Abstract:
The line interface circuit of a modem to be connected to a standard RJ11 jack of a digital PBX (Private Branch Exchange) is used along with software routines resident in the modem to detect the presence of a PBX connected to the modem. Upon going off-hook, the modem, by the software routines, determines whether the connection will harm the interface by one of several software routines. If it appears that the interface circuit may be damaged because a PBX line is present, the modem is instructed to go on-hook (hang up), or if the connection is not harmful to the interface, alerts the user that the modem is connected to a wrong jack.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 09/212,707 entitled, TELEPHONE LINE INTERFACE CIRCUIT WITHOUT HOOK SWITCH, filed Dec. 16, 1998, U.S. patent application Ser. No. 09/312,136, entitled, ELECTRONIC INDUCTOR WITH TRANSMIT SIGNAL TELEPHONE LINE DRIVER, filed May 14, 1999, and U.S. patent application Ser. No. 09/312,412, entitled TELEPHONE LINE INTERFACE CIRCUIT WITH INTELLIGENT LINE CURRENT AND VOLTAGE CONTROL, filed May 14, 1999. All of these applications are commonly owned by the assignee of the present application. The disclosures of all of these applications are explicitly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to improvements and telephone modems and, more particularly, pertains to new and improved digital telephone modems with line protection devices. 
     2. Description of Related Art 
     A telephone modem can be seriously damaged if it is accidentally connected to a digital Private Branch Exchange (PBX) terminal which are readily found in office environments. A digital PBX terminal (DPBX) looks and acts just like a standard RJ11 jack that is used for the public switch telephone network (PSTN), or an analog PBX. A PSTN terminal is equivalent to a 20 to 100 volt battery at the central office in series with a loop resistance which can vary between 200 ohms and 5,000 ohms. A DPBX terminal, on the other hand, is equivalent to a 10 to 100 volt battery with a resistance of less than 10 ohms. If a telephone modem is connected to the digital PBX terminal, the current through the modem will exceed the maximum expected limits without any drop in voltage at the tip and ring terminals. The resulting excess power will permanently damage the line interface circuit of the modem. Attempts to prevent damage to a telephone modem which is accidentally connected to a DPBX jack have used such devices as optoisolators to detect excessive current flow through the line interface circuitry of the modem. When excessive current is detected in this manner, the modem goes “on-hook,” thereby protecting the circuit from being damaged. Although these prior art devices may be satisfactory in their operation, the additional hardware required to implement the line protection function increases the cost of the modem inordinately. Furthermore, these prior art solutions cannot be used in a line interface configuration which limits the line current in itself. The present invention, on the other hand, uses the software-based architecture of its modem to implement a line protection device without adding additional hardware and is well-suited for use with all interface circuits. 
     SUMMARY OF THE INVENTION 
     To prevent damage to a modem that is accidentally connected to a digital PBX terminal which looks just like a standard RJ11 jack of the PSTN, the modem is equipped with software routines that use voltage readings from the modem telephone line interface circuit to identify a PBX connection and cause the modem to go on-hook. The ADC (analog-to-digital converter) of the modem measures the t-r line voltage through a resistor divider network. Based on this voltage, the software calculates either the power through the interface in off-hook state, or the rate of change of the line voltage over time while going off-hook, or the loop resistance of the PSTN. If the power is too high for the interface, the voltage rate of change is less than a predetermined amount or the loop resistance is less than an expected minimum, the software directs the modem to go on-hook. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The objects and many of the attendant advantages of this invention will be readily appreciated and better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
     FIG. 1 is a circuit diagram of the interface circuit and modem utilized to execute the present invention; 
     FIG. 2 is a flow chart showing the steps taken by the software in the modem to provide for line protection pursuant to one method of the present invention; 
     FIG. 3 is a flow chart of the steps taken by the software to provide line protection under another method of the present invention; and 
     FIG. 4 is a flow chart of the steps taken by the software to provide line protection under yet another method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Prior art systems guard against damage to a modem when it is connected to a digital PBX terminal by measuring the current through the modem&#39;s line interface circuit. When this current exceeds a predefined threshold, the modem concludes that the excessive current is caused by the presence of a digital PBX line. This method of detecting the presence of a digital PBX line has limited applicability and cannot be used with certain modem designs, especially those modems that utilize an interface circuit as shown in FIG.  1 . FIG. 1 shows an interface circuit  11  connected to a modem (not shown) having an ADC  37  and a controller  38 . The interface circuit  11  of FIG. 1 is powered by a fixed voltage supply. A Q 1  transistor  19  automatically limits the line current to the maximum current expected from a PSTN worldwide, about 150 mA. Therefore, the current measured through the modem would never exceed the maximum limit and the modem could not determine from this current reading whether it was connected to a digital PBX line. Some DPBX terminals have voltages as low as 10 volts. When a modem is connected to such a terminal, the current will be relatively small, i.e., 10-30 mA. A method based on measuring current would fail to detect the presence of such a DPBX terminal. 
