CA1105628A - Subscriber line-trunk circuit - Google Patents

Abstract

Abstract of the Disclosure A microprocessor controlled subscriber line circuit for inter-facing analog telephone lines and trunks to a digital switching system is disclosed. All processing of incoming analog signals is accomplished by the line circuit, including the interface between the line and a digital switch-ing matrix, measuring, monitoring, testing of the signals associated with the speech path, generating talking battery, ringing and other tones, equalization and providing the two-wire to four-wire conversion with acceptable echo performance and minimized cross-office transmission loss. The analog-to-digital and digital-to-analog conversions are accomplished under micro-processor control as are the tone and power generation functions in a programmable power supply configuration utilizing pulse duration Modulated feedback control in a programmable signal generator. The described subscriber circuit is particularly advantageous in that it is designed to maximize its implementation using solid state circuit techniques and LSI devices as opposed to conventional audio components and relays.

Description

~ %8 R. Treiber - 1 Background of the Invention -1. Field of the Invention The present invention relates to subscriber line/trunk circuits, in general, and to an improved analog subscriber line/trunk circuit wherein a micro- ~ ;
processor controlled line circuit for each line or group of lines in a multiple subscriber system compensates for differing transmission characteristics from line to line without the signal losses or complex and costly measure-ment and monitoring equipment of the prior art. All such functions are accompli.shed by the present subscriber circuit, without signal degradation, by utilizing either one microprocessor per line or for a plurality of lines to perform the foregoing and associated digital filtering for the subscriber circuit of the present invention.

2, Description of the Prior Art In the prior art, the problem of differing impedance and transmission characteristics of multiple line types has been approached by balancing the line impedance between the subscriber set and the switching network hy insextion of components on a per line basis to achieve a "compromise" characteristic. From a loss viewpoint, this resulted in a degradation of the analog line si.gnal of two decibels in the transmission direction and two decibels in the receiving direction. Further, if this problem is avoided by individually balancing each line, testing must be accomplished at each line, which is both expensive and time consuming. In accordance with the present system, the line balancing for each line is accomplished by means of an automatic computation in a data processor, such as a microcomputer, by sending a R. Treiber - 1 tone on the line, measuring the tone upon its return and storing data concerning the transfer function in a memory for use by a system of digital filters to compensate for attenuation versus frequency and reduce the magnitude of unwanted return signals to an acceptable level. Hereto-fore, subscriber circuits have utilized two~wire switching, which does not require -two-wire to four~wire conversion circuits, thus no undesired signal return is generated.
In contrast, the padding by subtraction technique of the present invention achieves the foregoing programmably using digital filters to compensate for variation in trans-mission parameters. Also, in accordance with the present invention, bulky components, such as audio transformers and electro mechanical relays are eliminated. Digital filters per se are well known in the prior art. A
description of the general design of such filters may be found in: Digital Processing of Signals, B. Gold and C. Rader, Lincoln Laboratory Publication, McGraw-Hill, 1969.
Summary of the Invention A line circuit interface incorporating a programmable power supply for interfacing the analog subscriber lines and trunks of a telephone system to the digital switching circuitry of a telephone exchange central office or to other digital circuitry is described wherein all A/D and D/A conversion, two-wire to four-wire conver-sion, talking voltage, ringing voltage and other tone generation, measuring, testing and line monitoring are done by the subscriber circuit, thereby enabling a more efficient central office operation. Continuous feedback under micro-processor control for a programmable signal generator is 6~3 R. Treiber - 1 provided to derive a pulse duration modulated control signal in response to monitored load changes on the subscriber tip and ring lines. Also, in the same manner, ring trip and off-hook detection is accom-plished without the need for external sensing devices.
Current sensitive equalization equipment as used in North America is effectively moved from the telephone subset to the central office. Except during program-mable measuring and testing, the subscriber circuit is isolated and floating with respect to dc ground, which substantially eliminates the common power supply cross-talk problem of the prior art. Speech signal - 3a -isolation is achieved under microprocessor control by digitally filtering the unwanted signal returns from the speech signals generating the talking battery such that there is no common audio impedance.
It is therefore a primary object of the present invention to provide an improved subscriber line/trunk circuit for a digitally switched local/central office of a telephone system which is particularly suited for manufacture utilizing LSI and solid state devices.
It is another primary object of the present invention to provide a method for implementingJ under programmable control at the subscriber circuit, a plurality of measuring, testing, monitoring, equalization and tone generation functions.
According to a broad aspect of the present invention there is provided in a telephone subscriber line circuit, a programmable signal generator for generating an electrically isolated OlltpUt voltage comprising:
means for coupling generated programmably regulated analog voltages to a two-wire line having signals: representative of speech modulation thereon;
analog-to-digital converter means coupled to said line for sensing said ana-log voltages and for deriving an electrically isolated digital output signal;
signal processing means responslve to said digital output signal for comparing said digital output signal with a reference signal and for deriving a pulse duration modulated feedback control signal in response to said comparing; and regulator means coupled to said two-wire line for controll7ng said pulse duration modulated feedback control signal to regulate said coupled program-mably regulated analog voltages on said two-wire line, said regulator means further including means for generating said analog voltages.
According to another broad aspect of the present invention, there is provided a method of providing a programmable signal generator for a bi-directional telephone s-~scriber line circuit comprising the steps of:
coupling generated and programmably regulated analog voltages to a two-wire line; sensing said analog voltages with an analog to digital converter coupled ~s~

to said line for deriving an electrically isolated digital output signal;
comparing said digital output signal with a reference signal in a signal processor responsive to said digital output signal for deriving a pulse duration modulated feedback control signal in response to said comparing;
and regulating said programmably regulated analog voltages on said two-wire line with a regulator coupled to said two-wire line for controlling said pulse duration modulated -feedback control signal and for generating said analog voltages.
The present ~n~ention and that of appli.cation Serial No.
297,983 will now be described in greater detail with reference to the accom-panying drawings, in which:
~igure 1 is a line circuit of the prior art illustrative of a typical interface between a subscriber telephone set and a digital local exchange;
Figure 2 ~s a block diagram showing the implementation of the two-wire to four-wire conversion using digital filtering teclmiques internal to the microcomputer, Figure 3 is a block and circuit diagram illustrative of greater detail of the line circuit excluding the microcomputer system;
and Figure 4 is a schematic diagram of the programmable signal generator described with reference to Figure 3, showing the progral~mable signal generator and driver circuits.

- ~a -R. Treiber - 1 Description of the Preferred Embodiment Referring now to Figure 1, a t;ypical sub-scriber line circuit of the prior art is illustrated for providing an interface between the analog tele-phone lines and the digital portions of a local exchange, such as the switching matrix thereof. The incoming analog signals on a two-wire analog line 10 from a typical subscriber telephone set i9 converted to a four-wire PCM encoded signal for connection to a digital local switchlng matrix via speech lines 12 and signalling lines 14. Input signals on line 16 and output signals on line 18 are isolated from each other by a two-wire to four-wire converter circuit 20, a circuit which does not compensate for the imperfect impedance match at the two-wire line interface. A
compromise balance circuit 21, typically 9~0 ohms and two microfarads in series, is used to attempt to balance the wide variation in two-wire line impedances. Due to its less than perfect action, some of -the signal 16 is transmitte~ back on line 18. In a typical connection between two line circuits via the switching matrix, their signals can cause an instability or near singing condition resulting in poor perceived transmission by the subscriber. The prior art seeks to resolve this problem by inserting an extra 2 db of loss in the four-wire path; (16, 18). This problem is avoided by one aspect of the present invention by more nearly matching the equivalent balance impedance to that of the line on an individual basis automatically.

~5~ R. Treiber - 1 Switches 22, 24, 26, 28 and others provide for switching in dc voltage for tip and ring signals, test signals, dc power supply and other line moni-toring signals, such as test signals via lines 30 and 32, a talking battery or other dc voltage via line feed loop detector 46, and a ringing supply via line 36 to a ring trip detector 38 and return dc via line 34. These switches, detectors, connections and lines when multiplied by the number of telephone subscriber sets in a typical telephone system are expensive; and, in fact, the subscriber line circuits typically represent as much as eighty percent of the equipment cost o~ telephone exchanges; hence, simpli-fication thereof represents a substantial cost saving.
All of the aforementioned switches, test lines, detec-tors, and the two-wire to four-wire converter 20 are eliminated in accordance with the present invention. -A coder-decoder (codec) 40 may perform the analog-to-digital and digital-to-analoy conversion. One such technique first encodes the analog signal into digital format using linear A/D conversion techniques. The resultant digital signal is then companded. Analog iltering is provided by filters 42 and 44 which are expensive bulky audio components. A line feed loop detector 46 provi~es the D. C. talking battery to the line and supplies a sensing signal to signalling logic, relay drivers and power switches 48, which serves to couple the digital output of the codec 40 to the speech 12 and the various signal tones to the signalling lines 14.

