AU4690493A - Isolation interface apparatus - Google Patents

Description

ISOLATION INTERFACE APPARATUS

The present invention relates to isolation interface apparatus. In particular the invention relates to isolation apparatus suitable for interfacing communications equipment to a communications network such as a telephone service network.

Throughout the world, telephone service providers insist that mains powered subscriber equipment be connected o their network via high integrity isolation apparatus. The isolation apparatus can form part of the subscriber equipment or it can be external and placed between the subscriber equipment and the telephone service network. The isolation apparatus is often called a Line Isolation Unit or LIU. In the USA, isolation apparatus are commonly known as Direct Access Arrangements (DAA's), whilst in other parts of the world they are also known as Network Termination Units (NTU's). DAA's, NTU's and LIU's prevent the coupling of dangerous voltages used in mains powered equipment to the telephone service network if and when serious faults occur in the subscriber equipment. They also lessen damage to the equipment and the risk of shock to users of the equipment if dangerous voltages are accidentaly imposed on the network. Telephone authorities are seriously committed to protecting both their subscribers and personnel who use and work on their networks. There is an equal commitment to preventing consequential damage to valuable plant and equipment should dangerous voltages be connected to the network. A concern also exists with respect to loss of credibility and revenue whilst the network is under repair.

Understandably, and without exception, telecommunication authorities impose strict regulations through legislation on products that are intended for connection to their networks. The process of evaluation against these regulations and the granting of a permit for connection is known as homoligation. The homoligation process varies from country to country to the extent that the regulations differ between countries and for many other technical reasons. It has therefore been necessary to vary LIU, DAA or NTU designs and to produce many product variants if the product is to be sold in a number of countries. Subscriber equipment designers have traditionally used bulky line transformers, relays, multiple optocouplers, fuses and other discrete devices to design and build LIU's, DAA's and NTU's.

The isolation apparatus of the present invention may possess improved flexibility to simplify the homoligation process in each country, thereby reducing the need to redesign the isolation apparatus for a number of markets. The isolation apparatus may have improved performance and at the same time be lighter and less bulky than prior art devices. In addition the apparatus of the present invention may be relatively inexpensive to manufacture.

Throughout the specification reference to a "transmit" direction is a reference to the direction from the subscriber equipment to the telephone network and reference to a "receive" direction is a reference to the direction from the telephone network to the subscriber equipment. Reference to a "line port" is a reference to terminals on the isolation apparatus which are adapted to connect to the telephone network. Reference to the isolation apparatus being in an "on hook" state is a reference to the state when a telephone line is "not looped" and reference to the isolation apparatus being in a "off hook" state is a reference to the state when the telephone line is "looped".

A principal purpose of the isolation apparatus is to facilitate transfer of voice, data and other communications signals. Various types of communications signals such as progress, voice and data signals need to be passed across the isolation apparatus to successfully execute communications on a standard two wire telephone line. The communications signals include:

(i) A ring signal to be passed from the telephone line to the subscriber;

(ii) A line loop or "hook switch" signal to be passed from the subscriber to the telephone line; (iϋ) A "ready for dialing" signal to be passed from the telephone line to the subscriber;

(iv) A decadic or DTMF (Dual Tone Multiple

Frequency) dialing signal to be passed from the subscriber to the telephone line; (v) Called party busy and called party ringing signals to be passed from the telephone line to the subscriber; and

(vi) Voice and data signals to be passed between the telephone line and subscriber as bidirectional audio signals.

According to one aspect of the present invention there is provided isolation apparatus for interfacing equipment to a communications network, said apparatus including: a first part connectable to said network; a second part connectable to said equipment; an isolation interface for electrically isolating said first and second parts, said interface including at least one isolating element for passing communications signals between said first and second parts, wherein at least one said isolating element is adapted to pass plural kinds of said communication signals whereby the number of said kinds of communications signals able to be passed between said network and said equipment exceeds the number of said isolating elements.

The first part of the isolation apparatus may be connectable to a two wire telephone line. The second part of the isolation apparatus may be connectable to subscriber equipment. In a preferred form of the present invention the isolation interface may provide electrical isolation between the two parts of the apparatus in excess of 3.5KV RMS. The or each isolating element may include an optical device such as an optocoupler. The optocoupler may include radiation transmitting means such as a light emitting diode (LED) and radiation receiving means such as a phototransistor. In a preferred form the apparatus may include a pair of isolating elements.

