FIELD OF THE INVENTION  
         [0001]    The present invention relates to a maintenance unit, and more particularly, to a remotely addressable maintenance unit (RAMU) for detecting and locating faults in digital subscriber loop (DSL) and/or plain old telephone system (POTS) environments. The RAMU of the present invention includes circuitry for setting and resetting one or more relays for either normal or testing/maintenance mode.  
         BACKGROUND OF THE INVENTION  
         [0002]    Recently, there have been dramatic changes in the telecommunications industry. For example, the Telecom Act of 1996 deregulated the local markets, which, in part, brought on the emergence of new technologies to this industry. These changes are also sparked by the growing demand for Internet access and for new technologies that deliver higher speed connections over existing infrastructure.  
           [0003]    As is well known in the industry, Digital Subscriber Loop, or DSL, is one of the most promising new technologies for delivering superior service and higher speed connections over existing infrastructure. In general, DSL uses the existing copper loop that is used for conventional telephony, but delivers much higher bandwidth. However, to achieve such high data rates, DSL operates at a higher frequency and is thus more sensitive to the length and quality of the copper loop. As a result, more sophisticated levels of testing, monitoring, and maintenance are required for successful DSL deployment.  
           [0004]    Currently, the transmission rates for DSL technologies are very much dependant on the distance between a telephone company and a customer. Moreover, depending on the type of DSL technology, the transmission rate downstream to the customer and upstream to a telephone company may vary. For example, for asymmetric DSL, the transmission rate is faster downstream to the customer than upstream to the telephone company. Asymmetric DSL is well suited for Internet usage and video on demand. For symmetric DSL, the transmission rate is about the same for both downstream and upstream.  
           [0005]    DSL uses packet switching technology that operates independently of voice telephone system, allowing telephone companies to provide the service and not lock up circuits for long distance calls. In addition, DSL can carry both voice and data signals simultaneously, in both directions, allowing the customer to log onto the Internet and make a telephone call at the same time.  
           [0006]    One major issue for those in this industry is the testing and maintenance of such systems. Currently, there exists a two-terminal testing device that is applied in the POTS (plane old telephone system) environment, which, in general, has been unsuccessful in the DSL environment.  
           [0007]    [0007]FIG. 1 illustrates a simplified diagram of a POTS environment having a conventional maintenance test unit (MTU). In the conventional POTS environment, a central office (CO)  2  is connected to a customer&#39;s telephone  8  using a pair of copper wires  4 . The CO  2  includes a testing instrument such as a test head  3  for performing the testing and maintenance functions. In between the CO  2  and the telephone  8 , there lies a network interface device (NID) such as the MTU  6 . There may be multiple telephones  8  and a single MTU  6  connected to one pair of wires  4  in the conventional POTS environment. In additional, other conventional devices (i.e., switches), which are not illustrated herein, may also be implemented in this environment.  
           [0008]    The MTU  6  illustrated herein is generally intended for a single party, loop-start line, voice frequency band POTS environment implementation. This is intended to be compatible with conventional test systems such as Local Test Disk (LTD), Mechanized Loop Testing (MLT), CK08555 (KS-8455) voltmeter, Automatic Line Insulation Test (ALIT), and the like.  
           [0009]    [0009]FIG. 2 illustrates a diagram of an existing circuit used in the POTS environment as shown in FIG. 1. In the CO  2 , the test head  3  (i.e., LTD, MLT) typically includes a power source  10  such as DC voltage V dc , current limiting resistor  12 , and two terminals  14 ,  16 , which are further connected to relay(s)  18 . The relay(s)  18  allows the two terminals  14 ,  16  to connect to a tip wire  20  (tip) and/or a ring wire  22  (ring). As is well known, tip and ring are terms used for describing the two wires that are needed to set up a telephony connection.  
           [0010]    The MTU  6  includes a pair of voltage sensitive switches (V ss ), where one V ss    24  is coupled to the tip  20  and the other V ss    26  is coupled to the ring  22 . In addition, a termination impedance Z t    28  is placed in between the two V ss    24 ,  26 , at a location near the customer&#39;s telephone  8 . The termination impedance Z t    28  is a signature impedance that works in conjunction with the CO  2  test systems for fault identification and localization.  
