IE54733B1 - Provate automatic branch exchange - Google Patents

Description

- 2 - - 2 - S4733 The present invention relates to a private automatic branch exchange apparatus.

According to the invention, there is provided a private automatic branch exchange comprising a power 5 supply and ring generating circuit, at least one extension line circuit, at least one public exchange line circuit, a switching matrix, a tone generator, a dual tone multi-frequency (DTMF) receiver, to receive a DTMF signal having a high group component and a low 10 group component, wherein the DTMF receiver comprises a filter to receive the DTMF signal and to separate the signal into its high group component and low group component, a decoder to receive high and low group component signals from the filter, and a detector 15 comprising a pair of voltage comparators, one to receive the high group component signal and the other to receive the low group component signal from the filter, and a NAND gate fed by the voltage comparators through respective rectifiers, the output of the NAND 20 gate being scanned by the control circuit. Preferably, the apparatus comprises a dual tone multi-frequency sender. - 3 - - 3 - S 4 7 3 3 Advantageously the apparatus comprises a means for hunting, so that incoming calls hunt from one extension line to the next, until a non-engaged line is available.

Preferably the hunting means permits hunting from one 5 extension line to the next when a line is not being answered.

Advantageously means are provided to alert a user of an engaged extension line that an incoming call is trying to get through.

Preferably means to receive up to ten extension line 10 circuits are provided.

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

Preferably, a ring generator comprises a means for generating a ring voltage from an AC supply, the 15 generating means comprising means for turning on and off the AC supply at various times throughout each cycle of the AC supply.

In a further embodiment of the invention the developed - 4 - - 4 - S 4 7 3 3 voltage decays across a capacitive circuit, on the AC supply reaching its peak value after the optocoupled triac is turned off, and the AC supply comprises two components 180° out of phase, and the optocoupled 5 triacs are turned 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: 10 Fig. 1 is a circuit diagram of a power supply and ringing generator of a private automatic branch exchange (PABX) according to the invention.

Fig. 2 is a control circuit diagram of the PABX of Fig. 1, 15 Fig. 3 is a circuit diagram of a switching matrix tone 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 Fig. 1, 20 - 5 - - 5 - 54733 F1g. 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 of the 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 of the 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 of the 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 to two 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. - 6 - 5 4 7 3 3 2. An extension circuit. 3. An exchange line circuit. 4. A control circuit, 5. A switching matrix and tone generator. 6. A dual tone multifrequency receiver. 7. A dual tone multifrequency sender.

Power Supply and Ring Generator The power supply circuit generates the following voltages: 10 (a) -48 Volt DC supply to power relays RL11 and RL12 in the exchange line circuit (Fig. 5), and the extension lines (Fig. 4). (b) -43 Volt DC supply to 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 voltages 20 for the transistors and the reference voltages for the voltage dividers used in the systems. - 7 - - 7 - S 4 7 3 (e) 5 Volt pulse, 100 Hz as an Interrupt for the control circuit (Fig. 2). (f) 50 Volt, 25 Hz which provides the ringing signal for the system.

A transformer T1 fed from the mains at 220 V provides three supply voltages, namely a 16V, 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 1.2V below earth by the voltage drop across diodes D38 and D39, which are biased on by a trickle current flowing through a resistor R138. The level of output voltage from the regulator REG 1 1s determined by a potential divider comprising resistor R139 and R140. 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 by the 12 V supply from the regulator REG 1.

Diodes 035 and 036 produce a full-wave rectified 100 Hz signal which is fed through a potential divider comprising resistor R135 and R136 to the input of an Inverter IC25. A resistor R137 holds the interrupt 8 line at 5 V. When the output of the inverter goes low, a negative going pulse on the interrupt line is produced. The duration of the pulse is determined by the time constant of the resistor R137 and a capacitor C57. When 5 the output of the inverter goes high again, the voltage of the 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 V supply, and in turn 10 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 V supply through a 5 V Zener Diode ZD 31.

Two 50 V AC windings on the transformer provides the AC 15 component of the ringing voltage. The ringing voltage 1s developed across capacitors C51 and C52 by turning the optocoupled triacs IC26 and IC27 on at various points in the cycle of the 60 Hz supply to produce a voltage with a fundamental frequency of 20 Hz. The 20 traics are switched on by a control signal from the microprocessors on output ports OUT 23 and OUT 24.

