Abstract:
An integrated telephone interface circuit for driving a telephone line includes a line current sensor and a phase converter, both associated with an output stage connected to the telephone line. The circuit is equipped with a protection device against the generation of spurious signals including a comparator connected between the converter and the output stage, a control and monitoring circuit linked operatively to an output of the comparator, and a plurality of switches associated with the input side of the converter, as well as with the current sensor and the output stage. The switches are linked operatively to respective outputs of the control and monitoring circuit to reverse polarity of the line supply upon a predetermined threshold value for the comparator being exceeded.

Description:
FIELD OF THE INVENTION 
     The invention is related in particular to the field of telephone circuitry which interfaces lines connected to telephone subscriber apparatuses with a central station. More particularly, this invention relates to an interface integrated circuit for driving telephone subscriber lines, being of a type which comprises a line current sensor and a phase converter, both associated with an output stage connected to the telephone line. 
     BACKGROUND OF THE INVENTION 
     There exists a demand in this specific field to improve the quality of the communications over the telephone lines and, at the same time, to expand the range of services offered to the users. In this respect, it can be mentioned that token-operated apparatuses installed in public telephone boxes produce noise (spurious signals) in the audio band each time that a token is accepted. This is due to a reversal of the polarity brought about at the line ends. Furthermore, in recent years, complex telephone services, such as &#34;videotel&#34;, are gaining widespread acceptance. Such services involve transmission of trains of digital information over the telephone lines. 
     To fill such increasingly more sophisticated demands, a constant effort to improve the quality of transmission over telephone lines has become mandatory. This effort concentrates, on the one side, on improving the structural quality of each line or of the telephone network as a whole. In fact, many national telephone companies are known to be involved in restoring their telephone networks through the use of fiber optics. On the other side, the improvement effort is directed toward the suppression of any noise in the audio band as well as interference in the band employed for transmitting digital information. To that aim, attention has been focused on the circuit devices which operate to interface the telephone subscriber lines. For example, it is a well-recognized fact that a controlled or &#34;soft&#34; reversal in the supply polarity to the telephone subscriber line could ensure suppression of the aforesaid noise caused by public telephones. 
     As used herein, &#34;polarity reversal&#34; or &#34;battery reversal&#34; means to exchange the polarities of the TIP and RING wires in a two-wire telephone line. In other words, to reverse polarity is to cause the TIP wire to take the voltage of the RING wire, and vice versa. It would indeed be appreciated if the interfacing telephone circuit connected to the line could perform a soft reversal of the supply polarity to the line. 
     However, interface telephone circuits have been implemented heretofore by integrated circuits which tend to be complicated both in construction and operation, and to which new functions cannot be integrated in a simple manner without compromising in some other areas of the circuit. In the specific instance considered, the reversal of polarity to the line, i.e. exchanging the TIP and RING wires, may cause a malfunction instead of fostering the suppression of line noise. Thus, no monolithically integrated telephone circuits are known at present which can reverse the supply polarity to the subscriber line in a soft manner. 
     An underlying technical problem of this invention is to provide an interfacing telephone circuit which is monolithically integrated and adapted to drive a telephone subscriber line, having further such structural and functional features as to allow a soft reversal of the line polarity, thereby overcoming the limitations of prior art approaches. 
     SUMMARY OF THE INVENTION 
     This problem is solved by an interface integrated circuit for driving a telephone subscriber line, being of a type which includes a line current sensor and a phase converter, both associated with an output stage connected to the line. The circuit includes a comparator connected between the phase converter and the output stage; a control and monitoring circuit means linked operatively to the output of the comparator; and a plurality of switches associated with the input side of the phase converter, as well as with the current sensor and the output stage. The switches are linked operatively to respective outputs of the circuit means to reverse polarity of the line supply upon a predetermined threshold value for the comparator being exceeded. 
     In one embodiment, the comparator has respective inputs connected to signal outputs of the converter. 
     An external capacitor is connected between a reference potential and a voltage input of the converter, and a switch is connected between the input and a pair of phase-opposed current sources independently connected between the switch and the reference potential. 
     The output stage includes a pair of output buffers, one for each line terminal, and the switches are connected between the signal outputs and corresponding inputs of the buffers. 