           [0011]    Testing in the conventional POTS environment is generally performed using only two terminals, tip  20  and ring  22 . The conventional testing method is generally acceptable in the POTS environment, but as will be described hereinafter, in the DSL environment, a more improved system and method is needed.  
           [0012]    The conventional testing system and method used in the POTS environment have many shortcomings and disadvantages. For example, one major disadvantage with the conventional system and method is that many fault conditions cannot be detected or located with exact precision, thereby requiring truck rolls. As a result, the conventional testing system and method require a great deal of time and resources to locate and determine the type of faults, which generally results in lost revenues and a more than desirable fix time for the operating company and the customer. An additional disadvantage using the conventional system and method is that phones are required to be connected to the tip and ring for testing for some type of fault identification and localization, which in many cases, can be quite burdensome.  
           [0013]    [0013]FIG. 3 illustrates a chart showing how the conventional MTU responds to different voltage levels as measured from tip to ring. For example, when the voltage difference from tip to ring is between +35 to +65 volts, this is indicative of normal or talk mode (ON mode), where the MTU is in a low impedance state. Conversely, the same behavior can be achieved when the voltage difference is reversed, as between −35 to −65 volts from tip to ring.  
           [0014]    When the voltage difference is dropped between −28 to +28 volts, the MTU  6  is turned off and it is in a high impedance state. Further, when the voltage difference from tip to ring is between +70 to +120 volts or between −70 to −120 volts, the voltage sensitive switches V ss    24 ,  26  are turned on, placing them in low impedance states. Furthermore, these voltages activate impedance signature Z t    28 , providing the so-called distinctive terminations to be detected by the test systems at the CO  2 . The impedance readings should be between 150K to 450K ohms when measured with +70 to +120 volts, and be equal or greater than 100 M ohms when the voltage is between −70 to −120 volts.  
           [0015]    As DSL technology continues to evolve, the conventional system and method using the MTU  6  is generally inadequate for testing/maintenance in the DSL environment. Most DSL circuits do not have batteries connected thereto (“dry circuit”), and thus the MTU  6  will typically not function properly under this environment.  
           [0016]    Another disadvantage of the conventional system and method is that the MTU  6  is typically implemented only with ILECs (Incumbent Local Exchange Carriers), which utilize their customized test systems to control and inter-work with the MTU  6 . The CLECs (Competitive Local Exchange Carrier), which are in direct competition with the ILECs in the marketplace, currently do not have such testing/maintenance systems to work with the MTUs. Installing such test systems is a very costly proposition for the CLECs.  
           [0017]    Thus, there is a need for a system and method for providing a remotely addressable maintenance unit for the DSL and POTS/DSL environments for both CLECs and ILECs. There is also a need for implementing a remotely addressable maintenance unit in existing infrastructures in current POTS environment for improved performance.  
         SUMMARY OF THE INVENTION  
         [0018]    It is an object of the present invention to provide a remotely addressable maintenance unit for the POTS, DSL, and POTS/DSL environments.  
           [0019]    It is another object of the present invention to provide a system and method for verifying connections between the central office and the remotely addressable maintenance units.  
           [0020]    It is yet another object of the present invention to provide a system and method for locating and determining faults between the central office and the customer&#39;s house/building.  
           [0021]    It is a further object of the present invention to provide a system and method for identifying the tip and ring wires using a “Signature.” 
           [0022]    It is yet another object of the present invention to provide galvanic isolation of in-house wiring from an outside plant.  
           [0023]    It is another object of the present invention to provide a system and method for detecting loop faults, including open and short circuits.  
           [0024]    It is another object of the present invention to provide sealing currents to the copper wires used between the central office and the customer&#39;s house/building using the remotely addressable maintenance unit.  
           [0025]    It is still a further object of the present invention to provide a system and method for remotely terminating service with a predetermined impedance to a particular customer using the remotely addressable maintenance unit for a loop only noise measurement.  
           [0026]    It is yet another object of the present invention to provide a system and method for determining loop resistance in existing copper loops.  