6;2~3 R. Treiber - 1 Referring now to ~igure 2, the two to four-wire portion and the analog-to-digital conversion portion of the subseriber line circuit for interfacing two-wire analog lines o~ trunks to a digital system is illustrated generally at 100. Undesired signal return as a result of an imperfeet two-wire to four-wire conversion is substantially reduced in accordance with the present invention without the two-wire to four-wire converter of the prior art, but rather by a padding by subtraction technique implemented under control of a microcomputer by digitally filtering the intelligence signal to eliminate such undesired signal return.
Padding is the unequal ~ttenuation of different frequency components to insure that all frequeneies transmitted over the telephone line are of equal power.
A more eomplete deseription of prior art padding appears in the ITT Referenee Data for Radio Engineers, Si~th Edition, 1975 at 11-8 to 11-9. All measurement and testing of analog signals associated with the speeeh path Erom a subscriber set are monitored and measured by the described mieroproeessor implemented line eireuit. The two~wire line 102 couples input analog speeeh signals to an analog-to-digital converter 104 whieh performs a digital coding function.
The coder 104 may comprise a voltage controlled oseillator for eonverting the ineoming analog signal to non-amplitude senstitive dc outputs. The dc encoded output is proeessed by microproeessor 126, within which microproeessor digital filters 106 and 124 and summa-tion eircuit 110 are implemented. After filtering by dlgital 6~

filter 106, which processes the coder 104 output with a stored and/or programmable algorithm in accordance with known digital filtering techniques, the receiver signal transfer function H(z) on line 108 is derived.
T~is signal is summed at a junction llO with the negative value of the transfer function GHz of the undesired signal return on line 112 This is imple-mented by microcomputer 126 which includes a standard microprocessor such as an Intel 8030 augmented with a high speed arithmetic capa~ility for digital filtering, such that the microprocessor 126 and digital :Eilters 12 and 106 perform all signal processing. In addition to the foregoing, the digital filter provides for varia-tion of and compensation for su~scriber line trans-mission parameters under program control. Further, ac signalling may be detected b~ digital filtering techniques.
The digital input signal corrected by subtraction of the unw~anted signal return is coupled from summing junction 110 via line 11~ to the digital portions of the sys*em, ~hich may comprise a digi.tal switching matrix such as that disclosed in United States Patent No. ~,17.3,713, ~hich is.-sued on November 6, 1979 to K. Giesken and J. Cotton, and was assigned to the same assignee as in the ins.~tant application.
The large audio components associated with the two to four-wire conversion have been eliminated, particularly the audio transformer associated with this function.