One isolating element may be adapted to pass ring signals in a receive direction when the apparatus is on hook and may be adapted to pass audio signals such as voice and data signals in the receive direction when the apparatus is off hook. Thus two communications signals may be passed by the one isolating element.

A further isolating element may be adapted to pass line loop control or decadic dialling signals in the transmit direction as a bias current. Both decadic dialling and line loop control may be achieved by switching the line port resistance to either a low value to loop the line or go off hook, or a high value to unloop the line or go on hook. When the isolation apparatus is off hook the further isolating element may be adapted to pass audio signals such as DTMF dialling, voice, and data signals as a modulation of the line port bias current. The bias of the current in the further isolating element may control loop status of the line, and modulation of the bias current in the further isolating element may control modulation of line current. Thus two communications signals may be passed by the further isolating element in a combined form. In this way the said pair of isolating elements may be adapted to pass four kinds of communications signals across the isolation interface.

As noted above, voice and data are carried between the telephone line and subscriber over a 2 wire telephone line as bi-directional audio signals. The term bi-directional denotes that both transmitted and received signals are carried simultaneously by the same electrical pair of wires. On the subscriber side of the isolation apparatus the audio signals are typically carried on two uni-directional circuits, one for transmited audio and one for received audio. The term uni-directional denotes that audio signals are carried in one direction only. The interface between the bi-directional audio circuit and the two uni-directional audio circuits may be carried out on the line side of the isolation apparatus by means of two-to-four wire converter means, also known as a "hybrid".

One purpose of the "2 to 4 wire" converter means or

"hybrid", is to take an audio signal applied to its

"transmit" input and transmit it to the telephone line with minimum "leakage" of this signal back to the "receive" output. The smaller this leakage, other things being equal, the better the hybrid "balance".

The isolation apparatus may include line impedance matching means for matching the impedance of the line port to that of the telephone line which may vary over a wide range. The impedance matching means may be externally programmable eg. by changing the value of an external impedance matching network. It is preferable that the impedance matching means is arranged such that it does not affect hybrid balance. In other words, it is preferable that hybrid balance be independent of the impedance matching means.

According to a further aspect of the present invention there is provided a hybrid circuit having an input port for receiving an input signal, an output port for conveying an output signal and a bidirectional port having a first impedance for connection to a transmission line, said transmission line having a second impedance and said circuit including: means associated with said bidirectional port for matching said first impedance to said second impedance; means associated with said bidirectional port for generating a current proportional to said input signal; means for producing said output signal such that said output signal is proportional to the sum of currents, other than currents which originate from said line, flowing through said bidirectional port including said impedance matching means, plus the inverse of half the current generated by said generating means, whereby when said first impedance means matches said second impedance means, said output signal cancells substantially to zero.

Many of the important characteristics of optocouplers, such as their current transfer ratio (CTR), exhibit considerable variation with respect to temperature, from unit to unit, and over time (ageing). Compensating, or cancelling out, these variations tends to be complex, so it is preferable to use the optocouplers such that the variations are allowed, but their deleterious effects are minimised. One way this may be achieved is to place the optocouplers outside of the impedance matching means. The latter may principally include the hybrid and its balance network.

Unless some of the received audio signal is fed back into the transmit audio input, which should not occur in most applications, the accuracy with which the isolation apparatus presents a "programmed" impedance to the telephone line may be essentially unaffected by variations in characteristics of the optocouplers. In this way the line impedance matching means may be independent of variations in optocoupler characteristics.

