ring voltage generator of a PABX apparatus according to another embodiment of the invention. In this case,the two 50 V AC supplies have frequencies of 50 Hz. The ring voltage generator is similar to that described with reference to the apparatus of Figs. 1 to 10, however, in this case the waveform fis generated as follows: When the positive-going control pulse is applied to the base oftransistorTR18 of Fig. 1,thetriaclC26 conducts and the voltage across the capacitor C51 and C52 follows the waveform of V1 until it reaches its negative peak value.At this point the current through the triac IC26 changes direction and the triac switches off. Both triacs remain in the non-conducting state for a shorttime, and the voltage across capacitor C51 and C52decaysata rangewhich depends on the current drawn by the load. When the positive going control signal is applied to the base oftransistorTRl 7, triac IC27 isturned on and the voltage across the capacitors C51 and C52 follows the waveform of V2 until it reaches the positive peak, see Fig. 7. At this point the triac switches off and the voltage decays slowly until the triac IC26 is again turned and the cycle repeats. Capacitors C53 and C54 are provided for suppression oftransientvoltages produced when the triacsturn off.

It will of course be appreciated that while specific components have been described for use in the PABX, other suitable components could be used.

Additionally, it will be appreciated that while specific circuits have been described forthe power supply, ringing generator, exchange line, extension line, control circuit and switching matrix, other suitable circuits could also be used. It will of course be appreciatedthatwhilethe apparatus has been described as being suitable for up to ten extension lines and two exchange lines more or less could be provided without departing from the scope ofthe invention.

Claims (16)

1. A private automatic branch exchange apparatus comprising: a power supply and ring generating circuit, at least one extension line circuit, at least one public exchange line circuit, a switching matrix, a tone generator, and a control circuit to control the operation ofthe other circuits.

2. Apparatus as claimed in claim 1 in which the apparatus comprises a means for hunting, so that incoming calls huntfrom one extension line to the next, until a non-engaged line is available.

3. Apparatus as claimed in claim 2, in which the hunting means permits hunting from one extension line to the next when a line is not being answered.

4. Apparatus as claimed in any of claims 1 to 3 in which means are provided to alert a user of an engaged extension linethatan incoming call is trying to getthrough.

5. Apparatus as claimed in any of claims 1 to 4, in which the apparatus comprises a dual tone multifre quency receiver,the receiver comprising a filter, a decoder and a fast detector.

6. Apparatus as claimed in claim 5 in which the fast detector comprises a pair of voltage comparators fed through potential dividers from the filter, and a logic NAND gate fed by the voltage comparators through a rectifier, which in turn is scanned bythe control circuit.

7. Apparatus as claimed in any preceding claim in which the apparatus comprises a dual tone multifrequency sender.

8. Apparatus as claimed in any preceding claim in which the control circuit comprises a microprocessor.

9. Apparatus as claimed in any preceding claim, in which means to receive uptoten extension line circuits are provided.

10. Apparatus as claimed in any preceding claim, in which means to receive up to two public exchange lines are provided.

11. Apparatus as claimed in any preceding claim in which the ring generator comprises a means for generating a ring voltage from an AC supply, the generating means comprising meansforturning on and off the AC supply at various times throughout each cycle ofthe AC supply.

12. Apparatus as claimed in claim 11 in which the generating means comprises a pair of optocoupled triacs and a capacitive circuit across which the ringing voltage is developed.

13. Apparatus as claimed in claim 12 in which the developed voltage decays across the capacitive circuit, on the AC supply reaching its peakvalue after the optocoupled triac is turned off.

14. Apparatus as claimed in claim 13 in which the AC supply comprises two components 180 out of phase.

15. Apparatus as claimed in any of claims 12 to 14 in which the optocoupled triacs are turned on and off by the control circuit.

16. A private automatic branch exchange apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings.