Subscriber line interface circuit with an off-hook detection circuit

Abstract: A circuit assembly is disclosed which comprises: an interface circuit (3') between the subscriber line (L) and exchange devices of the kind with two amplifiers connected in a bridge configuration (9A,9B) between the battery (UB) terminals and having a means (15) of detecting the output currents (IA,IB); a supply circuit (5') which includes the exchange battery (UB) and a ring generator (AC'); a switch-over means (7) for putting the line (L) through the interface circuit (3') or the supply circuit (5'); two bridge resistors (RP,RP') which are in series with the line (L) when the latter is connected to the supply circuit (5') and of which one (RP') is connected permanently between one (A) of the line terminals and the output (3A) of one (9A) of the amplifiers, which output is substantially at the potential of one of the battery (UB) poles; an additional resistor (RA), having a much higher resistive value than that of the bridge resistors (RP,RP') and being connected between the other (B) of the line (L) terminals and the output (3B) of the other (9B) of the amplifiers; and a processing means within the interface circuit (3'), which includes said detecting means (15) and is effective to isolate the useful signal (IT) from the line current, recognize a possible hook-off state, and send a corresponding signal (OH) to the exchange devices.

Description: This invention relates to telephone circuits, and more particularly, to an electronic subscriber link circuit assembly, i.e. one having an electronic interface between the telephone subscriber line and the telephone exchange, with a circuit for detecting the hook-off condition during the ring step. As is known, a telephone subscriber sets comprised essentially of a speech circuit and a bell connected in parallel to each other, is connected to a telephone exchange by means of a two-wire line. Connected in series to the speech circuit which would be only closed on hooking off, and serially to the bell, is a capacitor operative to DC uncouple the bell from the line. A telephone subscriber line is supplied a direct voltage from a source, having an alternate voltage source connected serially thereto, which forms the ring signal generator, when the exchange devices control special line connection contacts to close and issue said ring signals to the subscriber. The ring signals are sent over the line at predetermined intervals. When the hook-off occurs while a ring signal is being sent, a DC component overlaps the alternate ring current over the line. With today's telephone exchanges, the hook-off condition is detected by circuit means of an electronic type which sense the presence of direct current on the line due to hooking off, even where it overlaps the alternate current of a possible ring signal. Ring signals have usually a frequency in the 16 to 66 Hz range and voltage RMS values in the 50 to 105-Volt range, that is, much higher values than those of telephone speech signals. However, if the ring signals are not interrupted directly on hooking off, they are converted, by the speech circuit of the telephone subscriber set, to high-intensity acoustical signals which are harmful to the subscriber's hearing and the set itself. Accordingly, such circuit means are designed to timely sense the hook-off condition and issue an interrupt signal for the ring signals at once to the exchange devices. Implemented by electronic means are also other functions required for proper operation of the telephone network, such as handling the supply of direct current and sending ring signals, switching from two- to four-wire conversion, providing protection from overvoltages, checking the line operability, and so forth. These functions are performed by a circuit assembly which forms an interface between the telephone exchange control devices and the telephone subscriber line. This is commonly referred to as the electronic subscriber link or SLIC (Subscriber Line Interface Circuit). Most of the circuits of the electronic subscriber link are integrated to few, usually two, monolithic IC devices. The generator of ring signals is in some instances incorporated to one of the integrated circuits, and in some other instances, is a discrete component serving, as a rule, all of the subscriber lines to the exchange. This invention can be advantageously applied to the last-named situation. A circuit assembly of this kind, as designed and practiced by this Applicant, partly in the form of monolithic integrated circuits designated TDB 7711 and TDB 7722, is presented on page 380 of SGS-THOMSON MICROELECTRONICS handbook "Telecom Data Book", June 1989. In this application, to be discussed hereinafter, the hook-off recognition function is performed through the use of a discrete component, resistive network for detecting the line transverse current, and of circuit means internal to one of the integrated circuits to obtain the hook-off information from that current. The resistive network consists of six resistors which, for the circuit to operate properly, must be a high-accuracy (1 o/oo) type and are, therefore, relatively expensive, and the internal circuit means obviously require special access terminals for connection to the resistive network. It is a primary object of this invention to provide a circuit assembly as specified in the preamble of the first of the appended claims, which requires no high-accuracy resistors in order to recognize the hook-off condition during the ring step. Another object is to provide a circuit assembly as above, which can be advantageously implemented with monolithic IC devices having a reduced number of terminals. These objects are achieved, according to the invention, by the circuit assembly generally set forth in the first of the claims appended hereto. The invention will be better understood from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings, wherein: Figure 1 illustrates diagramatically the above-mentioned conventional circuit assembly; Figure 2 illustrates diagramatically a circuit assembly according to the invention; Figure 3 is a diagram showing a circuit for recognizing hook-offs during the ring step which utilizes the principles of this invention; Figure 4 illustrates the current transfer characteristic of a stage of the circuit in Figure 3; and Figure 5 shows in diagramatic form a modified embodiment of the circuit assembly according to the invention: Shown in Figure 1 is a telephone subscriber set U connected, via a line L, to a circuit assembly 1 of a telephone exchange, no further illustrated. The assembly 1 comprises an interface circuit 3 between the telephone subscriber and the exchange control devices, basically consisting of two integrated circuits in the application shown in the aforementioned publication, and having two terminals 3A and 3B for connection to the line L, a terminal 3C for connection to the exchange control devices (not shown), and two more terminals 3D, 3F. Understandably, the circuit 3 would actually include a multiplicity of additional terminals, not shown in the diagram of Figure 1 because immaterial to the function to be illustrated. In addition, a supply circuit 5 has been shown which comprises a battery or direct voltage source UB having its positive pole connected to the ground connection of the circuit assembly, and a ring current generator in series with the battery UB, being in the form of two identical, push-pull alternate current sources shown at AC. Switch-over means, such as a relay and respective contacts, generally indicated at 7, are represented by two switches 7A and 7B arranged to connect terminals A and B, respectively, of the telephone line L to the output terminals 3A and 3B of circuit 3 or the terminals of the supply circuit 5. The last-named connection is established through two resistors RP having the same resistive value, e.g. 300 ohms, and forming the so-called bridge resistances which, as is known, are required for the coupling at the exchange between the supply circuit and the subscriber line. Connected to the four terminals of the resistors RP is a group 6 of four identical resistors connected together in a crossed configuration, as shown in the drawing, and having two output terminals formed by the crossed internal terminals of the resistors, which are connected to the terminals 3D and 3F of circuit 3. The resistive network formed of these four resistors and the two bridge resistors is effective to separate the so-called transverse current, i.e. the useful component of the line current, from the so-called longitudinal current, i.e. the component from noise and stray phenomena. As is known, the network operation is based on that the transverse current flows in opposite directions through the two resistors RP, whereas the longitudinal current flows in the same direction through both of them. As mentioned hereinabove, in order for that operation to take place in an adequately accurate manner, the six resistors in the network should be all carefully sorted out such that their coupling can be a highly accurate one. The transverse current information thus revealed is processed within the circuit 3 by circuit means adapted to obtain information about a possible hook-off condition of the subscriber set U with the line L connected to the supply circuit 5 as a call or ring signal is being sent. In principle, such circuit means will recognize the presence of a DC component in the transverse current and send a signal to the exchange control devices whereby the ring signal is discontinued and the line of the calling subscriber is put through that of the called subscriber. Figure 2, where similar elements to those in Figure 1 are denoted by just the same references and comparable ones by the same references primed, illustrates a circuit assembly according to the invention. As shown therein, the supply circuit 5' is a