Abstract:
In one embodiment, a low cost, simple circuit for detecting an off-hook condition of a telecommunication line comprising tip and ring signal lines is provided. The circuit comprises a voltage divider for coupling between the tip and ring lines without an intervening transistor and having a node at which is presented a scaled version of a voltage across the voltage divider. The circuit further comprises a transistor having a control terminal coupled to the node, a first current flow terminal coupled to a voltage source, and a second current flow terminal coupled to an output terminal, wherein the output terminal bears a value that is indicative of a voltage across the tip and ring lines and thus whether the telecommunication line is off-hook.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of co-pending application Ser. No. 11/534,918 filed Sep. 25, 2006 as attorney docket no. Fischer 50-60-67, which is a continuation of application Ser. No. 09/605,953 filed Jun. 28, 2000 as attorney docket no. Fischer 33-45-25 and issued as U.S. Pat. No. 7,113,587, the teachings of both of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments of the invention pertain to telecommunication systems, and more particularly, to determining the condition of a telecommunication loop circuit, such as determining whether any equipment coupled to the loop is off-hook. 
       BACKGROUND OF THE INVENTION 
       [0003]    In recent years, there has been substantial expansion in the number and type of telecommunications equipment in common use in households and offices. For instance, it is not unusual for a subscriber to have multiple pieces of telecommunications equipment coupled to a single telephone subscriber loop, e.g., a single tip and ring wire pair. For instance, a household might have a telephone, a facsimile machine, a computer using a modem, and an answering machine hooked up to a single subscriber loop. Other than for voice communications using a telephone, it is possible for only one piece of equipment to use the loop at any given time. If a second piece of equipment goes off-hook while a first piece of equipment is using the loop to transmit or receive data, the noise and change in loop voltage due to the second piece of equipment going off-hook can cause data errors in connection with the first piece of equipment. Accordingly, many automated telecommunication apparatuses such as fax machines and modems are designed to detect the condition of the telephone line to which they are coupled before they go off-hook. Thus, for instance, if a facsimile machine coupled to a subscriber loop is receiving a facsimile (such that the line is off-hook), a computer modem having this feature will first check if the line is already in an off-hook condition before going off-hook itself and suppress an attempt to go off-hook if it detects that the subscriber loop is already off-hook (i.e, that another piece of equipment on the same line is off-hook). 
         [0004]    While there are many ways to determine whether a subscriber loop circuit is off-hook, probably the simplest way is to determine the DC voltage on the line. In the United States, subscriber loops are biased to approximately 48 volts DC (Direct Current) when the line is not in use (i.e, when no equipment coupled to that line is off-hook). If a telecommunication device on the line is off-hook, then the voltage drops typically to somewhere in the range of 20 volts or less. 
         [0005]    In the prior art, telecommunication equipment manufacturers have utilized voltage comparator circuits to detect the DC voltage across tip and ring of a subscriber loop and to use the comparator output as an on-hook/off-hook indicator signal.  FIG. 1  is an exemplary circuit of the prior art. In the circuit of  FIG. 1 , a full wave rectifier  12  is coupled across tip  14  and ring  16 . The tip′ output  14   a  from the rectifier is coupled to a voltage divider  17  comprising resistors  18  and  20 . The common node  22  of the voltage divider is coupled to one input of a comparator  24 . The other input of the comparator is coupled to a reference voltage  26 . The output of comparator  24  is coupled through a high voltage barrier circuit, such as optical coupler  28  to a digital signal processor (DSP)  25 . A less expensive electrical (as opposed to optical) coupling circuit, whether inductive or capacitive, would not function well in this application. Particularly, the signal across tip and ring can change too slowly to be distinguished from noise by an electrical coupling circuit. 
         [0006]    Of course, it will be understood by those of skill in the art that substantial additional circuitry is coupled across tip and ring that is not illustrated in  FIG. 1  in order to provide the functionality of the circuit (e.g., to send and receive facsimiles) and that  FIG. 1  merely shows the loop status detection circuitry. 
         [0007]    The reference voltage and values of the resistors  18  and  20  in the resistor voltage divider  17  are selected so that the comparator output changes state somewhere between the on-hook voltage (approximately 48 volts in the U.S.) and the off-hook voltage (approximately 20 volts in the U.S.). Accordingly, for example, the reference voltage and divider network resistor values can be selected so that the switching point of the comparator is at approximately 30 volts across tip and ring. The digital signal processor is programmed to disable the telecommunication apparatus from going off-hook when the voltage across tip and ring is less than 30 volts. Otherwise, the DSP allows the apparatus to operate normally. 