     Furthermore, the prior art method of current measurement to determine connection to a PBX line would be difficult to implement in a modem designed for worldwide use. The current threshold selected for the line interface circuit changes depending on the requirements of each country. Although the various current thresholds could be programmed for each specific country, such a solution would be complicated and costly to implement and could result in misdetection. 
     Modem line interface circuit  11  typically has a resistance of 100 to about 400 ohms when it is in an off-hook state. It is designed to dissipate less than 3 watts of power. A DPBX or digital private branch exchange (DPABX) terminal has the equivalent of a 10 to 100 volt battery, with loop resistance of less then 10 ohms. As a result, when a modem designed for the PSTN goes off-hook while connected to a DPBX tip and ring (t-r) terminal, the power dissipation in the line interface circuit  11  may exceed the maximum power rating and result in permanent damage. 
     Tip terminal  13  and ring terminal  15  are connected to the interface circuit  11  through a diode bridge  17  to a resistor divider circuit made up of R 1  resister  27  and R 2  resistor  29 . The values of resistances  27  and  29  are chosen to be very large in order to make the current through them negligible compared to the current through Q 1  transistor  19 . As a result, the current through Q 1  transistor  19  is equal to the line current. A U 2  operational amplifier  23  converts the voltage at node  38  to the current required to drive the base  18  of Q 1  transistor  19 . The negative input  22  of U 2  operational amplifier  23  is connected to the emitter  20  of Q 1  transistor  19  thereby effectively making the voltage at node  38  equal to the voltage at emitter  20  of Q 1  resistor  19 . Since the base current of Q 1  transistor  19  is negligible compared to the line current through Q 1  transistor  19 , line current can be calculated as voltage drop across Re emitter resistor  21 . 
     When the modem is on-hook, switch  35  is closed placing R 3  resistor  33  in parallel with R 2  resistor  29 . Switch  25  is also closed, shorting the base  18  of Q 1  transistor  19  to ground, effectively turning off QI transistor  19  and disabling U 2  operational amplifier  23 . 
     Switch  35  is closed to increase the dynamic range of the ADC with respect to Vtr by adding R 3  transistor  33  in parallel with R 2  resistor  29  so that a relatively large Vtr can be measured within the limited voltage range of the ADC (typically 0-4V). For example, if Vtr is expected to be 100 volts, R 3  is chosen so that the ratio (R 2 //R 3 )/R 1 +R 2 //R 3 ) is approximately 25 (100V/4 V). Switch  35  can also be enabled while off-hook, typically to comply with European specifications where the line current is limited to 60 mA and Vtr can be as high as 40 V in off-hook state. 
     When the modem goes off-hook switches  35  and  25  are open. The U 2  operational amplifier  23  becomes enabled and the voltage feedback from the tip and ring terminals  13 ,  15  at node  38  causes line current to flow through Q 1  transistor  19 . An analog-to-digital converter, ADC  37 , reads the voltage at node  38  while the modem is off-hook. Based on this reading, controller  39  determines the voltage at tip and ring  13 ,  15  according to the following equation: 
     
       
           Vtr=Vn [( R   1   +R   2   //R   3 )/( R   2   //R   3 )]  [1] 
       
     
     Since the voltage at node  38  while the modem is on-hook is a voltage at the tip and ring terminal  13 ,  15  without drawing any current from the telephone line, this voltage represents the battery voltage at the central office. 