This is described in detail with reference to Fig. 1. The waveforms and timing of the control signals are illustrated in Fig. 7. - 9 - - 9 - 54733 When the positive-going control pulse is applied to the base of transistor TR18 the triac IC26 conducts and the voltage across the capacitors C51 and C52 follows the waveform of VI until it reaches its negative peak value. At this point the current through the triac IC 26 changes direction and the triac switches off. Both triacs remain in the non-conducting state for a 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 transistor TR17, triac IC27 1s 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 VI until it reaches its positive peak value. At this point the triac switches off and the voltage again decays. Triac IC27 then turns on for a short time until V2 reaches its positive peak and then turns off again to 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 circuit for - 10 - - 10 - 5 4 7 3 3 the PABX 1s illustrated. For simplicity, all components designations will be those associated with extension 1, which in use 1s extension 21. Each leg of the line is fed from the 48 V supply through a 740 ohm resistance 5 R71 and a winding of an Inductance Li. 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 by turning on a 10 transistor TR1 from the output port OUT 9 of the control circuit, which is described below.

The operation of the 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 15 the off-hook detection 1s done by a potential divider comprising resistors R1 and Rll. The voltage at the midpoint of the divider is held high in the on-hook state giving a logic "1" on the input port INI of the control circuit, Fig. 2. When the telephone is taken off-hook a 20 low-resistance loop is connected across the a and b legs. The voltage on the a leg will therefore drop and pull the mid-point of the voltage divider low giving a logic "0" on the in-put port INI of the control circuit.

When ringing voltage is applied to the line the off-hook - 11 - - 11 - 54733 detection 1s done by a voltage divider which comprises resistors R21, R31 and R51. When the telephone is on-hook the voltage at the junction of resistors R21 and R31 is held at +5 V. The AC component of the ringing 5 current flows through resistor R51 but because of the long time constant of the resistor/capacltor network R21, R31 and Cl (-100 ms), the voltage at the junction of the resistors R21 and R31 does not vary to any significant extent.

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

Capacitors C21 and C31 isolate the switching matrix described below, from the signalling voltages used on the line. Zener diodes ZD1 and ZD11 protect the solid state switching arrays from voltage surges which may occur during connection and disconnection of the 20 ringing voltage or during decadic pulsing from the extension.

Exchange Line Circuit Referring now to Fig. 5 the exchange line is illustrated. - 12 - - 12 - 5 4 7 3 3 Relays RL11 and RL12 are continuously operated while power is connected. If the power supply fails or 1s disconnected, relays RL11 and RL12 release and connect the exchange lines directly to extensions 1 and 2 which 5 in this case is extensions 21 and 22.

The component designations used in the description below are those associated with exchange line 1.

Incoming Call Dealing firstly with incoming calls, when the public 10 exchange sends ringing on the exchange line it activates the ringing detector which comprises opto-coupler IC28 and its associated circuitry. Ringing current flows via capacitor C41, resistance R161, zener diodes ZD27 and ZD29 to the optocoupler. The ripple 15 component of the voltage on the base of the optocoupled transistor is removed by capacitor C43 so that the transistor conducts continuously during each burst of ringing current thus putting a low on input port IN 11. To answer an incoming call relays RL13 and RL15 are 20 operated.

This connects the exchange line through to the transmission bridge. The Darlington Pair transistors TR19 and TR21 act as a current sink for the DC current - 13 - - 13 - S4733 flowing in the line. When the DC line voltage is connected to the sink the voltage across capacitor C45 builds up and turns on TR19 and TR21 which then carry most of the DC current. The voltage on the base of TR19 is stabilised by the capacitor C45 which acts as a low impedance path for AC signals and by transistor TR23 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 at the current sink is correct regardless of the polarity on the exchange line.

A gas discharge tube 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 and VR3. The fastacting Zener Diodes ZD21 and ZD23 are placed at the input to 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 current sink. Loop-disconnect signalling pulses can now be transmitted to line. Before each pulse train - 14 - - 14 - 5 4 7 3 3 commences relay RL15 1s 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 5 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 10 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 resistor VR5.