           [0027]    These and other objects of the present invention are obtained by providing a remotely addressable maintenance unit (RAMU) that can be used in the POTS, DSL, and POTS/DSL environments. The RAMU of the present invention includes circuitry, working in conjunction with test systems located in the CO, for detecting and locating (sectionalizing between in-house and out-house) faults in the POTS, DSL, and POTS/DSL environments. The RAMU further includes circuitry for setting and resetting one or more relays for normal or test/maintenance mode. A test head is placed at the CO, which works in conjunction with a “Signature” in the RAMU for performing testing and maintenance tasks on the copper loop. The RAMU of the present invention can also be used to provide a sealing current to the wires in the DSL environment. Furthermore, the RAMU eliminates/reduces truck rolls through a more thorough and accurate diagnosis of fault types and localizations, thereby saving valuable time and resources for both the ILECs and the CLECs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    These and other objects and advantages of the present invention will become apparent and more readily appreciated from the following detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawings, of which:  
         [0029]    [0029]FIG. 1 illustrates a simplified diagram of a POTS environment having a conventional maintenance test unit;  
         [0030]    [0030]FIG. 2 illustrates a diagram of an existing circuit used in the POTS environment as shown in FIG. 1;  
         [0031]    [0031]FIG. 3 illustrates a chart showing how the conventional MTU responds to different voltage levels as measured from tip to ring;  
         [0032]    [0032]FIG. 4 illustrates block diagrams of various environments implementing the RAMU in accordance with the present invention;  
         [0033]    [0033]FIG. 5A illustrates a functional block diagram of the RAMU in accordance with the present invention;  
         [0034]    [0034]FIG. 5B illustrates a flow diagram showing the steps performed by the RAMU in accordance with the present invention;  
         [0035]    [0035]FIG. 6 illustrates an example of the RAMU signaling definition of the present invention;  
         [0036]    [0036]FIG. 7 illustrates a detailed circuit diagram in accordance with the present invention; and  
         [0037]    [0037]FIG. 8 illustrates a specific example of the RAMU signaling definition used in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    The present invention will now be described in greater detail, which will serve to further the understanding of the preferred embodiments of the present invention. As described elsewhere herein, various refinements and substitutions of the various embodiments are possible based on the principles and teachings herein.  
         [0039]    The preferred embodiments of the present invention will be described with reference to FIGS.  4 - 8 , wherein like components and steps are designated by like reference numerals throughout the various figures. Further, specific parameters such as potential differences, voltage values, circuit layouts, and the like are provided herein, and are intended to be explanatory rather than limiting.  
         [0040]    [0040]FIG. 4 illustrates block diagrams of various environments implementing the RAMU in accordance with the present invention. In the first environment, DSL, a test head  100   a  with a conventional processor board such as a P150 processor (e.g., CX100 Copper CrossConnect system of Turnstone Systems, Inc.) in the central office (CO) is connected to a loop (copper pair)  102   a  in the outside plant. The loop  102   a  is further connected to a RAMU  104   a,  which in turn is connected to the in-house wiring  106   a.  An ATU-R (ADSL transceiver remote unit)  108   a  or a similar end unit is further connected to the in-house wiring  106   a,  generally in the customer&#39;s house/building. Unlike the conventional system and method, the RAMU  104   a  is implemented instead of the MTU, where the RAMU  104   a  is used for testing and maintenance.  
         [0041]    In a similar manner, the RAMU can be implemented in a second POTS environment, again using a test head  100   b  having a processor such as a P150 processor, which is connected to a loop  102   b.  The RAMU  104   b  is connected to both the loop  102   b  and the in-house wiring  106   b,  which terminates with a conventional analog phone  109   a.  The system in the second POTS environment is similar to the first DSL environment except that the phone  109   a,  instead of the ATU-R  108   a,  is the end unit. These examples illustrate that the RAMU of the present invention can be easily implemented in both DSL and POTS environments.  
         [0042]    In yet another embodiment of the present invention, the RAMU can be implemented in an environment using both DSL and POTS concurrently. For example, in a third DSL/POTS environment, a test head  100   c  is again connected to a loop  102   c,  which in turn is connected to a first RAMU  104   c.  The first RAMU  104   c  is further connected to a POTS splitter  110 , which is used to split the wires for connection to a second RAMU  104   d  and a third RAMU  104   e.  As shown, the POTS splitter  100  can be implemented in a star topology, or in the alternative, other topologies such as a ring. The second and third RAMUs  104   d,    104   e  are further connected to the in-house wiring  106   c,  which terminate at the ATU-R  108   b  and phone  109   b,  respectively. The RAMUs  104   a,    104   b,    104   c,    104   d,    104   e,  are preferably placed at the network interface on the outside of the customer&#39;s house/building in which the outside plant copper wires for the access network terminate. In the alternative, the RAMUs  104   a,    104   b,    104   c,    104   d,    104   e,  can be placed inside the house/building structure.  