According to a still further aspect of the present invention there is provided isolation apparatus for interfacing equipment to a transmission line, said transmission line having a first impedance and said apparatus including: a first part having a second impedance connectable to said line; a second part connectable to said equipment; means associated with said first part for matching said second impedance to said first impedance; an isolation interface for electrically isolating said first and second parts, said interface including at least one isolating element for passing communications signals between said first and second parts; the or each isolating element being arranged such that it does not significantly affect the ratio of voltage to current in said line, whereby said impedance matching means is substantially independent of variations in characteristics of the or each isolating element. The isolation apparatus may include a ring sensing circuit for producing a ring detect output. It is desirable in many applications that the ring detect output produces one pulse for each cycle of a ring signal appearing at the line port of the apparatus. This normally requires that the capacitor used in the ring sensing circuit be large. On the other hand it is also desirable, in order to minimise the size and cost of the apparatus, that any capacitor(s) used in the ring sensing circuit be as small as possible. The configuration described herein may alleviate these conflicting goals. According to a still further aspect of the present invention there is provided a circuit connectable to a telephone network for detecting a ring signal carried by said network, said signal being superimposed on a DC bias voltage and comprising periodic bursts of a signal frequency, said circuit having a port for presenting a relatively high impedance to said network in the absence of said ring signal and including: filter means associated with said port and having at least one reactive element for passing said ring signal whilst substantially preventing passage of said bias voltage; and amplifying means associated with said filter means for receiving said ring signal and for providing at least the frequency of said ring signal at its output, said output having a relatively high current per unit of output voltage for driving a load having a relatively low impedance.

Electronic circuits generally require an uninterrupted supply of power to operate as linear signal processing systems. The isolation apparatus of the present invention is no exception. Since part of the isolation apparatus may derive its power from the telephone line, the line voltage preferably should not drop below a certain level to avoid (undesirable) distortion. The line "bias" (time average) voltage at which this will occur depends on the amplitude of the audio signal on the telephone line. If relatively large amplitude audio signals are expected then the line bias voltage will need to be set higher than if only small amplitude signals are expected. There are disadvantages with setting the line bias voltage too high, such as excessive power dissipation and difficulty meeting telecommunication regulations. An optimal bias setting will therefore depend on the particular application of the isolation apparatus.

To facilitate optimisation, the isolation apparatus of the present invention may be provided with a pair of pins between which a hold control element such as a resistor may be connected. The value of the hold control element may determine the telephone line bias voltage.

Line bias voltage regulation may be externally programmable e.g. by changing the value of an external hold control element. This may allow optimal tradeoff between signal amplitude and power dissipation to be set in specific product variants.

According to a still further aspect of the present invention there is provided a circuit connectable to a transmission line, said line having a line resistance, said circuit including means for applying a bias voltage to said line and means for regulating said bias voltage, said regulating means including: means associated with said line for producing a signal proportional to said bias voltage; means associated with said producing means for comparing said proportional signal to a reference voltage and for generating an error signal whenever said proportional signal exceeds said reference voltage; and means responsive to said error signal for reducing said bias voltage, such that said bias voltage is maintained relatively constant with respect to changes in said line resistance.

It is a requirement of some telephone network regulatory authorities that for a certain period immediately following establishment of a subscriber DC loop (the "seize" period) the maximum allowed resistance between the line port terminals (the line port resistance) should be less than that permissible for the remainder of the looped period (the "hold" period) . This requirement could be met by keeping the line port resistance below the seize limit for the entire looped period. However, this would impose a severe limitation on line audio amplitude which could be accomodated without distortion. A better scheme, which may be implemented in the isolation apparatus of the present invention may be to force the line port resistance to a low value during the seize period but allow it to rise to a value determined by the hold control element during the hold period. According to a still further aspect of the present

invention there is provided a circuit having a port connectable to a telephone line, said port having a port resistance and said circuit including means for switching said circuit between on hook and off hook states, said switching means including: first means for maintaining said port resistance at a first value when said circuit is in said on hook state; second means for reducing said port resistance to a second value lower than said first value for a duration defining a hold period; and third means for reducing said port resistance to a third value lower than said second value for a duration defining a seize period, said seize period being relatively shorter than said hold period and preceding said hold period.

The isolation apparatus of the present invention may be embodied as a solid state device. The apparatus may be adapted to fully meet requirements for interfacing subscriber equipment to telephone service lines using ring-in loop-out standards. The apparatus may also provide an impedance match to major telephone networks.

In some embodiments, the apparatus may be used to replace

2 to 4 wire conversion circuits used in telephone company products, thus further reducing design complexity, cost and space requirements.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein:

Fig. 1 shows a block diagram of one form of isolation apparatus according to the present invention;

Fig. 2 shows a block application circuit for the apparatus of Fig. 1;

Fig. 3 shows a schematic diagram of one form of loop switch control and audio circuit according to the present invention;

Fig. 4 shows a schematic diagram of one form of ring detection circuit according to the present invention;

Fig. 5 shows a schematic diagram of one form of telephone line bias voltage regulation circuit according to the present invention; Fig. 6 shows a schematic diagram of one form of seize circuit according to the present invention;

Fig. 7 shows a schematic diagram of an isolation apparatus incorporating the circuits of figures 3 to 6; and

Fig. 8 shows one form of application circuit for the apparatus of Fig. 7.