         [0008]    Except for the DSP, the circuit shown in  FIG. 1  is specifically dedicated to the aforementioned feature. The DSP typically would be a DSP that already exists in the circuit for performing some or all the functions of the actual device (e.g., facsimile machine) and would merely have additional functionality built into it for receiving the comparator output signal and selectively enabling/disabling the device from going off-hook responsive thereto. 
         [0009]    The telecommunication loop detection circuit itself must not disrupt the loop when checking the loop voltage. To do so would, of course, defeat its very purpose. 
         [0010]    Accordingly, it is an object of the present invention to provide a telecommunication loop condition detector that is simpler and lower in cost than prior art detectors. The telecommunication device must not disturb any call function with any audible noise injection and must not significantly alter the loop impedance. 
       SUMMARY OF THE INVENTION 
       [0011]    Embodiments of the invention provide a low cost, simple, circuit for detecting the condition of a telephone line. Particularly, an embodiment of the inventive circuit utilizes an existing low power analog-to-digital converter that is already incorporated into the telecommunication device and used for other functions such as caller ID and ring detection. The additional circuitry comprises a voltage divider coupled between tip and ring and a transistor having its control input (e.g., gate or base) coupled to the common node of the voltage divider, one of its current flow terminals (e.g., collector, emitter, drain, or source) coupled to the analog input of the analog-to-digital converter, and the other current flow terminal coupled to ground. The A/D converter is also coupled to tip and ring, respectively, through a pair of capacitors for detecting the AC voltage on the line for purposes of caller ID and/or ring detection. 
         [0012]    The resistors of the voltage divider are proportioned such that the common node voltage of the divider is above the threshold voltage of the transistor, thus turning it on, when the voltage across tip and ring is at the on-hook voltage of approximately 48 volts, and is below the transistor threshold voltage, thus turning it off, when the voltage across tip and ring is at the off-hook voltage of approximately 20 volts. Accordingly, when the transistor is on, the A/D converter input is driven to ground. When it is off, the A/D converter input goes to its self biased voltage. Accordingly, the digital output of the A/D converter is an indication of the voltage on the line and thus whether it is on-hook or off-hook. The output of the A/D converter can be coupled to a digital signal processor that disables the device from going off-hook if the A/D converter detects that the loop is off-hook. 
         [0013]    In an alternative embodiment, the voltage across one resistor of a resistor voltage divider that is coupled between tip and ring is provided to the analog input of the low power analog-to-digital converter through the current flow terminals of one or more transistors so that the A/D converter receives a scaled version of the actual tip to ring voltage rather than simply a two state on-hook/off-hook signal. The DSP may use the specific loop voltage information provided in this embodiment to determine additional information about the loop. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a circuit diagram of a telecommunication line status detection circuit of the prior art. 
           [0015]      FIG. 2  is a circuit diagram showing a telecommunication line status detection circuit in accordance with the first embodiment of the present invention. 
           [0016]      FIG. 3  is a circuit diagram showing a telecommunication line status detection circuit in accordance with a second embodiment of the present invention. 
           [0017]      FIG. 4  is a circuit diagram of one particular data access arrangement into which an embodiment of the present invention has been incorporated. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    An embodiment of the present invention is a low-cost, simple, circuit for detecting line voltage across tip and ring of a telecommunication subscriber loop that primarily utilizes circuitry that is commonly already incorporated in a data access arrangement (DAA) of telecommunication equipment. The circuit can be used to detect whether the line is in an off-hook condition and particularly can be used for disabling equipment from going off-hook if the line already is in use (i.e., off-hook) by another piece of telecommunication equipment. A preferred embodiment of the invention is particularly adapted for use in telecommunication equipment incorporating the CSP 1035 Silicon DAA manufactured by Lucent Technologies, Inc. of Murray Hill, N.J., the assignee of the present application. However, it will be obvious to those of skill in the telecommunication equipment field that embodiments of the invention can be utilized with other DAA designs. 
         [0019]    A DAA commonly includes circuitry for performing various functions including ring detection, DC loop hold, hook control and pulse, parallel phone sense and data/voice relay. Some or all of these functions may be performed by a programmed DSP. Some DAAs includes a low power analog-to-digital converter coupled to receive a differential signal from the tip and ring line pair so that a DSP can perform functions in connection with caller ID and ring detection. 