         [0043]    [0043]FIG. 5A illustrates a functional block diagram of the RAMU and FIG. 5B illustrates a flow chart of the steps performed by the RAMU in accordance with the present invention. Referring first to the RAMU  520  in FIG. 5A, two switches  502   a,    502   b  are used to indicate “reset” and “set” modes, which depends on the magnitude, polarity, and duration of an electric signal. For example, when a contact point a is connected to a contact point b via switch  502   a  and a contact point a′ is connected to a contact point b′ via switch  502   b  (as explicitly shown in FIG. 5A), the RAMU is in the “reset” or normal mode. In the alternative, when the contact point a is connected to the contact point c via switch  502   a,  and the contact point a′ is connected to the contact point c′ via switch  502   b,  the RAMU is in “set” or testing/maintenance mode. Thus, when the RAMU is subjected to an electrical impulse representing either the “reset” or “set” control signal of proper magnitude, polarity, and duration, the RAMU will switch to either the “reset” or “set” mode and will remain in that mode even upon removal or cessation of the control signal. The magnitude, polarity, and duration of the “reset” and “set” control signals for the RAMU are dependent on the specific application environment, and is generally defined/design into the RAMU and the CO  510  before deployment of the RAMU outside the CO  510 . A more detailed description regarding the specific circuit diagram and the conditions for the “reset” and “set” modes are disclosed later herein.  
         [0044]    In the CO  510 , there exists a power source V b    512  and two terminals  514 ,  516 , which can be connected to the tip (T), ring (R) and/or ground wires. One important feature of the present invention is that the two terminals  514 ,  516  can now be connected to the ground  518 . In this manner, the functional capabilities of the system is expanded as follows: (1) increases the addressing capabilities of the system, allowing multiple devices to be connected to the same copper pair; (2) de-couples the address signals (i.e., DC voltage levels that are used to control the RAMU) from voltages that are used for applications, such as for measurements and sealing current, and hence significantly minimizes interference between voltages representing control/addressing and applications; and (3) address or control bi-stable devices that can be used in the physical implementation of the RAMU.  
         [0045]    Thus, in the preferred embodiment, using appropriate voltage magnitude and duration, “set” mode is indicative when there is a positive polarity from tip to ground, ring to ground, or tip and ring to ground. Conversely, again using appropriate voltage magnitude and duration, “reset” mode is indicative when there is a negative polarity from tip to ground, ring to ground, or tip and ring to ground. In addition, signals that have an amplitude equal to at least twice or greater than the amplitude of the “set” or “reset” control signals, applied between the tip and ring, will not have any effect on the “present” state of the RAMU.  
         [0046]    [0046]FIG. 6 illustrates an example of a RAMU signaling definition of the present invention. In greater detail, FIG. 6 shows eight different conditions/quadrants for addressing the RAMU: (1) V tg  (voltage from tip to ground) with positive polarity; (2) V tg  (voltage from tip to ground) with negative polarity; (3) V rg  (voltage from ring to ground) with positive polarity; (4) V rg  (voltage from ring to ground) with negative polarity; (5) V trg  (voltage from tip and ring to ground) with positive polarity; (6) V trg  (voltage from tip and ring to ground) with negative polarity; (7) V tr  (voltage from tip to ring) with positive polarity; and (8) V rg  (voltage from tip to ring) with negative polarity. Certain conditions/quadrants (e.g., 1, 2, 3, 4, 5, 6) can be used for addressing and other conditions/quadrants (e.g., 7, 8) can be used for applications. Voltage range between −50V to +50 for all combination of tip, ring, and ground configurations (V tg , V rg , V trg , V tr ) have been reserved for measurements and other industry applications such as parameter measurements and telephonic functions.  
         [0047]    The “reset” mode can be actuated with a negative polarity using a V tg , V rg , or V trg  condition, using arbitrary voltages range (i.e., preferably, at a range less than −50V but greater than −120V), which ranges are pre-defined/pre-designed. Conversely, the “set” mode can be actuated with a positive polarity using a V tg , V rg , or V trg  condition, using arbitrary voltage ranges (i.e., preferably, at a range less than +120V but greater than +50V), which ranges are pre-defined/pre-designed. In additional, a particular voltage range that actuates the “set” mode should include a reciprocal voltage range to actuate the “reset” mode (i.e., 55.8V to 68.2 V for “set” mode and −68.2V to −55.8V for “reset” mode).  
