IES940369A2 - A private automatic branch exchange (PABX) telephone system having improved exchange line and extension line interfaces - Google Patents

Abstract

A private automatic branch exchange (PABX) telephone system of modular construction has improved exchange line and extension line interface circuitry. The extension line interface circuit is connectable to extension lines having differing parameters by modifying current feed/voltage feed parameters according to the length of the extension line. The exchange line interface circuit comprises over-voltage protection, suppression of radio frequency interference (RFI) and impedance matching circuitry together with tone generators and conference bridging circuitry.

Description

A PRIVATE AUTOMATIC BRANCH EXCHANGE (PABX) TELEPHONE SYSTEM HAVING IMPROVED EXCHANGE LINE AND EXTENSION LINE INTERFACES The present invention relates to a private automatic branch 5 exchange (PABX) telephone system and more particularly to a modular PABX system having improved interface capabilities adapted to interface with incoming exchange lines and outgoing extension lines.

PABX systems must conform to a number of rigourous standards for line parameters and signal circuitry. High quality transmission is always sought but usually involves high cost and extreme system complexity. Extension telephones adapted for use with such systems are supplied by extension line feed circuits which generally comprise a large number of circuit components. 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 at either factory level or at national distribution points of the PABX system exchange line interface circuit. Additionally, adaptable compensation circuitry is often required to overcome differences in national network and PABX system parameters .

It, is an object of the present invention to seek to alleviate the above disadvantages and to provide a private automatic branch exchange (PABX) telephone system having improved exchange line and extension line interfaces.

Accordingly, the present invention provides a private automatic branch exchange (PABX) telephone system of modular construction comprising at least a power supply unit (PSU), a dual tone multi-frequency (DTMF) transmitter 35 and receiver circuit, a tone gen^atxui,,-a^,on.txQlXecT-~a»---·· -940369 exchange line interface circuit, at least one extension line interface circuit and a crosspoint switch matrix suitable for routing path signals between the or each extension line interface circuit, the exchange line interface circuit, the tone generator, the DTMF receiver and the DTMF transmitter, wherein the PABX is configured to provide a modifiable extension line interface circuit connectable to a number of extension line types of differing parameters, the interface circuit being switchable so that current feed/voltage feed parameters are modified according to the length of the extension line.

Conveniently, the power supply unit generates standard DC voltage rails and RMS ringing signals in additional to a DC supply controlled by voltage regulators which supplies power to the exchange line and extension line interface circuits and decreases the component count and relative complexity of the interface circuitry.

Advantageously, the power supply unit (PSU) may be modified to generate a negative feed voltage to at least one extension line interface circuit to provide long-line extension capability.

Preferably, the extension circuit is adjustable between a short extension mode and a long-line extension mode by manipulating at least one and preferably two switches or jumpers to select suitable parameters for each mode to minimise heat dissipation within a driver or feed circuit of the line interface.

In one embodiment the power supply unit is adapted to disable the long-line capability. This modification comprises a breakable link or switch which when connected provides the required power and current parameters to 0359 ’ provide long-line capabilities to the driver circuit of the extension line interface.

Preferably, the exchange line interface is provided with 5 over-voltage protection, suppression of radio frequency interference (RFI), impedance matching circuitry for when the exchange line is in off-line (idle) and on-line states, dial tone generation and additional tones-to-line together with conference bridging circuitry connected to the θ exchange line interface.

The invention also provides conference bridge circuitry suitable for use with private automatic branch exchange (PABX) telephone systems which comprises a single dedicated circuit which is active during all operations and has switchable line load parameters which switch to match the system parameters for the required number of conference lines.

Advantageously, the conference bridge circuitry provides impedance matching while two or more extensions are on a conference call whereby two or more extensions are connected to the exchange line via their respective interfaces. Additionally, tone-to-line signals may be injected at the conference bridge circuit to provide tones along the speech path to the exchange line interface circuit.

Conveniently, a negative resistance generator is incorporated in the conference bridge circuit to facilitate impedance matching especially where tones are injected to the speech path.

Preferably, the extension line interface circuit has an a,b wire feed circuit, said circuit comprising an a-leg” 940369 constant current feed circuit and a b-leg balancing circuit which balances the a,b wires by providing a voltage transformation resulting in improved dynamic range.

Conveniently, the ringing signal is passed to the a-leg of the extension circuit via an opto-triac which together with clamping diodes protects the constant current source against ringing voltages.

The invention will now be described more particularly with reference to the accompanying drawings which show, by way of example only, one embodiment of PABX telephone system according to the invention. In the drawings: Figure 1 is a schematic circuit diagram of the telephone system; Figure 2 is a schematic circuit diagram of a power supply unit, an exchange line interface, a first extension line circuit and a tone and DTMF signal generator circuit shown in more detail than that of Figure 1; and Figure 3 is a schematic circuit diagram of a controller, a crosspoint matrix, a two-four wire convertor and extension line circuits including an extension line circuit having long-line capability which are shown in more detail then that of Figure 1.