     The control and monitoring circuit means is a logic circuit having a first input connected to the comparator output and a second input adapted to receive an enable signal from an exchange incorporating the telephone circuit. The logic circuit has respective control outputs for each of the electronic switches. 
    
    
     The features and advantages of a circuit according to the invention will become apparent from the following detailed description of an embodiment thereof, given by way of example and not of limitation with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing schematically a monolithically integrated interface telephone circuit connected to a telephone subscriber line. 
     FIG. 2 is a diagram showing schematically an interface circuit according to this invention and effective to permit reversal of the supply polarity to the telephone line. 
     FIG. 3 shows a set of graphs plotted along a common time base and related to voltage signals appearing in the circuit of FIG. 2. 
    
    
     DETAILED DESCRIPTION 
     With reference to the drawing views, generally and schematically shown is a line driver circuit 1 included in a monolithically integrated telephone circuit 25 called SLIC (Subscriber Line Interface Circuit) for installation in telephone exchanges. 
     The line driver circuit 1 is powered by a voltage -Ve and connected to a two-wire telephone line 3 which is connected to a subscriber apparatus 8 and comprises a pair of conductors A and B. The voltage Ve is a voltage produced as described in U.S. Pat. No. 4,800,589 to Siglioni, et al., where Ve corresponds to the supply voltage V B  of U.S. Pat. No. 4,800,589. The line driver circuit 1 drives the telephone line 3 through a pair of outputs, namely: a first terminal TIP connected to the conductor A, and a second terminal RING connected to the conductor B. Respective line currents I A  and I B  are supplied at the terminals TIP and RING. 
     Advantageously, according to the invention, the circuit 1 is equipped with a protection device effective to perform a soft reversal of the supply polarity of the line 3 wires, and this without generating spurious signals or interferences in the audio transmission bands. For this purpose, the circuit 1 is constructed with different circuit portions, each exhibiting predetermined functional characteristics as detailed herein below. 
     With reference to FIG. 2, a first circuit portion 2 comprises a voltage-current converter 13 having a voltage input 7 and a pair of signal outputs 4, 5. Correspondingly associated with each of said outputs 4, 5 is a current source A1, A2. Such voltage-current converters and current sources are known in the art. 
     The first signal output 4 is connected to ground, downstream from the current source A1, via a resistor R1. The second signal output 5 is connected to a supply pole at a negative voltage -Ve via a resistor R2. Preferably, the resistors R1 and R2 have the same value, whilst the current supplied from the current source A1 is equal to the negative of the current supplied by the current source A2. 
     The signal outputs 4 and 5 are also connected to respective inputs of a comparator 12. The single output COMP from the comparator 12 is applied to an input of a logic circuit 15. 
     The voltage input 7 of the converter 2 is AC coupled, on the one side, to ground via a capacitor C, and on the other side, to a switch 9. Advantageously, the capacitor C is the only external component part of the integrated circuit 1. The switch 9 is, of course, an electronic switch as is known, and can be switched between a first position, where it connects the input 7 to one end of a first current source I o1  having the opposite end grounded, and a second position where it connects the input 7 to one end of a second current source I o2  having the opposite end grounded. The current sources I o1 , I o2  associated with the switch 9 are in phase opposition with each other. The switch 9 is linked operatively to an output CSC from the logic circuit 15, which circuit receives a control enable signal at a second input COMBI. 
     A second circuit portion 20 comprises an output stage which incorporates a pair of output buffers 16, 17, one for each of the TIP and RING line terminals. Each output buffer 16 and 17 has its output fed back to one of its buffer inputs. The other input of each buffer is connected to a corresponding switch 18, 19. Each switch 18, 19 is an electronic type and has two stable states, each associated with the respective signal output 4, 5 of the converter 13. The switches 18 and 19 are linked operatively to a second output BI of the logic circuit 15. 
     A third circuit portion 30 comprises a line current sensor connected between the supply pole at the negative voltage -Ve and ground. Such signals are correlated by an appropriate sign with the line currents I A  and I B . The third circuit portion 30 processes such current signals IA and I B  to output further signals which are respectively proportional to the difference and the sum of the line currents, i.e., (I A  -I B )/K and (I A  +I B )/K, where K is a constant of proportionality. 