so-called unbalanced bell type, that is one in which the ring generator consists of a single alternate current source AC' and is, from the functional standpoint, the equivalent of the conventional one with two push-pull sources. This construction for the power supply is preferable in that it affords a simpler circuitry, but in principle, the invention could be also practiced with a conventional balanced bell supply, as explained hereinafter with reference to Figure 5. The bridge resistors are connected, the one RP, between one contact of the switch 7B and the supply circuit 5', specifically the negative terminal of the battery UB through the source AC', in series, similarly to the circuit of Figure 1, and the other RP', between the line terminal A and the output terminal 3A of circuit 3'. An additional resistor RA is connected between the line terminal B and the output terminal 3B of circuit 3'. The resistive value of resistor RA should be much higher than that of resistors RP, RP' for example equal to 4 kOhms, for a reason to be explained. The terminals 3A and 3B of circuit 3' are the output terminals of two bridge-connected operational amplifiers in a buffer configuration. This construction of the output stage of circuit 3' allows the line current flowing through the stage output terminals 3A and 3B to be detected. With that, moreover, the output terminal 3A will be, during the ring step, at a DC potential a few volts below ground level (typically 5 Volts), and the output terminal 3B will be at a potential a few volts above battery UB voltage (typically 5 Volts). This ability to "sense" the line current and these voltage conditions of terminals 3A and 3B are utilized, in accordance with the invention, to perform the function of hook-off recognition during the ring step. It should be noted, in fact, that when the contacts 7A and 7B of the switch-over means 7 occupy the position indicated by dash lines in Figure 2, the telephone line L is connected to the supply circuit 5' through the bridge resistors RP, RP', as prescribed. The one difference from the conventional connection, such as that shown in Figure 1, is that the bridge resistor RP' related to wire A in the line would be connected to a terminal which is at a slightly more negative potential than ground, but this is immaterial from a functional standpoint. As for the line current information, which in the conventional circuit of Figure 1 was obtained through the net 6 of six precision resistors, according to the invention, it is obtained directly from the final stage of the interface circuit 3' which is coupled, as shown, to the telephone line L even with the latter connected to the supply circuit 5', via the bridge resistor RP' and the additional resistor RA. It should be noted that the resistive value of the last-named resistor is sufficiently large not to significantly alter the desired level for the ring signal unused on the subscriber line, but still capable of supplying on terminal 3B the required line current information. The final stage of circuit 3' may be illustrated schematically by two operational amplifiers, indicated at 9A and 9B in Figure 2, which have their respective inverting inputs connected to the respective outputs 3A and 3B, and the non-inverting terminals biased to a voltage value VA slightly below ground and a voltage value VB slightly above battery UB voltage, respectively. A circuit block 15, e.g. of the kind described in Italian Patent Application No. 19983-A/85, filed on 20. March 85 by SGS Microelettronica S.p.A., is connected to the amplifiers 9A and 9B so as to obtain, from currents IA and IB as by add and subtract operations, a current kIT which is an image of the transverse current IT. This is supplied to a detection circuit, indicated at 17 in Figure 2, which obtains, from the image of the transverse current IT, information of'the presence of a DC component on the line current, and accordingly, detects any hook-offs and sends a corresponding signal (OH) to the exchange control devices. It should be also noted that the resistor RA is not strictly required. In fact, with ring voltage values below 60 Volts RMS, or more generally, with any value of the ring voltage where high-precision sensing circuits are employed, it is not necessary that current IB be other than zero, since the current IT information may be alternatively obtained from just current IA by virtue of the detection being carried out by performing the half-sum of currents IA and IB. The hook-off detecting circuit 17 may be a conventional one, such as that disclosed in Italian Patent No. 1,212,838 to this Applicant, but could be advantageously configured as shown diagramatically in Figure 3. A logic circuit LG will receive, from the exchange control devices, a ring command signal RC and, as a result, cause a switch 5, in series with the input of a transconductance operational amplifier OTA, to close. The current kIT, being the image of the line