In the block diagram shown in Fig. 1, two isolating elements provide optocoupled paths for transferring four kinds of communications signals across an isolation interface, also known as an isolation "barrier". A receive isolating element such as an optocoupler is represented by signal path S12 and a transmit isolating element such as an optocoupler is represented by signal path S4.

Line loop control and decadic dialling are achieved by switching bias current to line port pins LI, L2 to a high value to loop the line (ie. go off hook) or to a low value to unloop the line (ie. go on hook). To place the apparatus in an off hook state a logical high signal is applied to the dial hold pin (D/H) of the apparatus. This signal is propagated via signal paths S5, S6 and actuates or closes switches SW4 and SW5. A bias signal (represented via current source CS1 in Fig. 2) applied to the audio input pin (Al) propagates via signal path S7 and passes across the isolation interface via transmit signal path S4 and via signal path S2 actuates or closes switch SW1. This causes the positive side of the line to be connected (via diode bridge D1-D4) to the line positive pin (T+) and has a steady bias current applied to it via current source CS11.

In the off hook state audio signals such as DTMF dialling, voice and data are transmitted as a modulation of the line port current. The audio signal is represented by signal source SSI in Fig. 2. Audio signal SSI is applied to the Al pin via a capacitor C4 (for blocking DC) and series resistor R4 (for setting signal level) . The audio signal propagates via signal path S7, passes across the isolation interface via transmit signal path S4 and modulates current source CS11 to cause a corresponding

[ v modulation of line port current.

To place the apparatus in the on hook state a logical low signal is applied to the D/H pin of the apparatus. Switches SW4 and SW5 are in the unactuated states shown in Fig. 1. This interrupts passage of the bias signal applied to the Al pin and connects signal path S2 to its reference level via transmit signal path S4. Switch SW1 is therefore open or unactuated and little or no bias current is applied to line port terminals LI, L2. When the apparatus is on hook a ring signal of sufficient amplitude and frequency applied between the shunt (SH) and line (L2) pins of the apparatus (note: the SH pin is shunted to LI in most applications - refer Fig. 2) will cause each comparator CPl and CP2 to pulse high once per cycle. Due to the offsetting effect of voltage sources VS11 and VS12 the high periods of the two comparators will not overlap, so the output of OR gate Gl will pulse high twice per cycle.

Capacitor Cll ensures that the ring output pin (RO) is not spuriously activated by high DC voltages on the line. Capacitor Cll also blocks the DC bias on which the ring alternating voltage is superimposed.

It is desirable that the ring output be arranged such that it only pulses while the (instantaneous) polarity of the ring signal is the same as the polarity of the DC bias. This is achieved by: sensing the line voltage after rectification by the diode bridge D1-D4; comparing it with a reference voltage VS13 via comparator CP3 (VS13 is set slightly higher than the maximum peak ring voltage minus the line DC bias); and using AND gate G2 to generate the logical AND of the result of this comparison and the output of OR gate Gl, to cause a pulsating signal to appear on signal path SI.

The value of reference voltage VS13 is set such that the ring output will only produce one pulse per cycle of ring input. Current source CS12 represents the current which flows through the receive isolating element (LED of optocoupler) during ring detection. Although this current needs to be relatively large, capacitor Cll can be small, due to the buffering and current gain between it and the LED .

The pulsating ring signal on signal path SI propagates via summing block SB3, passes across the isolation interface via receive signal path S12 and appears at signal path S13 on the subscriber side. If the subscriber power supply voltage (shown as VS1 on Fig. 2) applied between the C+ and CR pins is adequate, the pulsating ring signal appears on line S14 and the RO pin

(represented as VI in Fig. 2). A 2 to 4 wire converter or hybrid is included in the apparatus of Fig. 1. The hybrid includes current source

CS11, summing block SB1 and resistors Rll, R12. The purpose of the hybrid, is to take the audio signal (SSI in figure 2) applied to the hybrid via the Al pin and signal path S4, and transmit it to the telephone line with minimum "leakage" of this audio signal back to the AO pin

(and V2 in figure 2) via signal path S3. As noted above the smaller this leakage the better the hybrid "balance".