~ ~ R. Treiber - 1 Since the digital filter (106) is capable of compensating for line loss, the current sensitive :
equalizer may be moved from the t~lephone subsets to th~ central office, since a selective frequency attenuation is added into the line circui.t under micro-processor control to insure that the total loss will be equal for all transmitted frequencies and distances :
from the central ofEice. A typical equalizer circuit of the prior art and description thereof ~ppears in Basic Carrier Telephony, David Talley, revised second .~ ~;
edition, Hayden Book Co., at page 121 thereof. Function heretofore normally requiring separate access, i.e. ~
ta.lking batteries, ringing voltages, test measurements, tone dial pulse coding, etc. have been eliminated; and these functions can be performed without separate access as shown in Figure 3. Thus, complete remoting ~:
of the line/trunk circuit from the switching matrix and modularity thereof on a per line basis is provided, therebv resulting in a standard interface to the switching system independent o the type of transmission circuit from which signals must be processed. The programmability of the microprocessor provides the flexibility for program modification for adapting to different characteristics and requirements of the line/trunk circuit, vis-a-vis the input linesl such as line 1~2. The digital return signal is coupled from microprocessor 126 on line 116 to a decoder 118 which performs the digital-to-analog conversion and which may comprise any suitable digital-to-analog converter such as the well-known weighted resistive network type g _ z~
R. Treiber - 1 D/A converter. After the analog filtering by filter 120, the analog speech signal is coupled through summing impedance 122 out onto the two-wire line 102.
Microprocessor 126 includes a memory capa-city for program storage accessible as needed which are illustrated as accessible over a high speed data bus 116. A permanent storage of data which may be accomplished by means of a read-only memory incor-pora-ted within the microprocessor provides for the storage of programs not readily transferred to the line circuit on an as-needed basis. However, the storage requirement is minimiæed by the use of the high speed data bus, since the memory associated with each line circuit must exist for all line circuits;
hence, any cost savings associated with reduced memory capacity results in a substantial cost saving when considered in the light of the system as a whole. Typically, however, the central storage of non-easily accessible programs would be associated with a sixty-four line block of subscriber line circuits; hence, one central memory would serve sixty-four two-wire lines, such as line 102, in an actual telephone system implementation.
Referring now to Figure 3, a hardware imple-mentation of the more generalized subscriber line circuit illustrated by the block diagram of Figure 2 is illustrated generally at 200. The key element of Figure 3 is a programmable signal generator 202 for generating the subscriber line circuit ac and dc voltages as required under microcomputer control.

~ R. Treiber - 1 Essentially, the incoming analog signals, including the tip and ring signals on lines 204 and 206 of the two-wire line 102, are coupled through a suitable overvoltage protection circuit 208 to the afore-described coder 104 which senses the instantaneous voltage on the tip and ring lines 204 and 206, respectively; digitizes the sensed voltage and routes the digitized voltage through the microprocessor, wherein a pulse duration modulated drive signal is generated and fed back via driver 210 to the progra~nable signal generator 202 to generate the ~ i re~uired talking battery and ringing voltages. In addition, control is provided to a plurality of switches as will be described hereinaEter and with a resultant minimization of system hardware require-ments. In effect, the coder is an efficient means for controlling a programmable voltage source having dc continuity and ac and dc isolation characteristics even though the coder is transformer coupled. This is accomplished by modulating the signal including the dc component and transformer coupling the output.
Demodulation is accomplished by recovering the digital signal in the microprocessor wherein a digital control signal for driving the programmable signal generator is derived.
A11 testing is done by appropriately controlling the programmable signal generator and switches.
The programmable signal generator 202 is isolated ~rom the remainder of the illustrated circuitry ~ 6Z~ R- Treiber - 1 by means of a ferrite transformer. The tip and ring lines are coupled via lines 212 and 214, respectively, to th~ digital to-analog decoder 216 and to analog~
to-digital coder 218 via lines 220 and 222, without the heretofore re~uired two-wire to four-wire converter.
A highly effic~ent feedback control with pulse width duration modulation is provided by the digital voltage control output from the microprocessor on line 224, which signal is applied as the base drive to transistor amplifier 226, the pulsed output of which is transformer coupled across a small ferrite transformer 228 to the programmable signal generator 202. A polarity control signal i5 provided via lines 230 from the microprocessor and is transormer coupled to signal generator 202. A
battery isolating impedance 231 is provided and a line ;.
matching impedance 232. :~
The aforedescribed microprocessor generated pulse duration modulated control signal is preferably in the range of 50 to 100 kHz. The aforedescribed programmable signal generator control signal on line 224 is derived by first measuring the output of signal generator 202 in the coder circuit 218 wherein the incoming analog signal on lines 220 and 222 is converted to a non-analog sensitive digital output :~
and -transformer coupled via transformer 234 to the microprocessor. The instantaneous value of this trans-former coupled digital signal is then compared with a .
reference value stored internally in the microprocessor such that any deviation therefrom serves to generate a corrective increase or decrease, as the case may be, ~ 8 R. Treiber - 1 within the microprocessor to vary the pulse duration of the signal output from the microprocessor on line 224. Thus, the microprocessor functions as feedback regulator circuit to vary the pulse duration of the base drive for driver 226 in order to generate the desired signal according to the internal reference stored in the microcomputer. An exemplary means for generating such a pulse duration signal is by counting down a preset value stored in a counter associated with the microprocessor. When such stored value reaches zero, the pulse, to be transmitted on line 22~, is terminated. Of course, the preset counter is controlled by the digital feedback increase/decrease in~ormation derived by the microprocessor from the measured dc output. Other techniques for controlling the preset values stored in such a counter, such as a look-up table stored in the microprocessor memory may also be utilized. The high fre~uency operation of the aforedescribed programmable power supply enables the generation of relatively smooth waveforms and the consequent use of small ferrite-type transformers and capacitors, thereby avoiding the heavy and bulky battery feed coils and audio transformers of the prior art. An isolated dc-to dc converter 236 of conventional design may serve as the power supply for decoder 216 and coder 218. Bipolar signals are obtained by means of a floating isolated bridge which reverses the polarity of the secondary output of the switching regulator.