         [0020]    The circuitry of one embodiment of the present invention is shown in  FIG. 2  and appears within dashed box  30  in  FIG. 2 .  FIG. 2  also illustrates some of the circuitry that already commonly exists in a DAA and particularly the circuitry that is relevant to the operation of the inventive circuit  30 . The detection circuit  30  comprises resistors R 1  and R 2 , diode D 1  and transistor Q 1 . In particular, resistors R 1  and R 2  comprise a resistor voltage divider coupled between tip′ and ring′. Note also that ring′ is coupled to analog ground. The common node N 1  between resistors R 1  and R 2  is coupled to the control terminal (the gate, in the case of a MOSFET) of transistor Q 1 . Node N 1  also is coupled to a control signal line  40  through diode D 1 . 
         [0021]    This control signal line  40  pre-exists in many DAAs and is designed to remain at a logic low level until just before the equipment attempts to go off-hook, at which time the control signal goes to logic high. As will become clear from the description below, operation of embodiments of the present invention may interfere with other functions of the equipment, such as caller ID and ring detection. Accordingly, this control line is used to disable operation of the inventive detection circuit until just before the equipment attempts to go off-hook so that, for instance, caller ID can operate without interference until the equipment is ready to go off-hook. 
         [0022]    The current flow terminal of transistor Q 1  (source and drain in the case of a MOSFET) are coupled between ring′ (ground) and one of the inputs of differential, low power, A/D converter  36 . 
         [0023]    The low power A/D converter is a differential converter that converts a differential analog voltage input across its two inputs to a digital value. Accordingly, the “+” and “−” inputs of the converter are coupled to tip and ring of the telephone line through capacitors C 1  and C 2 , respectively. 
         [0024]    Capacitors C 1  and C 2  block DC current on tip and ring from the A/D converter so that only the AC current across tip and ring reaches the A/D converter through those paths. The digital output of the A/D converter is coupled to a digital signal processor that reads the AC information and performs caller ID and ring detection functions as known in the prior art. 
         [0025]    Tip and ring are also coupled to a full wave rectifier BR 1  to produce tip′ and ring′ signals. The full wave rectifier is employed simply to assure that tip′ is always positive compared to ring′. Specifically, it is possible that the polarity of tip and ring can be reversed. Rectifier BR 1  assures that tip′ is always more positive than ring′. 
         [0026]    When the control signal is at logic low, node N 1  is essentially coupled to ground thus keeping transistor Q 1  turned off. With transistor Q 1  turned off, detection circuit  30  has no affect on the analog input of A/D converter  36 . Accordingly, the AC signals from tip and ring are received by the A/D converter  36  without interference, which signals can be used for caller ID, ring detection and similar functions. 
         [0027]    When the signal line goes high, diode D 1  is essentially open circuited and the resistor divider formed by R 1  and R 2  will selectively turn transistor Q 1  on or off. In particular, resistors R 1  and R 2  are ratioed relative to each other so that the voltage at node N 1  is greater than the threshold voltage of Q 1  when the voltage between tip′ and ring′ is at the on-hook voltage of the line and will be below the threshold voltage of transistor Q 1  when the voltage between tip′ and ring′ is at the off-hook voltage level. 
         [0028]    As previously mentioned, the standard on-hook DC voltage across tip and ring for a subscriber loop in the United States is approximately 48 volts, while standard off-hook DC voltage across tip and ring is approximately 20 volts. Accordingly, resistors R 1  and R 2  can have a ratio relative to each other so that the common node voltage will be the threshold voltage of transistor Q 1  when the voltage between tip and ring is anywhere between just above 20 volts and just below 48 volts. 
         [0029]    The inputs of the A/D converter are both biased to the common mode voltage. Accordingly, when transistor Q 1  is turned off, the DC voltage across the differential inputs of A/D converter  36  is approximately 0 volts. However, when transistor Q 1  is turned on, the − input terminal of the A/D converter is driven to ground through the current flow terminal of transistor Q 1  while the + input terminal remains at common mode voltage. Accordingly, when transistor Q 1  is turned on, A/D converter  36  detects one half full scale voltage. 
         [0030]    Accordingly, in operation, when the control signal  40  goes high, thus open circuiting diode D 1 , the A/D converter will detect 0 volts across its differential inputs if the line is off-hook (and thus transistor Q 1  is turned off). However, if the line is on-hook, transistor Q 1  is turned on so that A/D converter  36  will detect one half full scale voltage across its differential inputs. 