The apparatus is designed so that its line port input impedance can be varied over a wide range, to match the telephone line impedance, by means of a single external network (Zn in figure 2) without affecting, or requiring re-adjustment of, its hybrid balance. This is achieved by making the output of the hybrid represented by signal path S3 proportional to the sum of currents, other than currents which originate from the telephone line, flowing through elements of the line port impedance

(comprising Rll and R12 in figure 1 and Zn in figure 2) plus the inverse of half the current generated by CS11. It will be noted that when the line port impedance matches the impedance of the telephone line, half the current generated by CS11 will flow through the elements of the line port impedance, namely Rll, R12 and Zn. The latter currents are sensed by the non-inverting inputs to SB1. Hence when the apparatus is connected to a telephone line with impedance equal to the isolator line port input impedance, the sum of the signals which originate from the

Al pin entering summing block SB1 will cancel out to zero.

Signals which originate from the telephone line will not cancel out but will propagate along signal paths S3,

SUBSTITUTE SHEET S15, and S12 and appear as the received signal on the AO pin and across V2 in figure 2.

The apparatus of Fig. 1 is arranged such that the

"programmed" impedance which the apparatus presents to the telephone line is essentially unaffected by variations in characteristics of the isolating elements which provide the signal paths across the isolation interface. This is achieved by judicious choice of networks which are connected across the telephone line. In Fig. 1 the networks connected acorss the telephone line which have a significant effect on line impedance comprise passive components such as: resistors Rll, R12 (plus Zn in Fig. 2); fixed voltage sources (the forward voltage drop of bridge rectifier diodes D1-D4); and a current source (CS11) , the current of which is entirely controlled by the signal fed into the Al pin. This ensures that unless some of the received signal from the AO pin is fed back into the transmit Al pin, the isolating elements will not affect the ratio of line port voltage to line port current. Hence the accuracy with which the apparatus presents the "programmed" impedance to the telephone line is essential unaffected.

Line bias setting in figure 1 is programmable by connecting external hold control resistor Rl (shown in Fig. 2) between the T+ and HC pins of the apparatus. The value of resistor Rl determines or programs the line bias voltage during the hold state.

Line bias voltage regulation is implemented as a negative feedback loop. The voltage across the line is sensed via a resistive divider comprising resistor R13 in figure 1 and hold control resistor Rl in figure 2. The voltage at the junction of these resistors is compared with a reference voltage VS14 via summing block SB2. The output of SB2 may be regarded as an "error" signal which is low pass filtered by resistor R14 and capacitor C12, and applied to the control input of current source CS13, the current terminals of which are connected across the line. Switch SW2 is open during the hold state, so resistor R15 is not relevant to bias voltage regulation. As may be apparent, any increase in bias voltage above the level where the two input voltages to SB2 are equal will tend to be opposed by a proportional increase in the current flowing through CS13, thus keeping the line bias voltage relatively constant with respect to changes in resistance of the line.

The apparatus of Fig. 1 incorporates an automatic seize function whereby line port resistance is forced to a low value during the seize period but rises to a value determined by hold control resistor Rl during the hold period.

The seize function is implemented by a first section which generates a pulse starting immediately after the telephone line is looped and having a duration corresponding to the desired seize period, and a second section which presents a low resistance to the line for the duration of, and under the control of, this pulse.

The first section consists of external seize control capacitor Cl (shown in figure 2), resistors R16, R17, R18, voltage source VS15, and comparator CP4. During the on hook period switch SW1 is open and seize control capacitor Cl will discharge, so when the line is looped and switch SW1 is closed, the voltage on the positive input of comparator CP4 will initially be close to T+ and then ramp down towards the line ref. voltage. The voltage at the negative input of comparator CP4 is biased part way between T+ and the line ref. voltage, so the output of comparator CP4 will go high when the line is looped then drop low to define the end of the seize period.

The second section consists of switch SW2, resistor R15, capacitor C12, and current source CS13. When the output of the comparator CP4 is high, switch SW2 will be closed and the control input of current source CS13 will be connected to T+ via resistor R15.