Referring to the drawings, the power supply unit (PSU) accepts a 220V RMS mains input and, using a linear power supply circuit, generates the required regulated +5V and -19V DC rails and a 55V RMS ringing signal RING. The AC mains input circuit comprises a mains transformer TR1 and protective components comprising a fuse FS1 and a varistor VR8, VR8A. The two alternate positions of varistor are 940369 provided to accommodate international safety criterion.

The mains input is provided via a connector CN6. The centre-tapped winding on the main transformer TR1 is fed to a bridge rectifier comprising diodes D17-D20, and smoothed by capacitors C21, C43, to give raw + 30V DC supplies.

These DC supplies are controlled by first and third regulators REG1, REG3. The output of the first regulator REG1 is used to power all of the circuits in the PABX. Two resistors R81, R82 are used to tap off a 50Hz mains interrupt signal INT which is AC coupled via a capacitor C54 to the microcomputer U1 at pin 26 (maskable interrupt/zero crossing detector), shown in Figure 3. As the first regulator REG1 is a variable regulator, two resistors R75, R76 are used to set the output voltage. A second regulator REG2 is used to provide Ground voltage to all the +5V DC circuits which are positively supplied via the first regulator REG 1. A -19V rail taken from the input of the second regulator REG2 is used by the extension line interface circuits and to each of the relay drivers. This topology provides reduced component count for the extension line interface circuits.

The third regulator REG3 is used to generate an optional negative feed voltage for the b-leg of one of the extension line interface circuits to give the interface long-line capability. The voltage from this regulator REG3 is set by two resistors R65, R74. Further resistor values R63, R64 can be selected by the user or installer via switches or jumpers JP1, JP2 to set the parameters for a given extension length. This arrangement is necessary to minimise heat dissipation in the extension line interface circuits .

The ringing signal RING is provided by one of the secondary windings on the mains transformer TRI . Two resistors R50, 940369 R51 define the ringer source impedance while a capacitor C30 and diode D38 act to AC couple the ringing signal and centre it around the +5V rail. A resistor R6 and capacitor C41 provide protection from spikes and assist in turning off each triac, or opto-triac (as described in more detail below), particularly when driving inductive loads. If the long extension option is not fitted, a link LK1 is fitted and the components from the -43V Rail (C43,C44, R63-R65,R74,D39,REG3,JP2) are not fitted. In conjunction with a standard mains transformer (i.e. without centre tap) the DC power supply will provide only +5V and -19V.

The exchange line A+B wires are brought into the system via a connector CN1. The exchange line interface comprises firstly a pair of varistors VR5, VR6 to provide overvoltage protection and optional capacitors C39, C40 are provided to suppress radio frequency interference. A relay RL1 is used to switch the exchange line between the off-line (idle) and on-line states. When the system is off-line, the idle impedance is defined by passive components R39, D28, D29, C28. When the system is on-line, a hold circuit consisting of a bridge rectifier D24 - D27 and a current sink circuit, comprising a transistor Q5, a zener diode D23 and their associated passive components, is connected across the line.

Tone-to-line and the AC path signals are coupled via a second transformer TR2, and a two-to-four wire converter around an integrated circuit (IC) U9, to the crosspoint matrix and the dial tone generator. In addition, a conference bridge circuit constructed around an integrated circuit (IC) U6a is connected at this point. Clamping diodes D21, D22 protect the crosspoints against spikes below 0 Volts and above +5 Volts. 940369 The purpose of the conference bridge circuit is to provide impedance matching while two extensions are on a conference call with the exchange line and also to buffer tones, On Hold tones, DTMF signals from a transmitter IC U3 and the output of the Music-On-Hold chip U10/U10A, to be fed out to the exchange line.

Tone injection is controlled via resistors R29, R30 and a port PC1 on a microcomputer U1 based controller.

Music-On-Hold is controlled via a port PC2 on the microcomputer U1 in conjunction with resistors R83-R85.

The tone and Music-On-Hold signals are turned off and on by setting the port to output low and input (High impedance) respectively.

The action of positive and negative feedback around the conference bridge integrated circuit U6A results in a negative 600 Ohm impedance looking into the conference bridge circuit.

A tone detection circuit based around another integrated circuit U6B is used to measure the level and frequency of tones on the exchange line for dial tone detection, etc. During decadic dialling, a first opto-coupler OP1 is turned on which shunts the hold circuit. The associated relay RLI is turned off and on to generate the make and break conditions on the line.

A buffer amplifier U6C and resistors R16-R19 are used to monitor the line status for detecting incoming ringing, line presence or absence and on/off conditions on the line .