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

A summary of the Circuit conditions for each line state are give in Table 1 below: - 15 - Table 1 - Input/Output Ports Line State IN 11 OUT 21 (RL13) OUT 22 (RL15) Idle High Low (Rel) Low (Rel.) Seized High High (Op.) High (Op.) Loop-dls. Hake 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.) 54733 Control Circuit Fig. 2 illustrates the control circuit. The control circuit comprises four sections: 15 1) Central Processing Unit (CPU) IC1 and associated circuitry, ii) Programme Store, IC2, iil) Data Store, IC3, iv) Input/Output Section (1/0). - 16 - - 16 - 547 3 3 The CPU IC1 is an RCA 1802 COSHAC microprocessor. It is a single chip CMOS 8-bit register-orientated microprocessor. A detailed guide to the architecture and programming technique is contained in the 'User Manual 5 MPM-201C'.

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

The I/O 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 low for a period determined by the time constant of resistor R145 and capacitor C6Q. This ensures that the programme does not run until all parts of the circuit are fully powered-up. When the CLEAR line goes high program 15 execution begins from the first location in the Program Store. A 4-pole dual-in line switch SW1 selects various programme options.

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

The microprocessor has eight address lines. These eight lines supply a 16-bit address in the form of two consecutive 8-bit bytes. The higher order address byte - 17 - - 17 - 5 4 7 3 2 appears on the address lines first followed by the lower order byte. The timing pulse TPA is used to strobe the higher order address byte into an address latch consisting of integrated circuits 1C4 and IC5.

The allocation of addresses in the system is as follows: Programme Store 0000 - IFFF 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 of the groups by sending out the identity of the group on the N lines, NO, N1 and N2. The identity is converted from parallel binary format to one-out-of-f1ve in the decoder chip IC6. When a group of ports is selected in this way information can be transferred between the eight ports and the eight lines of the processor data bus. In the case of the input ports the direction of the information flow is from port to data bus. In the case of the output ports the information flows in the opposite direction.

The processor can determine the state of various - 18 - - 18 - 54733 elements in the PABX circuit via 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 of the input and output ports is given at the end of this description.

Switching Matrix Fig. 6 Illustrates the switching matrix schematically. The matrix comprises two types of switches, namely 10 4x4x2 arrays which are IC17, IC18 and IC19, and 2x2x2 arrays which are IC20, IC21, IC22, IC23 and IC24. ICs 17 to 19 are used for setting up connections between two extensions or for connecting a tone to an extension. A call between two extensions must use one 15 of the two internal paths. Two sets of crosspoints are operated therefore to set up such a connection.

Resistor group RG8 - RG12 are connected 1n series with the extensions.

In conjunction with the parallel RC circuits consisting 20 of resistor R157 and capacitor C66 (Internal Path 1) or resistor R158 and capacitor C64 (Internal Path 2) they form an attenuation pad on extension to extension calls - 19 - - 19 - 54733 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, Rlngback Tone 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 extension to the 425 Hz signal in such a manner as to produce the cadence of the required tone.

The 425 Hz frequency 1s derived by applying the output of a square-wave generator through a low pass filter. IC25b and IC25e form an astable multivibrator which oscillates at 425 Hz. The output of this is applied via the buffer IC30a and a voltage divider to the active low-pass filter formed by IC30b, C62, C63 and R156. IC30c acts as a buffer between the filter 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 supply for the Control Circuit. IC13 and IC14 act as voltage level shifters at the interface. The Control data for operation and - 20 - - 20 - 5 4 7 3 3 release of crosspoints is sent from output ports OUT 1 -OUT 8 and the Q output from the microprocessor. The data is in parallel binary format. OUT 1 - OUT 4 are decoded by binary to decimal decoders IC15 and IC16 to 5 select the appropriate crosspoint IC.

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

The DTMF Receiver comprises three elements, namely, a 10 filter, a decoder and a fast detector.

The filter provided by integrated circuit IC53 separates the incoming DTMF signal into its high group and low-group components. The resulting pair of sine-waves FHT are limited to produce square waves having the same 15 frequencies as the individual input tones. 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 20 signals from the Filter and provides a tri-state 4-bit binary output. When both high-group and low-group signals have been simultaneously detected a flag EST - 21 - - 21 - 54733 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) 5 for a minimum time before being considered valid.