         [0048]    Each individual RAMU can be defined/designed to only respond to a certain voltage range. For example, one can “set” one particular RAMU to respond in the +55.8 to +68.2 voltage range, and “reset” in the −55.8 to −68.2 voltage range. In the same manner, another RAMU can be “set” and “reset” to respond in the +73.8 to +90.2 voltage range, and −73.8 to −90.2 voltage range, respectively. In this manner, multiple RAMUs can be remotely addressable from the CO  510  by providing different voltages and polarities to the tip and/or ring to ground. Further, installing multiple RAMUs at different locations between the CO  510  and the customer&#39;s house/building provides a way to determine the exact location and type of fault in any environment.  
         [0049]    Additionally, a sealing current can be provided to the wires using the V tr  condition and in conjunction with a properly designed signature impedance. Thus, voltage between 0 to +65 volts can be applied from tip to ring using the RAMU to provide a sealing current to the copper wires. Other applications such as loop powered tone generation can be performed by providing other voltages to the ring and tip in the V tr  configuration. It is also noted that in FIG. 6, the voltage ranges are for illustrative purposes only and other practical voltages ranges can be substituted for those illustrated therein.  
         [0050]    Referring back to FIG. 5A and as mentioned earlier herein, the RAMU can be placed at the network interface outside of the customer&#39;s house/building in which the outside plant copper wires for the access network terminate, which wires are connected to contact points a and a′. In the DSL or POTS environment, there is typically a DTU (Digital Termination Unit)/phone set  540  connected to the contact points b and b′ through a pair of copper wires. When the DTU/phone set  540  is connected to the contact points a and a′, via the contact points b and b′, this represents the “reset” or normal mode. Conversely, when the RAMU is in the “set” or testing/maintenance mode, the DTU/phone set  540  is disconnected from the contact points a and a′, and such contact points are connected to a Signature  532  via the contact points c and c′.  
         [0051]    The Signature  532  is preferably a passive network or active circuit elements that perform a specific function, as described in more detail hereinafter. The Signature  532  contains specific circuitry designed to identify the presence of the RAMU. The Signature  532  can also perform the function of a resistance, which is used to detect fault conditions in the tip and ring, loop length measurements, and a DC path for a sealing current to “wet” the loop.  
         [0052]    The RAMU  520  further includes the following functional blocks: Signal Detector  522 , Level Comparator  524 , “Adjustable” Fixed Voltage Driver  526 , Reference Voltage  528 , and Relay  530 . As shown, the RAMU  520  can be connected to the CO  510  at the T and R wires, and to the customer&#39;s house/building via the T′ and R′ wires.  
         [0053]    Reference will now be made to both FIGS. 5A and 5B concurrently for a more complete understanding of the present system and method. First, in step  602 , a voltage is applied to T and/or R. In step  604 , the Signal Detector  522  coupled to the T and R detects the polarity of the signal and voltage level at the T and/or R with respect to ground. The polarity of the detected signal is then fed into the Reference Voltage  528  to select a polarity for the reference voltage in step  606 . The Reference Voltage  528  contains pre-defined voltage amplitude whose polarity selections are controlled by a signal from the Signal detector  522 . The Reference Voltage  528  can also be adjustable depending on the type and values of the hardware components used in a particular circuitry.  
         [0054]    The voltage with the proper polarity from the Reference Voltage  528  is then inputted into the Level Comparator  524 , which is also coupled to the Signal Detector  522  and the Reference Voltage  528 . The Level Comparator  524  compares the T and/or R voltages with the reference voltage from the Reference Voltage  528  in step  608 . In step  610 , when the value of the voltage from the T and/or R is greater than the reference voltage, an enable signal is generated by the Level Comparator  524  and inputted into the Fixed Voltage Driver  526 , for enabling the same.  