i6'~
R. Treiber - 1 The incoming speech signals are thus switched from the two-wlre line 102 through coder 218 to the microprocessor and to the receiver output for coupling to a suitable digital switching matrix, while the microprocessor control decoder 216 output is isolated from the transmissions through coder 21~ bu~ is still switched through the circuit to lines 204 and 206 to enable the normal two-way telephone configuration.
Switches S1 through S7 are used for testing operations. They provide for grounding either/or both sides of the line through S1 and S2, measuring current output (the voltage across 231) through S7, S4 and S3, and disconnectin~ the low terminating impedance 232 through S5 and S6, when line leakage type measurements must be made~ Test voltages are generated by the programmable signal generator.
While switches Sl through S7 are illustrated without control inputs thereto for simplicity of description, ~;
it is to be understood that such control inputs are coupled to switches Sl through S7 from microprocessor 126.
Switches Sl through S7 may be implemented in like manner as switches 320, 322, 324 and 326 described hereinafter with reference to Figure 4, with any desired switching se~uence being obtainable from the microprocessor~
Referring now to Figure 4, the programmable signal generator 202 is illustrated. The circuit provides a floating isolated bridge for switching the circuit on the secondary output 302 of ferrite trans-former 300. For a given value of Vcc applied to the primary 304 of transformer 300 and to drive transistor 306 R. Treiber - 1 of driver 226, and for a yeneration of a fixed dc voltage at the output of signal generatc)r 202, the pulse duration coupled to the base of transistor 306 is constant. Should the load change on the output (tip and ring lines 204 and 206, respectively), the sensing of such change causes the pulse duration of the base drive to vary correctively, as aforedescribed.
This enables the detection of changes in hook status of the subscriber telephone. Further, for ring-trip detection, the average value of the pulse duration (the e~uivalent dc value) may be utilized to measure changes in dc line current. Switches 308 and 310 may be utillzed to ground either side of the line (tip or ring) for test purposes, while inductance 312 serves to isolate the relatively low impedance power supply and signal generator from the line. Monitoring of gain stability is accomplished internally in the microprocessor by sensing the line voltages.
Operationally, the primary 304 stores energy in accordance with the well~known relationship E = 1/2 Li .
When transistor 306 is ON (the indicated positive polarity dots become negative polarity), diode 314 does not conduct. When transistor 306 is OFF, diode 314 conducts, charging shunt capacitor 316 and transferring the energy stored in primary 304 to the secondary 302, i.e., to capacitor 316. Capacitor 316 also serves as a ripple voltage filter. Energy transfer from primary 304 to secondary 302 is controlled by the switching of transistor 306 while the amount of energy trans~erred, i.e., the effective output voltage, is controlled by R. Treiber - 1 the duty cycle of the switching of transistor 306, which in turn is controlled by the pulse duration modulated signal applied to the base thereof from the microprocessor. A regulated energy feedback power supply is thus provided, but at a high enough fre~uency (100 KHz) to avoid the costly and bulky audio transformers and relays of the prior art.
Switches 320, 322, 32~ and 326, which may be implemented by VMOS, DMOS, bipolar or other semi-conductor switches of known configuration, are driven by isolated, transformer coupled microprocessor controlled switching pulses under the control of the microprocessor programming. To generate an ac signal, a hal-wave rectiied signal and appropriate switch is thus employed. When switches 320 and 322 are ON, switches 324 and 326 are OFF and vice versa. Illus-tratively, when switches 320 and 322 are ON, the negative polarlty from capacitor 316 is coupled to the tip line 204 and the positive polarity from capacitor 316 to the ring line 206. Conversely, when switches 324 and 326 are ON, the tip has coupled thereto the positive polarity from capacitor 316 and the ring has coupled thereto the negative polarity ~rom capacitor 316. An ac signal from the programmable signal generator under microprocessor control is thus generated from a dc signal. The effect is signiicant, since the heretofore required ac power supply and switches for switching the ac power into the circuit of the prior art are eliminated. The present circuit generates all o the subscriber line ac and dc voltages requi:red for ~5~Z~3 R. Treiber - 1 tones, operation and testing.
While the present invention has been described in connection with a preferred embodiment thereof, it is to be understood that additional embodiments, modi-fications and applications which will become obvious to those skilled in the art are included within the spirit and scope of the invention as set forth by the claims appended hereto.