Resistor R15 is much smaller than resistor R14 so the control input to current source CS13 will be forced close to T+ causing CS13 to conduct a relatively high current per unit of line voltage. This causes the line port of the apparatus to present a low resistance to the telephone line. When the output of comparator CP4 goes low, switch SW2 will open and the apparatus will enter, and remain in, the hold state for as long as the line remains looped.

Fig. 3 shows the loop switch control and audio circuit and forms part of the isolation apparatus shown in Fig. 7. The isolation apparatus can force the telephone line into one of two states, looped or on hook, depending on whether the dial hold pin (D/H) is in the logic high or logic low states respectively.

When the D/H pin is high, transistors Q22 and Q23 will be on, so current will flow through the LED in optocoupler U2 causing a proportional current to flow in the phototransistor in U2, which will bias transistor Q12 on, and cause it to carry a current in proportion to that of this phototransistor. The sum of the currents in this phototransistor and in Q12 form the entire transmit audio current, and part of the line bias current. The current flowing through Q12 will also switch transistor Q9 on, thus allowing current to flow through R37 and the rest of the line side circuit. Transistor Q3 will be biased for linear operation, and any variation of telephone line voltage due to received audio signal will cause proportional variation in Q3 collector current and hence the current flowing in the LED of optocoupler Ul. This variation will be reflected in the phototransistor of Ul and hence in the voltage developed across resistor R65 and at the emitter of Q20, so the voltage appearing at the audio output pin (AO) will vary in proportion to the received audio signal.

Provided that the impedance of the line port terminals (LI, L2) is "programmed" to match that of the telephone line, audio signals fed into the audio input pin (Al) will not appear at the AO pin, since there will be no net transmit audio signal appearing between the base of Q3 and the line reference due to cancellation by the network consisting of R37, R38, R48, R49 and an external impedance matching network Zn comprising resistor R2 and capacitor C2 (shown in Fig. 8) connectable between the line positive pin (T+) and the impedance matching network pin (IN). When the D/H pin is low, transistors Q22 and Q23 will be off, so no current will flow through the LED or the phototransistor of optocoupler U2, hence Q12 will be off, so Q9 will be off. There will therefore be no current supply to bias Q3 on, or the rest of the line side circuit of the isolation apparatus, so no current will flow between the line port terminals (LI, L2) unless a ring signal is arriving from the line. The line will therefore be unlooped and the isolation apparatus will be

"on hook". Audio signals will not be propagated by the apparatus in this condition.

Fig. 4 shows the ring detection circuit and forms part of the isolation apparatus shown in Fig. 7. This circuit is designed to operate when the D/H pin is low and the isolation apparatus is in the on hook state. When an alternating voltage of sufficient amplitude and frequency superimposed on a suitable DC bias is applied to the line port terminals LI, L2 of the isolation apparatus, one of the transistors Ql or Q2 will switch on for part of each cycle. During the part of each cycle when the magnitude of the line voltage is increasing, whichever of these transistors is connected to the most positive side of the line, will switch on. If the instantaneous value of the line voltage is sufficiently high during these periods then transistor Q3 will be caused to switch on and supply current to the LED in optocoupler Ul, which will switch on the phototransistor in Ul thus switching on Q21 which will pull the ring output pin (RO) low. If the switching thresholds of this circuit are set appropriately the RO pin will pulse low exactly once per ring cycle over the full range of required operating conditions.

Capacitor Cll is included in the ring detection circuit to give it frequency discrimination so it will not respond to very low frequencies even if they are of a high amplitude. Due to the "buffering" effect of Ql, Q2, and Q3, a large LED current can be obtained even if capacitor Cll is relatively small.

Fig. 5 shows the line bias voltage regulation circuit and forms part of the isolation apparatus shown in Fig. 7. The purpose of this circuit is to minimise variation, from the externally programmed setting, of the bias, or time average, voltage across the line port terminals LI, L2 of the apparatus in the hold state, due to variation in the resistance of the telephone line between the exchange and the apparatus.

The circuit of Fig. 5 has similar electrical behavior to an inductor, a small resistor, and a programmable voltage source, all in series, connected to the rectified side of diode bridge D1-D4. The purpose of the inductance is to give the circuit a high impedance to audio frequency signals but a low impedance to DC. In this circuit the inductor is simulated by an active circuit including a voltage controlled current source.