Capacitor C2 (Fig. 8) is charged via resistors R2 and R3 in series from EST when a DTMF tone pair is detected. After a period determined by the time constant of capacitor C2, and resistors R2 and R3, the voltage on 10 ST exceeds its input threshold which sets an internal flag indicating the detected signal is valid. A 4-bit binary code word corresponding to the received character 1s 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 component. The FHT and FLT outputs of the Filter are connected to voltage 20 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 - 22 - - 22 - 5 4 7 3 3 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 5 DTMF. If the first digit received is decadic the receiver is disconnected.

The handling of DTMF digits depends on the number of extensions which are in a 'DTMF Signalling State1 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-digit time supervision expires.

If more than one extension is in a 'DTMF Signalling State' the extension number is placed in a queue. If the MF receiver is free it is connected to the extension at the top of the queue. In the next primary interval, see Fig. 9, the Fast Detector will indicate the presence 20 (or absence) of a high-group and low-group tone.

If the expected signal is absent then the receiver is disconnected, the extension number is returned to the end of the queue and the receiver is connected to the - 23 - - 23 - 54733 extension at the top of the queue.

If the expected signal Is present then the receiver remains connected for a further two primary intervals to allow validation of the signal. When the signal 5 is received the receiver is disconnected, the extension number Is returned to the end of the queue and the receiver is connected to the extension at the top of the queue.

When all digits have been received or when the inter-10 digit time supervision expires the extension is removed from the queue.

Dual Tone Multifrequency Sender Fig. 8 also illustrated the DTHF sender. DTMF signals are generated by a CMOS integrated tone dialler IC58 15 primarily intended to interface directly to a standard telephone keypad and generates all dual tone multi-frequency pairs required in tone dialling systems.

When the appropriate signals are applied to the control Inputs Rl - R4 and Cl - C4, see Fig. 8, the tone pair 20 is applied via the emitter follower circuit TR1 to the drain input of the switching FET's TR2 - TR5. The DTMF signal may be applied to either exchange line circuit by operating two of the FET's. 24 - 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 l-out-of-4 decoder, to the correct form for application to the inputs of the DTMF generator. The coding of OUT 25 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's which operates 1s determined by the state of OUT 30.

DTHF Sending The DTHF generator is used only on calls where last number redial or the abbreviated dialling has been invoked. On all other extension to exchange line calls the MF signalling comes directly from the extension.

When the system requires generation of an HF signal, the control data for the digit is set up on output ports OUT 25 - OUT 28 according to the arrangement in Table 3. Simultaneously OUT 30 is set to select FET pair for connection of the sender to the required exchange line. In the next primary Interval OUT 29 goes low to initiate sending of the DTHF signal and to turn on the FET pair. - 25 - S4733 Approximately 70 milliseconds later OUT 29 goes high again to terminate the signal and to turn off the FET's. A minimum pause time of 70 milliseconds 1s 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 10 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 15 8 Extension 7 Line Condition 9 Extension 6 Line Condition 10 Extension 5 L i ne Condition 11 Exchange Line 1 Ringing Detector (Ringing on = 0; Ringing off = 1) 20 12 DTMF Decoder 13 Exchange Line 2 Ringing Detector - 26 - 5 47 3 3 Table 2 - Input/Output Ports (Contd.) Input Port Function 14 DTMF Fast Detector 15 Spare 5 16 Spare Output Port Function 1 Operation of Switching Matrix 2 Operation of Switching Matrix 3 Operation of Switching Matrix 10 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 15 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 20 13 Extension 9 Ringing Relay - 27 - S4733 Table 2 - Input/Output Ports (Contd.) Output Port Function 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 - 28 - 5 4 7 3 3 Table 2 - Input/Output Ports (Contd.) Output Port Function 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 28 27 26 25 R4 R3 R2 R1 C4 C3 C2 Cl 1 L L L L H H H L H H H L . 2 L L L H H H H L H H L H 10 3 L L H L H H H L H L H H 4 L H L L H H L H H H H L 5 L H L H H H L H H H L H 6 L H H L H H L H H L H H 7 H L L L H L H H H H H L 15 8 H L L H H L H H H H L H 9 H L H L H L H H H L H H 0 H H L H L H H H H H L H k H H L L L H H H H H H L Ik H H H L L H H H H L H H 20 A L L H H H H H L L H H H B LHHH HHLH L Η Η H - 29 - - 29 - 54733 Table 3 - OUT 25 » OUT 28 : Coding of Data (Contd.) Output Ports Digit 28 27 26 25 R4 R3 R2 R1 C4 C3 C2 Cl C HLHH HLHH LHHH D HHHH LHHH LHHH The use of the PABX will now be described.