         [0031]    The output of the A/D converter  36  is coupled to the digital signal processor  25 . The digital signal processor  25  is programmed to prevent the equipment from going off-hook if it receives approximately 0 volts at this instant and to allow the equipment to go off-hook if it receives one half full scale voltage at this instant. 
         [0032]    Accordingly, the inventive circuit provides a non-disruptive line condition detection function with a minimum of additional circuitry. In the embodiment shown in  FIG. 2 , for instance, the circuit adds only two resistors, a diode and a transistor to the DAA. 
         [0033]      FIG. 3  illustrates an alternative embodiment  50  of the present invention in which the A/D converter detects a scaled version of the actual voltage across tip and ring rather than merely a two state (on-hook/off-hook) signal. The alternative circuit is shown in box  50 . It comprises resistors R 3  and R 4 , diode D 2  and transistors Q 2  and Q 3 . Resistors R 3  and R 4  form a resistor divider network coupled between tip′ and ground, just as in the  FIG. 2  embodiment, except that diode D 2  is coupled between the bottom of resistor R 4  and ground. Diode D 2  assures that the source terminals of transistors Q 2  and Q 3  are referenced to ground. In essence, diode D 2  acts as a regulator, keeping the voltage at the source terminal of transistor Q 3  at or above 0.7 volts (the bias voltage of the diode). 
         [0034]    Transistor Q 2  has its current flow terminals coupled between the common node N 2  between resistors R 3  and R 4  and one of the differential inputs of the A/D converter  36 . Transistor Q 3  has one of its current flow terminals coupled between resistor R 4  and diode D 2  and its other current flow terminal coupled to the other differential input of the A/D converter. The control terminals (gates) of both transistors Q 2  and Q 3  are coupled to the aforementioned control signal  40 . 
         [0035]    In operation, transistors Q 2  and Q 3  are turned off until the control signal  40  goes high just before the device attempts to go off-hook. With transistors Q 2  and Q 3  turned off, circuit  50  will have no effect on the operation of A/D converter  36 . However, when the control signal  40  goes high, transistors Q 2  and Q 3  will be turned on. Accordingly, A/D converter  36  will detect the voltage across resistor R 4  at its two differential input terminals. Since resistors R 3  and R 4  (and diode D 2 ) form a voltage divider across tip′ and ring′, this voltage is simply a scaled version of the voltage between tip′ and ring′. Accordingly, the DSP which receives the output of the A/D converter will receive a scaled version of the DC line voltage and can react accordingly. Thus, with the addition of one extra transistor over the embodiment of  FIG. 2 , the embodiment of  FIG. 3  provides the actual tip to ring DC voltage to the DSP. The DSP can use this more specific information about the DC condition of the loop as needed. 
         [0036]      FIG. 4  is a partial block, partial schematic diagram of an exemplary DAA  100  incorporating an embodiment of the present invention. The circuitry of the second embodiment of the present invention is shown in dashed box  50 . The aforementioned DSP and low power A/D converter are shown at  25  and BR 1 , respectively. The DAA  100  further includes a digital bit output controller  107  which is the source of the aforementioned control signal  40  as well as other control signals in the DAA. The DAA further includes a full power receive A/D converter  101  and transmit D/A converter, both of which couple to the tip and ring line pair through circuitry  105  for conditioning signals. Circuitry  105  performs various function, including hook switch line modulation, shunt regulation A/D and D/A interfacing. The DSP  25  receives the digital output data from the A/D converter  36  through a digital transmitter, shown as part of circuit  109 , and a high voltage interface circuit  111 . 
         [0037]    Since the DC line voltage is encoded by the low power A/D converter  36 , the high voltage interface circuit may be a less expensive and complex electrical high voltage interface circuit and need not be optical. In essence, the A/D converter modulates the DC voltage signal. Thus, there is no slow moving voltage that must go through the high voltage interface that would preclude the use of an electrical, as opposed to an optical, high voltage interface. 
         [0038]    The DSP also sends digital information to various circuits through the high voltage interface circuit  111  and a digital data receiver portion of circuit  109 . For instance, the DSP  25  communicates with the digital output bit controller  107  through circuits  111  and  109  and thus can control signal  40  as needed. The DSP  25  includes algorithms for performing various functions, including line status detection in accordance with an embodiment of the present invention, ring detection, and caller I.D. functions based on the signals that are received through the low power A/D converter  36 . DAA  100  of  FIG. 4  is merely an exemplary DAA employing an embodiment of the present invention. It should be clear to those of skill in the related arts that embodiments of the invention can be employed in other DAAs also. 
         [0039]    Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.