     The third circuit portion 30 includes plural current mirrors 27 having suitable gain values to output said signals proportional to the difference and the sum of the line currents. A description of the construction and operation of such circuits, excluding electronic switches 23, 24 which provide isolation for downstream circuits, is presented in U.S. Pat. No. 4,782,507 to Siglioni et al. 
     The third circuit portion 30 is also provided with the pair of electronic switches 23 and 24 linked operatively to the output BI of the logic circuit 15. The switches 23, 24 connect to lines 37, 38 when the signal BI is low and connect to lines 38, 37 when the signal BI is high. 
     The operation of the inventive circuit will now be explained. The battery reversal command, as shown in the first line of FIG. 3, is supplied to the input COMBI of the line driver circuit 1 from exchange equipment. This command is then passed by the logic circuit 15 as its own output CSC to enable, by appropriately controlling operation of the switch 9, the capacitor C to become charged with a constant current from one of the current sources I 01 , I 02 . 
     As a result of the charging of the capacitor C, a ramp voltage Vc is generated which will, through the voltage input 7 of the voltage-current converter 13, be amplified and converted to two signals in phase opposition within the circuit portion 2. The two phase-opposed signals are transmitted to the line buffers 16, 17, respectively, through the outputs 4 and 5 of the circuit portion 2. 
     The comparator 12, being in turn connected to the outputs 4 and 5, will detect the zero value of the line voltage between the outputs 4 and 5. The comparator 12 will, in response, transmit, to the logic circuit 15, a signal which activates the command to switch for the input stages of the buffers 16 and 17. 
     The output BI from the logic circuit 15, in fact, operates the switches 18 and 19, and in parallel therewith, the switches 23 and 24 in the line current sensor 30. 
     Simultaneously therewith, the output CSC will also change over and operate the switch 9 as well, thereby reversing the direction of the current through the capacitor C and causing the capacitor C to be discharged. The voltage Vc in the capacitor will consequently ramp back down, as shown in the lower portion of FIG. 3. 
     The line conductors A, B are then supplied once again the original current, and through the resistors R will be driven to their original voltages proportional to Vc, but with the line polarities reversed, as shown in FIG. 3 with respect to the patterns of the signals VTIP and VRING. Thus, the magnitude of the signal VRING will continue to increase as the capacitor voltage Vc ramps back down, and the signal VTIP will continue to decrease. As is known, the voltages at the output of the buffers 16, 17 will be identical to the voltages at their inputs so that the curves for VTIP and VRING in FIG. 3 also represent the voltages at the terminals TIP and RING. 
     One skilled in the art will recognize that the above operations are repeated upon the battery reversal signal being presented to the input COMBI by the exchange equipment being switched back to the original condition. 
     In essence, the circuit of this invention utilizes the suppression of the line voltage to switch over the various stages, thereby avoiding discontinuities in the line voltage and current. By virtue of the polarity recovery at the sensor 30, the operations connected with the battery reversal can be confined to the first, second and third circuit portions 2, 20 and 30. But the integrated telephone interface circuit 25 comprises more circuit portions, not shown because conventional, which may be arranged to handle the voice signal, for example. 
     Consider, for example, that circuit portion which provides the telephone hybrid coil, or that handling the DC supply to the telephone line. These additional circuit portions are in no way affected by the operations described above in connection with the battery reversal. It follows that these additional circuit portions may be made even simpler by omitting the stages for phase reversal or rectifying circuits which would be required in the absence of the above described interface circuit 25. Where such portions are separate from the interface circuit 25, they would not even require the handling feature associated with battery reversal. 
     The circuit of this invention does solve the technical problem described above, and affords a number of advantages, such as improved communications quality can be ensured over lines utilizing the battery reversal function. Furthermore, the inventive circuit drastically cuts down interferences (bit rate error) with data communications. Another important advantage is that the invention is compatible to those communications systems which employ unbalanced powering of the telephone line. 
     Notice, moreover, that the capacitor C is an external component part of the circuit 1, and that its value may be decided upon by a user in designing the telephone exchange. Accordingly, the user will be able to also select the rate at which the circuit is to reverse the line polarity, the charge/discharge current for the capacitor C being, in fact, a predetermined constant value. 
     The circuit of this invention is not only effective to produce a soft reversal of the line polarity but also allows the rate of such a reversal to be programmed to suit individual design requirements. Understandably, many changes and modifications may be made unto the circuit of this invention within the scope of the appended claims.