The current source consists of Q5, Q6, Q7, R42, R43, and R44, across the line, controlled by the voltage across capacitor C12. Capacitor C12 in turn is fed current in proportion to the instantaneous line voltage by Qll, R51, R52, and an external hold control resistor Rl (shown in Fig. 8) connectable between the line positive pin (T+) and the hold control pin (HC) . Q10, R53, R54, and R55 form a voltage reference which biases Qll to act as a "common base" voltage amplifier. This amplification makes it possible to obtain a high, but controlled, gain in the voltage regulation feedback loop, allowing tight regulation of line bias voltage.

Fig. 6 shows the seize circuit and forms part of the isolation apparatus shown in Fig. 7. This circuit minimises the resistance presented to the telephone line by the isolation apparatus for a short period after the line is looped (the seize period) by switching a small resistor R44 across the line (via diode bridge D1-D4) under control of a monostable multivibrator.

The multivibrator, consisting of Q4, Q8, Q13, R69, R70, R71, R72, R75 and R77, uses an external timing capacitor Cl (refer Fig. 8), connectable between the seize control pin (SC) and the line positive pin (T+), and is reset by the discharge of timing capacitor Cl during on hook periods. When the line is looped timing capacitor Cl will slowly charge up, the voltage across its terminals will "ramp" up towards the hold state equilibrium value, and the charging current will decline towards zero, so transistor Q8 will switch on immediately after line looping, and remain on until the charging current falls below a threshold. Q13 and Q4 are controlled by, and provide voltage and current gain for Q8, so during the seize period they will be on, and they will force transistors Q5, Q6, and Q7 to saturate (switch on hard). Q9 (refer Fig. 7) represented by a broken line in Fig. 5 is always switched on while the line is looped, so during seizure there is a series path between the line port terminals LI, L2 of the apparatus, which consists of two diodes Dl and D4 or D2 and D3, four saturated transistors, Q5, Q6, Q7, Q9 and one small resistor R44, thus providing a low resistance and voltage drop between the line port terminals LI, L2. After the seize period Q8, Q4, and Q13 will switch off.

Fig. 8 shows one application of an isolation apparatus 80 according to the present invention. The line side of apparatus 80 is connected to a two wire telephone line. The subscriber side of apparatus 80 is connected to subscriber equipment 81 such as a modem, fax machine, personal computer or the like.

On the line side of apparatus 80 an impedance matching network Zn comprising capacitor C2 and series resistor R2 is connected between the line positive pin (T+) and the impedance matching network pin (IN). A hold control resistor Rl is connected between the T+ and hold control (HC) pins and a seize control capacitor Cl is connected between the T+ and seize control (SC) pins. The two wire telephone line is connected to line port pins LI, L2. A bidirectional zener diode BZ1 is connected across line pins LI, L2 to protect apparatus 80 from high voltage transients appearing on the telephone line. Shunt pin (SH) is connected to line port pin LI. On the equipment side of apparatus 80 pin C+ is connected to a suitable power supply voltage as is pin V+ of equipment 81.

The ring output pin (RO) is connected to the "ring detect" pin of equipment 81. The audio input pin (Al) is connected to the transmit pin (Tx) of equipment 81 via resistor R4, for setting transfer gain, and capacitor C4, for blocking DC. The audio output pin (AO) is connected to the receive pin (Rx) of equipment 80 via resistor R5, for providing a load, and capacitor C5 for blocking DC. The dial/hold pin (D/H) is connected to the dial/hold pin of equipment 81. The subscriber reference pin (CR) is connected to the ground pin (GND) of equipment 81.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

Claims (1)

1. 1. Isolation apparatus for interfacing equipment to a communications network, said apparatus including: a first part connectable to said network; a second part connectable to said equipment; an isolation interface for electrically isolating said first and second parts, said interface including at least one isolating element for passing communications signals between said first and second parts, wherein at least one said isolating element is adapted to pass plural kinds of said communication signals whereby the number of said kinds of communications signals able to be passed between said network and said equipment exceeds the number of said isolating elements.

   2. Isolation apparatus according to claim 1 wherein the or each isolating element includes an optocoupler having a light emitting diode (LED) and a photo transistor.