transverse current, is compared within input node NI to a threshold current IS having a predetermined value, to thereby prevent spurious hook-off recognitions due to noise on the line. If kIT > IS, a current IIN = kIT - IS will flow through a capacitor CRT cascade connected to the output of the amplifier OTA. The latter has a transfer characteristic of the kind illustrated by Figure 4, that outputs a current ICRT which is proportional to the input current IIN, for values of this current falling between two given threshold values, and maintained constant at a positive or negative value Io as the input current varies, upon attaining such values. Prior to hooking off, the mean value of current kIT would be less than the threshold current IS, and accordingly, the capacitor CRT left in the discharged state. Upon hooking off, due to the direct current which overlaps the ring alternate current, the mean value of kIT will exceed the threshold current IS, and the capacitor CRT will become charged. The output of the circuit OTA is connected to the input of a comparator CP which has another input at a predetermined reference voltage VREF and its output connected to the logic circuit LG. On the voltage across the capacitor CRT exceeding the value of the reference voltage VREF, the comparator CP will send a signal to the logic circuit LG, which transmits a hook-off condition signal OH to the exchange control devices. Shown in Figure 5, where similar elements to those in Figure 2 are denoted by the same references, is a circuit assembly according to the invention, wherein the ring signal is supplied by two identical, push-pull AC sources being both designated AC. This circuit differs from the one shown in Figure 2 by that the bridge resistor, here designated RP" and relating to the line terminal A, is connected to the supply circuit 5" through contact 7A, as was the case with the conventional circuit in Figure 1, and in that a second resistor RA', having the same resistive value as RA, is connected between the terminal A and the output 3A of circuit 3'. Thus, in this embodiment, the line current information is supplied through the two resistors RA and RA', and the above-described characteristic of circuit 3' is utilized whereby the terminals 3A and 3B are, during the ring step, at voltages close to ground and negative battery pole levels, respectively. Of course, in this embodiment of the inventive circuit, the resistors RA and RA' are both required at all times. It will be readily appreciated from the foregoing that the circuit assembly of this invention fully achieves its objects. In fact, for recognizing the hook-off condition during the ring step, it requires no discrete high-precision resistors to detect the line current, since it utilizes current information already present on the interface circuit, and hence, no dedicated electrical connections between external components and the interface circuit. Thus, the latter can be advantageously integrated with a reduced number of terminals. The few additional discrete components involved, which as explained are not invariably necessary, are no critical values, and form, therefore, low-cost components.

Claims: A telephone circuit assembly with a circuit for recognizing the hook-off condition during the ring step, comprising an interface circuit (3') between the telephone subscriber line (L) and exchange control devices; a supply circuit (5') including a DC voltage source (UB) and a ring AC generator (AC') in series with each other; a switch-over means (7) effective, under control by the exchange control devices, to put the telephone subscriber line (L) through the interface circuit (3') or the supply circuit (5'); two resistive means (RP,RP') having substantially the same resistive value and being each connected serially between one (B,A) of the telephone subscriber line (L) terminals and one of the DC voltage source (UB) terminals, with the telephone subscriber line (L) being connected to the supply circuit (5') by the switch-over means (7); and a processing circuit means responsive to the current on the telephone line (L), which current comprises a transverse, or useful, component, and effective to isolate the useful component from said line current, recognize any DC component therein during the ring step, and send upon this recognition a signal to the exchange control devices; the interface circuit (3') comprising a pair of amplifiers (9A,9B) connected in a bridge configuration between the terminals of the DC voltage source (UB) and having their respective outputs (3A,3B) connected each to a terminal (A,B) of the telephone subscriber line (L) upon the switch-over means (7) putting the latter through the interface circuit (3'), and a current detecting means (15) associated with said amplifiers (9A,9B) to detect the output current (IA,IB) from the amplifiers (9A,9B) and produce a current (kIT) proportional to the useful component of the current on the telephone line (L); characterized in that one (RP') of the resistive means is connected permanently between one (A) of the telephone line (L) terminals and the output (3A) of one (9A) of the interface circuit (3') amplifiers, thereby said connection to the respective terminal of the DC voltage source (UB) occurs through said amplifier (9A); and the processing circuit means is incorporated to the interface circuit (3'), includes the current detecting means (15) associated with the amplifiers (9A,9B), and is effective to perform the half-sum of the output currents (IA,IB) from the amplifiers (9A,9B) as detected by the current detecting means (15). A circuit assenibly according to Claim 1, characterized in that it comprises an additional resistive means (RA), having a much higher resistive value than that of said two resistive means (RP,RP') and being connected between the other (B) of the telephone line (L) terminals and the output (3B) of the other (9B) of the interface circuit (3') amplifiers. A telephone circuit assembly with a circuit for recognizing the hook-off condition during the ring step, comprising an interface circuit (3') between the telephone subscriber line (L) and exchange control devices; a supply circuit (5") including a DC voltage source (UB) and a ring AC generator (AC) in series with each other, said ring generator comprising two identical push-pull sources; a switch-over means (7) effective, under control by the exchange control devices, to put the telephone subscriber line (L) through the interface circuit (3') or the supply circuit (5"); two resistive means (RP,RP") having substantially the same resistive value and being each connected serially between one (B,A) of the telephone subscriber line (L) terminals and one of the DC voltage source (UB) terminals, with the telephone subscriber line (L) being connected to the supply circuit (5") by the switch-over means (7); and a processing circuit means responsive to the current on the telephone line (L), which current comprises a transverse, or useful, component, and effective to isolate the useful component from said current, recognize any DC component therein during the ring step, and send upon this recognition a signal to the exchange control devices; the interface circuit (3') comprising a pair of amplifiers (9A,9B) connected in a bridge configuration between the terminals of the DC voltage source (UB) and having their respective outputs (3A,3B) connected each to a terminal (A,B) of the telephone subscriber line (L) upon the switch-over means (7) putting the latter through the interface circuit (3'), and a current detecting means (15) associated with said amplifiers (9A,9B) to detect the output current (IA,IB) from the amplifiers (9A,9B) and produce a current (kIT) proportional to the useful component of the current on the telephone line (L); characterized in that it comprises two additional resistive means (RA,RA'), substantially identical with each other, having much higher resistive values than that of said two resistive means (RP,RP") and being individually connected between one (A,B) of the telephone line (L) terminals and the output terminal (3A,3B) of one (9A,9B) of the interface circuit (3') amplifiers, and the processing circuit means is incorporated to the interface circuit (3') and includes the current detecting means (15) associated with the amplifiers (9A,9B). A circuit assembly according to any of the preceding claims, characterized in that the processing circuit means comprises a circuit (17) having a first input terminal adapted to receive said proportional current (kIT) to the useful component of the telephone line (L) current, a second input terminal connected to the exchange control devices for receiving a ring command signal (RC) therefrom, and an output terminal connected to the exchange devices to supply them said signal (OH) of recognition of the direct current during the ring step, said circuit (17) comprising a threshold current source (IS), a current comparator (NI) input-wise connected to the first input terminal and the threshold current source (IS), a transconductance operational amplifier (OTA) input-wise connected to the output of the current comparator (NI) through a controlled switch means (S) and output-wise to a charge storage means (CRT) and a first input terminal of a voltage comparator (CP), said voltage comparator (CP) having a second input connected to a reference voltage source (VREF) and an output connected to an input of a logic circuit (LG) which is connected to said second input terminal and said output terminal (OH) of the circuit (17), said logic circuit (LG) also having an output terminal connected to the controlled switch means (S) to control it to close upon receiving a command signal (RC) on said second input terminal thereof.

Inventors: SAVIOTTI VANNI, SILIGONI MARCO, SAVIOTTI, VANNI, SILIGONI, MARCO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H04M19
IPC codes: H04M19, H04M3, H04Q3