         [0055]    The Fixed Voltage Driver  526  consists of a switching function and a fixed voltage circuitry. The switching function controls the application of a fixed voltage to drive the Relay  530 . The combination of the fixed voltage with a coil resistance of the Relay  530 , which is generally fixed for a given relay type, forms a “constant current source.” The current flowing through the Relay  530  is not affected by further increases in the voltage at the T and/or R. In other words, once the circuitry detects a voltage at the T and/or R, which exceeds a pre-defined value, the Fixed Voltage Driver  526  applies a constant voltage and actuates the Relay  530 . The resultant current flowing through the Relay  530  is also unaffected by further increase in voltages at the T and R. This is an essential feature of the present invention to provide optimum operations. In addition, the fixed voltage driver value can be further adjusted to accommodate a different voltage rating of a particular relay.  
         [0056]    The Relay  530  is preferably a latching-type relay (includes memory), which could be either electromechanical or semiconductor solid state. The latching-type feature is essential in that once the applied voltage exceeds a pre-defined reference voltage, a constant voltage is applied to actuate the Relay  530 . Once the Relay  530  is actuated, the “set” or “reset” control voltage can be subsequently removed, resulting in minimum or no power dissipation. The latching-type feature is also essential because it can then allow application of a voltage across the T and R to perform other functions without affecting the Relay  530 .  
         [0057]    [0057]FIG. 7 illustrates an example of a detailed circuit diagram that can be used in the present invention. Functionally, the Signal Detector  522  includes the diodes D1  730 , D2  732 , D3  734 , D4  736 , and transistors Q1  740 , Q2  742 , Q3  744 , Q4  746 . The Level Comparator  524  uses the voltages associated with zener diodes Z1  712 , Z3  718  or Z2  714 , Z4  716 , in conjunction the voltages associated with zener diodes Z5  722 , Z6  724 . Next, the Reference Voltage  528  is set by choosing a combination of voltages associated with zener diodes Z1  712 , Z3  718  or Z2  714 , Z4  716 , with voltages associated with zener diodes Z5  722  or Z6  724 . The zener diodes Z1  712 , Z2  714 , Z3  718 , Z4  716 , Z5  722 , Z6  724  perform dual functions for the Level Comparator  524  and the Reference Voltage  528 .  
         [0058]    The Fixed Voltage Driver  526  includes zener diodes Z5  722 , Z6  724 . Adjustments/modifications to the Fixed Voltage Driver  526  can be made by selecting the voltages associated with zener diodes Z5  722 , Z6  724 . For symmetrical operation, zener diodes Z5  722 , Z6  724  can have identical values such as approximately 20V. Once the zener diode voltages are known, the voltage required to drive the Relay  530  is also known (i.e., 20V).  
         [0059]    The Relay  530  is defined by K1  760 , which can have the following characteristics: 24V rated, single coil, two form-c contacts, and latching-type. In other embodiments, a different rated voltage relay can be used in the circuitry of FIG. 7. Note that since the Relay K1  760  is rated at 24V, the actual voltage to drive/actuate it is about 19.8V as defined by Z5  722  (20.0V), in conjunction with forward drop of Z6  724  (0.7V), plus the V be  (base to emitter voltage drop) of Q4  746  (0.7V), minus the V be  of Q2  742  (0.7V), minus the V ce  (collector to emitter) saturation voltage of Q4  746  (0.2V), minus the forward drop (V be ) of D4  736  (0.7V).  
         [0060]    The Signature  532  includes resistors R5  750 , R6  752 , capacitor C2  770 , and zener diodes Z7  780 , Z8  782 . Voltages for the zener diodes Z7  780  and Z8  782  are selected to be different from each other to enable identification of either the tip or ring. Resistors R5  750 , R6  752  and capacitor C2  770  together provide the proper AC termination impedance for testing operations. This also allows a DC current path to be present for the “wetting or sealing current” of the loop.  
         [0061]    The circuitry also includes a capacitor C3  772  for lightning surge suppression and resistors R1  710 , R2  708  for limiting a surge current. The capacitor C1  774  is used to filter out ringing signals that may be present, and for preventing erroneous circuit operations due to transient voltages that may be present. The resistor R7  754  is used to dissipate charges stored in the capacitor C1  774 .  