The CMOS Op-Amps used in the two-to-four wire converter 940369? and the conference bridge require capacitors C56-C58 between their inverting input and output for stability.

As each of the extension interface circuits are similar, only one such extension will be described in detail. Resistors R13, R14 bias a central transistor Q1 to provide a 25mA constant current feed to the extension a-leg. A capacitor C49 decouples the bias signal to the base of the transistor Q1 to provide a measure of noise immunity. The base voltage on the transistor Q1 is monitored via Port ANO on the microcomputer U1 to determine the hook status of the extension line. When the extension is on hook, the voltage across the base of the transistor Q1 is around 5 Volts, and falls to nearly 0 Volts when the extension is off hook.

This action is also used to detect a ring trip. A diode D15 protects the extension interface circuit from being pulled below -19V and another diode D16 and varistor VR1 together protect the circuit against overvoltage damage.

An optional capacitor C38 provides RFI protection.

A capacitor C6 is used to couple path signals and tones between the crosspoint matrix and the extension feed circuit. Diodes D13, D14 provide spike protection.

The opto-triac OPT1 is used to switch the ringing signal onto the a-leg. Opto-triac semiconductor switches are utilized instead of relay switches in the ringing circuits to provide better conditioning of the ringing signal and to prevent difficulties associated with powering inductive loads. Diodes D15, D16 protect the constant current source against the ringing voltage. A relay RL2 is used (on one extension only) to provide a power-fail connection to the exchange line interface circuit, as shown in Figure 2.

Another extension on the unit, such as that shown in Figure 3, is provided with a selectable negative feed voltage 940369* which may be set by a user or installer by modifying two 3-pin jumper links JP1, JP2 in the access area. The first jumper link JP1 selects whether the b-leg is 'connected to the -19V rail (to give 24V feed) or to the negative supply which can be set to give 32v, 36v, and 48V feed voltages. The nominal feed current on this extension is 21mA, instead of 25mA as on the other extensions. This arrangement together with the selectable feed voltage is to minimise the heat dissipated by the transistor Q4.

The settings for the jumpers JP1, JP2 are as follows: 24V feed: JP1: Position 2 JP2: Position 1 or 2 32V feed: JP1: Position 1 JP2: Position 1 36V feed: JP1 : Position 1 JP2: Position 2 48V feed: JP1: Position 1 JP2: Remove shorting link.

The DTMF transmitter circuit is based around a binary interface DTMF tone generator IC U3. The output of this device is injected into the conference bridge via a resistor R28, to be fed out to the exchange line interface. The DTMF receiver IC U2 is used to scan each extension via the crosspoint IC (J4 and present the binary equivalent to the microcomputer U1. A capacitor C18 AC couples the DTMF 0 3 6 9 * - 10 signal and resistors R34, R41 set the gain of the input amplifier on the receiver. A resistor R35 and capacitor C19 are used to set the minimum valid burst length that the device will detect. A 3.579545MHz crystal XT2 controls the internal clock oscillator IC U2. Capacitors C16, C17 aid stability. The output of this oscillator is used to drive the clock input of the transmitter IC U3.

The tone generator is driven by a counter on the microcomputer chip U1 which generates a 425Hz square wave signal appearing on port PC4. This is fed to a 3rd order low pass filter based around an IC U6d. The output of this circuit is a clean sine wave at 425Hz.

The microcomputer U1 is a single chip 78C18 device with 32k of ROM, 1k of RAM, 1x16-bit plus 2x8-bit timers, 1x8 channel ADC port and 5x8-bit I/O ports. On power up, the microcomputer U1 is held to reset by a resistor R53 and capacitor C31 until the +5 volt rail has stabilised. Optionally, these two components may be replaced by a watchdog circuit U8 . After power-up, the microcomputer has to strobe the circuit U8 via port PD7 once every 10ms to avoid being reset. The watchdog circuit U8 also monitors the +5V rail and if the voltage deviates outside a limit of +10% this deviation will also cause a reset. Resistors R1 , R2 set the microcomputer U1 to access the internal ROM and RAM memory. Crystal XT1 (12.00 MHz) is used to control the internal clock oscillator on the microcomputer via pins 30 and 31. Capacitors C1, C2 aid stability. A 256-bit serial EEPROM device U7 is used to store information such as a barring table, etc. which must be retained if power to the system is temporarily disconnected (i.e. during brownout). Internal RAM memory on the microcomputer device is used as temporary program parameter storage. 4 0 3 6 9- 11 Under control of the program stored in the internal ROM memory, the microcomputer scans the extensions and the line via its analogue and digital ports then via its digital ports, turns on and off relays and crosspoints, tone and ringing signals, and so on, to change the status of the system.