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a telephone subscriber line circuit, a programmable signal generator for generating an electrically isolated output voltage comprising:
means for coupling generated programmable regulated analog vol-tages to a two-wire line having signals representative of speech modulation thereon;
analog-to-digital converter means coupled to said line for sensing said analog voltages and for deriving an electrically isolated digital output signal;
signal processing means responsive to said digital output signal for comparing said digital output signal with a reference signal and for deriving a pulse duration modulated feedback control signal in response to said comparing; and regulator means coupled to said two-wire line for controlling said pulse duration modulated feedback control signal to regulate said coup-led programmably regulated analog voltages on said two wire line, said regulator means further including means for generating said analog voltages.

2. In a telephone subscriber circuit in accordance with claim 1, a programmable signal generator for generating an electrically isolated output voltage wherein said analog voltages on said two-wire line include ac signal-ling voltages.

3. In a telephone subscriber circuit in accordance with claim 1, a programmable signal generator for generating an electrically isolated output voltage wherein said signal processing means comprises a microcomputer.

4. In a telephone subscriber circuit in accordance with claim 3, programmable signal generator for generating an electrically isolated output voltage wherein said regulator means includes:

switching circuit means having a duty cycle controlled by said feedback control signal and having a switched output signal; and transformer means for transferring energy from the primary there-of to the secondary thereof in response to said switched output signal from said switching circuit such that the amount of energy transferred from said primary to said secondary of said transformer is proportional to the duty cycle of said switching circuit.

5. In a telephone subscriber circuit in accordance with claim 4, further comprising:
means for testing said two-wire line with test voltages generated by said programmable signal generator.

6. In a telephone subscriber circuit in accordance with claim 4, wherein said regulator means further includes:
signal isolation means for isolating the secondary of said trans-former from said two-wire line, and wherein said switching circuit is a switch-ing transistor.

7. In a telephone subscriber circuit in accordance with claim 4, wherein said transformer is a ferrite transformer and wherein said isolation means includes a floating bridge circuit.

8. In a telephone subscriber circuit in accordance with claim 7, wherein said floating bridge circuit is comprised of a plurality of switches controlled by said microcomputer for generating an output ac voltage from said regulated analog voltage.

9. A method of providing a programmable signal generator for a bi-directional telephone subscriber line circuit comprising the steps of:
coupling generated and programmably regulated analog voltages to a two-wire line;
sensing said analog voltages with an analog to digital converter coupled to said line for deriving an electrically isolated digital output signal;
comparing said digital output signal with a reference signal in a signal processor responsive to said digital output signal for deriving a pulse duration modulated feedback control signal in response to said comparing;
and regulating said programmably regulated analog voltages on said two-wire line with a regulator coupled to said two-wire line for controlling said pulse duration modulated feedback control signal and for generating said analog voltages.

10. A method in accordance with claim 9, wherein said regulating step includes generating an electrically isolated, regulated output voltage in accordance with the following substeps:
switching a solid state circuit having a duty cycle controlled by said feedback control signal and having a switched output signal; and transferring energy from the primary of a transformer to the secondary thereof in response to said switched output signal from said switch-ing solid state circuit such that the amount of energy transferred from said primary to said secondary of said transformer is proportional to the duty cycle of said switching circuit.

11. A method in accordance with claim 10 further including the step of;
isolating the secondary of said transformer from said two-wire line.

12. A method in accordance with claim 9 including the further step of testing said two-wire line with test voltages generated by said program-mable signal generator.