         [0062]    During operation, when a positive voltage with respect to the ground is applied to the T  704  while the R  706  is left open, and the applied voltage is greater than the combined voltages set by zener diodes Z2  714 , Z5  722  and the forward diode voltage drops for zener diodes Z4  716 , Z6  724  (i.e., typically 0.7 volts), current will begin to flow through the resistor R2  708 , zener diodes Z2  714 , Z4  716 , resistor R3  720 , zener diodes Z5  722 , Z6  724 , and resistor R4  726 . As the voltage from T  704  continues to increase, so does the voltage at resistor R4  726 . When the voltage level increases to a value of V be  (which is approximately 0.7V) for the transistor Q4  746 , a base current is injected, which turns on transistors Q4  746 , Q2  742 . Current then begins to flow through relay K1  760 , actuating the relay K1  760  in the “set” mode. The relay current path is through resistor R2  708 , zener diodes Z2  714 , Z4  716 , diode D2  732 , transistor Q2  742 , relay K1  760 , diode D4  736 , and transistor Q4  746 . The voltage across the relay K1  760  is defined by the voltage of zener diode Z5  722 , V be  drops for zener diode Z6  724 , transistors Q2  742 , Q4  746  and diode D4  736 . Since the voltage of the zener diode Z5  722  is usually much greater than the combined V be &#39;s, the voltage across the relay K1  760  is essentially the voltage of zener diode Z5  722 . The voltage is “impressed” across the relay K1  760  with a coil fixed resistance and produces a fixed current through the relay K1  760 , which is independent of the voltage. Another key feature of the present invention is the combined effect of a fixed voltage across the relay K1  760 , which is turned on only when the T and/or R voltage reaches a pre-defined voltage, and thus results in an “interlock” mechanism. This interlock mechanism applies to all subsequent operations that are discussed hereinafter.  
         [0063]    When a negative voltage with respect to ground is applied to the T, while the R is left open, and the applied voltage is greater than the combined voltages set by zener diodes Z6  724 , Z4  716  and the forward diode voltage drops for zener diodes Z5  722 , Z2  714 , current will begin to flow through resistor R4  726 , zener diodes Z6  724 , Z5  722 , resistor R3  720 , zener diodes Z4  716 , Z2  714 , and resistor R2  708 . As the applied voltage continues to increase across the T, so does the voltage at resistor R4  726 . When the voltage level increases to a value of V be  for the transistor Q3  744 , current is injected, turning on transistors Q3  744 , Q1  740 . Current then begins to flow through relay K1  760 , actuating the relay K1  760  to the “reset” mode.  
         [0064]    When a positive voltage with respect to ground is applied to the R, while the T lead is left open, and the applied voltage is greater than the combined voltages set by zener diodes Z1  712 , Z5  722  and the forward diode voltage drops for zener diodes Z3  718 , Z6  724 , current will begin to flow through resistor R1  710 , zener diodes Z1  712 , Z3  718 , resistor R3  720 , zener diodes Z5  722 , Z6  724 , and resistor R4  726 . As the voltage increases at R, the voltage at the resistor R4  726  also increases. When the voltage increases to the V be  value for the transistor Q4  746 , base current is injected, turning on transistors Q4  746 , Q2  742 . Current then begins to flow through relay K1  760 , actuating the relay K1  760  in the “set” mode. The relay current path is through resistor R1  710 , zener diodes Z1  712 , Z3  718 , diode D2  732 , transistor Q2  742 , relay K1  760 , diode D4  736 , and transistor Q4  746 . The voltage across the relay K1  760  is defined by the voltage of zener diode Z5  722 , V be  drops for zener diode Z6  724 , transistors Q2  742 , Q4  746 , and diode D4  736 . Again, since the voltage associated with the zener diode Z5  722  is usually greater than the V be &#39;s, the voltage across the relay K1  760  is essentially the voltage of zener diode Z5  722 .  
         [0065]    When a negative voltage with respect to ground is applied at the R  706 , while the T lead  704  is left open, and the applied voltage is greater than the combined voltages set by zener diodes Z6  746 , Z3  718  and the forward diode voltage drops for zener diodes Z5  722 , Z1  712 , current will begin to flow through resistor R4  726 , zener diodes Z6  724 , Z5  722 , resistor R3  720 , zener diodes Z3  718 , Z1  712 , and resistor R1  710 . As the applied voltage increases, the voltage across the resistor R4  726  will also increase. When it reaches the V be  drop of transistor Q3  744 , current is injected, turning on transistors Q3  744 , Q1  740 . Current then begins to flow through relay K1  760 , actuating the relay to “reset” mode.  