   3. Isolation apparatus according to claim 1 or claim 2 wherein one isolating element is adapted to pass in a receive direction ring signals when said apparatus is in an on hook state and is adapted to pass in the receive direction audio signals when said apparatus is in an off hook state. 4. Isolation apparatus according to any one of the preceeding claims wherein one isolating element is adapted to pass in a transmit direction line loop/decadic dialling signals as a bias current and is adapted to pass in the transmit direction audio signals as a modulation of said bias current.

   5. A hybrid circuit having an input port for receiving an input signal, an output port for conveying an output signal and a bidirectional port having a first impedance for connection to a transmission line, said transmission line having a second impedance and said circuit including: means associated with said bidirectional port for matching said first impedance to said second impedance; means associated with said bidirectional port for generating a current proportional to said input signal; means for producing said output signal such that said output signal is proportional to the sum of currents, other than currents which originate from said line, flowing through said bidirectional port including said impedance matching means, plus the inverse of half the current generated by said generating means, whereby when said first impedance means matches said second impedance means, said output signal cancells substantially to zero.

   6. A hybrid circuit according to claim 1 wherein said impedance matching means includes a network external to said circuit.

   7. A hybrid circuit according to claim 5 or 6 wherein said producing means includes a summing junction and an inverter.

   8. Isolation apparatus for interfacing equipment to a transmission line, said transmission line having a first impedance and said apparatus including: a first part having a second impedance connectable to said line; a second part connectable to said equipment; means associated with said first part for matching said second impedance to said first impedance; an isolation interface for electrically isolating said first and second parts, said interface including at least one isolating element for passing communications signals between said first and second parts; the or each isolating element being arranged such that it does not significantly affect the ratio of voltage to current in said line, whereby said impedance matching means is substantially independent of variations in characteristics of the or each isolating element.

   9. A circuit connectable to a telephone network for detecting a ring signal carried by said network, said signal being superimposed on a DC bias voltage and comprising periodic bursts of a signal frequency, said circuit having a port for presenting a relatively high impedance to said network in the absence of said ring signal and including: filter means associated with said port and having at least one reactive element for passing said ring signal whilst substantially preventing passage of said bias vo ltage ; and amplifying means associated with said filter means for receiving said ring signal and for providing at least the frequency of said ring signal at its output, said output having a relatively high current per unit of output voltage for driving a load having a relatively low impedance.

   10. A circuit according to claim 9 wherein operation of said circuit is substantially independent of the polarity of said bias voltage.

   11. A circuit according to claim 9 or 10 wherein said reactive element comprises a capacitor having a relatively small value.

   12. A circuit according to claim 9, 10 or 11 wherein said filter means and said amplifying means are powered by said DC bias voltage.

   13. A circuit according to any one of claims 9-12 wherein said load comprises a light emitting diode associated with an optocoupler. 14. A circuit connectable to a transmission line, said line having a line resistance, said circuit including means for applying a bias voltage to said line and means for regulating said bias voltage, said regulating means including: means associated with said line for producing a signal proportional to said bias voltage; means associated with said producing means for comparing said proportional signal to a reference voltage and for generating an error signal whenever said proportional signal exceeds said reference voltage; and means responsive to said error signal for reducing said bias voltage, such that said bias voltage is maintained relatively constant with respect to changes in said line resistance. 15. A circuit according to claim 14 wherein said means for applying said bias voltage includes a first current source having a first polarity connectable to said line and said means for reducing said bias voltage includes a second current source having an opposite polarity connectable to said line. 1₆. A circuit according to claim 14 or 15 wherein said producing means includes a network external to said circuit.

   17. A circuit having a port connectable to a telephone line, said port having a port resistance and said circuit including means for switching said circuit between on hook and off hook states, said switching means including: first means for maintaining said port resistance at a first value when said circuit is in said on hook state; second means for reducing said port resistance to a second value lower than said first value for a duration defining a hold period; and third means for reducing said port resistance to a third value lower than said second value for a duration defining a seize period, said seize period being relatively shorter than said hold period and preceding said hold period.

   18. _A circuit according to claim 17 wherein said third means includes timing means for generating a control signal having a duration corresponding to said seize period, and a current source for applying to said port a relatively high current per unit of line voltage in response to said control signal, such that said port resistance is reduced to said third value for the duration of said seize period.

   SUBSTITUTE SHEET