Making Calls Making an Internal Call 1. Lift the handset and wait for 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 by the user. Also the previous external number called may be redialled 15 automatically.

- System Selection of Exchange Line 1. Lift the handset and wait for 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 is free, that free line is selected.

If both lines are free, that line which was not selected for the last external call is selected.

User Selection of Exchange Line 10 1. Lift the handset and wait for internal dial tone. 2. Dial '31' to select exchange line No. 1 or '32' to select exchange line No. 2. Wait for public exchange dial tone. 3. Dial required number (18 digits maximum).

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

To re-dial this number automatically, lift the handset, wait for internal dial tone, dial '5'. The -31 - -31 - 54733 system will automatical 1y select and seize an exchange line as in system selection of exchange Tine above, and after a 2.5 second pause, will send out the digits of the last number dialled on an external call. The 2.5 second pause is to ensure that digit sending 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 lift the handset and speak. Different ringing candences are given at the called extension for calls originating internally and externally (See Page 38).

Handling of Incoming External Calls 1. If Night Service has not been invoked (See Page 37) all incoming exchange line calls are presented initially to Extension 21 if it is free. The extension answers by lifting the handset. 2. If Extension 21 is engaged or fails to answer within 20 seconds, the call is presented to Extension 22 if it is free. -32- -32- δ 47 3 :: 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, 5 the alert tone 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 10 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 15 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 20 hangs up.

. If Night Service has been invoked (See Page 37 ), incoming exchange line calls ring Extension 23 only, - '33 - - '33 - 54733 or a central bell if fitted at this position.

There is no hunting as described in 1 - 4 above.

Universal Answer Any extension may answer an incoming exchange line 5 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/Enquiry/Transfer/Conference - External Calls The following assumes that speech connection has already 10 been established between an extension and an exchange line.

The connection may have been set up by the extension dialling an external number or by the extension answering an incoming external call. 1. Put outside 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, - 34 - - 34 - 4. 5 5. 6. 10 7 15 8 Z0 called extension rings. On answer, confer in private (enquiry).

Revert to outside line by dialling '1'. This puts the 'enquiry' extension on hold.

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

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

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 of the other extension to the outside line) by hanging up.

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. - 35 - - 35 - 5 4 7 3 Can Hold/Enquiry/Conference - Internal Calls The following assumes that speech 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, revert to the 2nd extension by dialling Ί1. 4. Shuttle back and forth as required between 2nd 10 extension and 3rd extension by dialling '1' each time.

. To have a conference (own extension + 2nd extension + 3rd extension) dial '4' while in speech contact with the 3rd extension. Any of the 3 extensions 15 may subsequently leave the conference (without affecting the other two) by replacing the handset. - 36 - - 36 - 5 4 7 3 3 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. Lift the handset and wait for 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 by the diverting extension whenever he lifts his handset as a reminder that he has diverted 15 incoming calls.