         [0066]    When a positive voltage is applied to both the T and R concurrently with respect to ground, current will begin to flow when the voltage is at the combined value set by either (1) zener diodes Z2  714 , Z5  722  and the forward diode voltage drops for zener diodes Z4  716 , Z6  724 , and/or (2) zener diodes Z1  712 , Z5  722  and the forward diode voltage drops for zener diodes Z3  718  and Z6  724 . As the applied voltage increases, the voltage across resistor R4  726  also increases and when it reaches the V be  of transistor Q4  746 , transistors Q4  746 , Q2  742  turn on, allowing a current to flow through relay K1  760 , thereby actuating the relay K1  760  in the “set” mode. The current path through the relay K1  760  is via (1) resistor R1  710 , zener diodes Z1  712 , Z3  718 , diode D2  732 , transistor Q2  742 , relay K1  760 , diode D4  736 , and transistor Q4  746 , and/or (2) resistor R2  708 , zener diodes Z2  714 , Z4  716 , diode D2  732 , transistor Q2  742 , relay K1  760 , diode D4  736 , and transistor Q4  746 .  
         [0067]    When a negative voltage is applied to both the T and R concurrently with respect to ground, current will begin to flow when the voltage is at the combined value set by either (1) zener diodes Z4  716 , Z6  724  and the forward diode voltage drops for zener diodes Z2  714 , Z5  722 , and/or (2) zener diodes Z3  718 , Z6  724  and the forward diode voltage drops for zener diodes Z1  712  and Z5  722 . As the applied voltage increases, the voltage across the resistor R4  716  also increases and when it reaches the V be  of transistor Q3  744 , transistors Q3  744 , Q 1   740  turn on, allowing a current to flow through relay K1  760 , thereby actuating relay K1  760  in the “reset” mode. The current path through the relay K1  760  is via (1) resistor R1  710 , zener diodes Z1  712 , Z3  718 , diode D1  730 , transistor Q1  740 , relay K1  760 , diode D3  734 , and transistor Q3  744 , and/or (2) resistor R2  708 , zener diodes Z2  714 , Z4  716 , diode D1  730 , transistor Q 1   740 , relay K1  760 , diode D3  734  and transistor Q3  744 .  
         [0068]    As described above, when relay K1  760  is in the “reset” mode, the T and R are connected to the DUT/phone set  540 , which represents the normal operation mode. In the alternative, when relay K1  760  is in the “set” mode, the T and R are connected to the Signature  532 , which represents the testing/maintenance mode.  
         [0069]    [0069]FIG. 8 illustrates a specific RAMU Signaling Definition for the circuit as shown in FIG. 7, which represents one RAMU and one specific voltage level that the RAMU can be signaled/addressed.  
         [0070]    From the previous discussion, it is easy to understand that multiple RAMUs can be connected on the same line functioning at different addressing voltage levels in order to locate and determine the type of fault. By having a sequence of RAMUs, one can determine the exact location of a fault, by invoking consecutive RAMUs. In addition, by changing the level of the voltage values of the zener diodes, a particular RAMU can be actuated by providing a particular voltage. In other words, each RAMU can be defined/designed with different levels of voltage sensitivity. In other embodiments, other components can be substituted for the specific components described herein so long as these components perform essentially identical functions as described herein.  
         [0071]    In the previous descriptions, numerous specific details are set forth, such as specific functions, components, etc., to provide a thorough understanding of the present invention. However, as one having ordinary skill in the art would recognize, the present invention can be practiced without resorting to the details specifically set forth.  
         [0072]    Although only the above embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiments are possible without materially departing from the novel teachings and advantages of this invention.
The invention presented herein is directed to a remotely addressable maintenance unit (RAMU) working in conjunction with a test head at the central office for detecting and locating faults in digital subscriber loop (DSL) and/or plain old telephone system (POTS) environments. The RAMU includes circuitry for setting and resetting one or more relays for either normal or testing/maintenance mode. The present invention provides a system and method for addressing the RAMU by applying either positive or negative voltages from the tip to ground, from ring to ground, and from tip and ring to ground. In this manner, individual RAMUs can be defined/designed to respond in certain voltage levels and polarities. Accurate fault detection and sectionalization is achieved by the combination of the addressing capabilities enumerated herein, and the impedance signature designed into the RAMU, working in concert with a test head in the central office.