The crosspoint switch matrix U4,U5 is used to route the path signals between the extensions, the line, tone generator, DTMF receiver and the DTMF transmitter. The matrix is arranged as two 4x4 crosspoint chips. The upper device U5 is used to route dial tone to each extension interface via a resistor (dial tone to each extension has to be fed through a series impedance so as not to swamp tones fed to the DTMF receiver). The remaining three horizontal paths on this device are used to route internal (extension to extension) path connections. Horizontal path X0 on the lower chip U4 is used to make connection to a doorphone interface for example.

Path X1 is used to scan each of the extensions for DTMF signalling and path X3 is used to connect the extensions to the line. Horizontal paths XO on the upper device U5 is connected to a reference voltage Vref to provide a low impedance Silence to keep the telephone impedance matched into the system when the dialtone source has been disconnected (i.e. during internal dialling).

An interface circuit for a standard doorphone unit is provided as an option on the PCB. The interface comprises a 600 Ohm path connection, from the crosspoint matrix U4, fed out through coupling capacitors C33, C34 and appearing at a connector CN5. Diodes D33-D36 and a variable resistor VR7 provide overvoltage protection. Two relay outputs RL3, RL4 are associated with the doorphone interface circuit. 940369 - 12 The first relay RL3 is used to control the amplifier AMP in the doorphone unit and the second relay RL4 is used to control a solenoid door opener DOOR.

An option to provide an on-board music source is provided on the PCB. One of two devices may be fitted. The first (low-cost) is a 3-pin device (type UM66T) which is a simple sixty four note melody generator, and the second (more costly) alternative is a one thousand and twenty four note (UM3491) device which gives a higher quality output. Both devices (UM66T, UM3491) require a 3 volt DC supply, provided by a potential divider R78, R79. Resistors R83, R84 form an attenuator to control the output level to be fed into the conference bridge circuit. This attenuator is used along with a resistor R85 and a port PC2 on the microcomputer U1 to mute the output to the conference bridge when not required.

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 invention as defined in the appended claims.

Claims (5)

1. A private automatic branch exchange (PABX) telephone system of modular construction comprising at least a power supply unit (PSU), a dual tone multi-frequency (DTMF) transmitter and receiver circuit, a tone generator, a controller, an exchange line interface circuit, at least one extension line interface circuit and a crosspoint switch matrix suitable for routing path signals between the or each extension line interface circuit, the exchange line interface circuit, the tone generator, the DTMF receiver and the DTMF transmitter, wherein the PABX is configured to provide a modifiable extension line interface circuit connectable to a number of extension line types of differing parameters, the interface circuit being switchable so that current feed/voltage feed parameters are modified according to the length of the extension line.

2. A PABX telephone system as claimed in claim 1, in which the extension line interface circuit is adjustable between a short extension mode and a long-line extension mode by selecting suitable parameters for each mode to minimise heat dissipation within a driver or feed circuit of the line interface.

3. A PABX telephone system as claimed in claim 1 or claim 2, in which the exchange line interface is provided with over-voltage protection, suppression of radio frequency interference (RFI), impedance matching circuitry for when the exchange line is in off-line (idle) and on-line states, dial tone generation and additional tones-to-line together with conference bridging circuitry connected to the exchange line interface. 940369 - 14

4. A PABX telephone system as claimed in any one of the preceding claims which includes any one or more of: a power supply unit (PSU) generating standard DC voltage rails and RMS ringing signals in additional to a DC supply controlled by voltage regulators which supplies power to the exchange line and extension line interface circuits and decreases the component count and relative complexity of the interface circuitry; a power supply unit (PSU) modifiable to generate a negative feed voltage to at least one extension line interface circuit to provide long-line extension capability; a power supply unit which is adaptable to disable long-line extension capability by disconnecting a breakable link or switch which when connected provides the required power and current parameters to facilitate long-line capabilities to the driver circuit of the extension line interface; conference bridge circuitry which comprises a single dedicated circuit which is active during all operations and has switchable line load parameters which switch to match the system parameters for the required number of conference lines; conference bridge circuitry which provides impedance matching while two or more extensions are on a conference call whereby two or more extensions are connected to the exchange line via their respective interfaces; conference bridge circuitry in which tone-to-line signals are injectable at the conference bridge circuit to provide tones along the speech path to the exchange line interface circuit; 940369 - 15 a negative resistance generator incorporated in a conference bridge circuit to facilitate impedance matching especially where tones are injected to the speech path; an extension line interface circuit having an a,b wire feed circuit, said circuit comprising an a-leg constant current feed circuit and a b-leg balancing circuit which balances the a,b wires by providing a voltage transformation resulting in improved dynamic range; and 1 0 an extension line interface circuit in which a ringing signal is passed to the a-leg of the extension circuit via an opto-triac which together with clamping diodes protects the constant current source against ringing 15 voltages.

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