. Diversion will not be accepted if an attempt is made to divert calls to an extension which has already diverted incoming calls. Busy tone is given if this is attempted. - 37 - - 37 - 54 733 6. An extension which is already acting as 'Host1 for another extension cannot divert calls. Busy tone is given if this is attempted. 7. The 'Host' extension is allowed to call the 5 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 available whereby incoming exchange line calls ring at 1 extension only (Extension 23) or at a central bell if fitted in the position. Night Service may only be invoked or revoked from Extension 21. 1. To invoke Night Service, lift the handset of Extension 21, wait for internal dial tone, and dial '44'. Dial tone returns as acknowledgement. 2. To revoke Night Service, lift the handset of Extension 21, wait for internal dial tone and 20 dial '45'. Dial tone returns as acknowledgement. - 38 - - 38 - S4733 TECHNICAL DATA Internal Tones The various tones encountered when using the apparatus are as follows: 5 Dial Tone : 425 Hz continuous Busy Tone : 425 Hz - 0.38 sec. on, 0.38 sec. off Ringback Tone: 425 Hz - 1.0 sec. on, 2.55 sec. off Alert Tone : 425 Hz - 0.1 sec. on, 2.50 sec. off Special Dial 10 Tone : 425 Hz - 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 : 25 Hz - 1.0 sec. on, 2.0 sec. off External : 25 Hz - 0.4 sec. on, 0.2 sec. off 0.4 sec. on, 2.0 sec. off - 39 - 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-5 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 particular to Fig. 10 f, 10 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 15 reference to the apparatus of Figs. 1 to 10, however, in this case the waveform f is generated as follows: When the positive-going control pulse is applied to the base of transistor TR18 of Fig. 1, the triac IC26 conducts and the voltage across the capacitor C51 and 20 C52 follows the waveform of VI until it reaches its negative peak value. At this point the current through the triac IC26 changes direction and the tiMac switches off. Both triacs remain in the non-conducting state for a short time, and the voltage across capacitor C51 and - 40 - 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 transistor TR17, triac IC27 is turned on and the voltage across the 5 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 10 suppression of transient voltages produced when the triacs turn 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, 15 it will be appreciated that while specific circuits have been described for the 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 appreciated 20 that while the 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 of the invention.

Claims (14)

1. A private automatic branch exchange 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, a dual tone multi-frequency (DTMF) receiver, to receive a DTMF signal having a high group component and a low group component, wherein the DTMF receiver comprises a filter to receive the DTMF signal and to separate the signal into its high group component and low group component, a decoder to receive high and low group component signals from the filter, and a detector comprising a pair of voltage comparators, one to receive the high group component signal and the other to receive the low group component signal from the filter, and a NAND gate fed by the voltage comparators through respective rectifiers, the output of the NAND gate being scanned by the control circuit.

2. A private automatic branch exchange as claimed in Claim 1 in which the apparatus comprises a dual tone multi-frequency sender.

3. A private automatic branch exchange as claimed in any preceding claim in which the apparatus comprises a - 42 - 42. S 4 7 3 3 means for hunting, so that incoming calls hunt from one extension line to the next, until a non-engaged line is available.

4. A private automatic branch exchange as claimed in 5 Claim 3 in which the hunting means permits hunting from one extension line to the next when a line is not being answered.

5. A private automatic branch exchange as claimed in Claim 3 or 4 in which means are provided to alert a 10 user of an engaged extension line that an incoming call is trying to get through.

6. A private automatic branch exchange as claimed in any preceding claim in which the control circuit comprises a micro-processor. 15

7. A private automatic branch exchange as claimed in any preceding claim in which means to receive up to ten extension line circuits are provided.

8. A private automatic branch exchange as claimed in any preceding claim, in which means to receive up to 20 two public exchange lines are provided.

9. A private automatic branch exchange as claimed in any preceding claim in which the ring generating - 43 - - 43 - 54733 circuit comprises a means for generating a ring voltage from an AC supply, the generating means comprising means for turning on and off the AC supply at various times throughout each cycle of the AC 5 supply.

10. A private automatic branch exchange as claimed in Claim 9 in which the generating means comprises a pair of optocoupled triacs and a capacitive circuit across which the ringing voltage is developed. 10

11. A private automatic branch exchange as claimed in Claim 10 in which the developed voltage decays across the capacitive circuit, on the AC supply reaching its peak value after the optocoupled triac is turned off.

12. A private automatic branch exchange as claimed in 15 Claim 11 in which the AC supply comprises two components 180° out of phase.

13. A private automatic branch exchange as claimed in any of Claims 10 to 12 in which the optocoupled triacs are turned on and off by the control circuit. 20

14. A private automatic branch exchange substantially as described herein with reference to and as illustrated in the accompanying drawings. 5 4733 - 44 - Dated this 4th day of September 1984 F.F. GORMAN & CO. BY:_EXECUTIVE Agents for the Applicants, Clifton House, Lower Fitzwilliam Street, Dublin 2