     When the modem goes off-hook, the ADC  37  continues to read the voltage at node  38  and the controller  39  monitors the line voltage as the line current increases. Based on the assumptions discussed above, the controller can also measure the line current at any point in time by the following equation:              Itr   =     Ve   Re             [   2   ]             since                         Vn   =   Ve                         then                         Itr   =     Vn   Re                                              
     As a result, the controller  39  can monitor both line voltage and line current by using the Vn reading at node  38  obtained by ADC  37  at any point in time. 
     As can be seen by the above equation, although the line current can be monitored by reading the voltage across Re resistor  21 , this line current quantity cannot be used by the controller to make a decision as to whether a digital PBX line is present. The U 2  operational amplifier  23  is powered by a fixed voltage supply of 5 volts. The output of the U 2  operational amplifier  23  can never exceed 5 volts by definition. The emitter  20  of Q 1  transistor  19 , as a consequence cannot exceed 4.3 volts, assuming a typical base to emitter voltage of 0.7 volts. Therefore, Q 1  transistor  19  automatically limits line current to a maximum value which is intentionally selected to be the maximum current expected from a PSTN worldwide, about 150 mA, by choosing an appropriate value for Re resistor  21 . Furthermore, if the DPBX voltage is 20V and the line interface resistance is 400 ohms, for example, the current through the interface would be only 20V/400 Ω=50 mA, with total power dissipation at 20V×50 mA=1 watt. The modem, by way of the ADC  37  and controller  39 , thus would not be able to determine from these current readings whether a digital PBX line is present. 
     The present invention contemplates several different software processes to detect a digital PBX line. The process utilized would depend, in part, upon the interface circuit  11  configuration. 
     The interface circuit  11  of FIG. 1 is particularly adaptable to a voltage gradient method which is illustrated in FIG.  2 . 
     The flow chart of FIG. 2 illustrates the voltage gradient method  39  of detecting whether a digital PBX line is connected to the interface circuit of the modem. The process involves the general steps of the ADC  37  and controller  39  measuring the line voltage at the tip and ring terminals  13 ,  15  immediately before the modem goes off-hook while it is still on-hook, and repeatedly after going off-hook. This voltage is measured in the manner explained above by monitoring the Vn voltage at node  38 . If the voltage measured in this way does not change fast enough as a function of time over a predetermined amount of time, the modem concludes that an unusual condition is present, like a PBX line, and alerts the modem of a fault condition causing the modem to go on-hook and notifying the user. 
     Generally, the presence of capacitor  31  connected between Vn and ground, which is required to filter out AC signals from Vn node  38 , causes a slow transient of the voltage at tip to ring while going off-hook, typically on the order of 100 ms. Inductance in the PSTN can also cause such a transient. 
     The specific voltage gradient method  39  illustrated in FIG. 2 consists of an on-hook state  41 , while a controller measures the line voltage. Upon receiving a user command  43 , the controller instructs the modem to go off-hook  45 . The controller continues to measure the Vn voltage at node  38  and at the same time presets  47  a protection counter. The controller continues to read the voltage at node  38 . For each voltage measurement of node  38 , the controller calculates  51 , the derivative of the voltage with respect to time and compares  53  each calculation with a predetermined minimum. If the derivative equals the minimum, the controller increments  55  the protection counter. The controller periodically compares  57  the contents of the protection counter with a predetermined maximum count. If that maximum count is reached, the controller alerts the modem  59  that a fault condition exists, causing the modem to go on-hook and notify the user. 
     The protection counter limit is determined by the sampling period T of the power P, and is chosen so that P×T results in an acceptable energy dissipation in the line interface circuit, over the total sampling time. 
     Another method of determining whether a PBX line is connected to the modem can be thought of as a power method which can be utilized with any number of interface circuits, as well as interface circuit  11  of FIG.  1 . In the power method, the controller repeatedly measures the Vn voltage at the node  38  in the manner discussed above, after going off-hook. This voltage is multiplied by the current through Q 1  interface transistor  19  to obtain the instantaneous power through the interface circuit. The instantaneous power dissipation in the interface, the calculated power, is compared to a predetermined maximum rating for that interface. If the calculated power exceeds that rating for a predetermined number of readings the modem concludes that a digital PBX line is present and causes the modem to go off-hook. 
     The power method of detecting a PBX line  61  connection to the interface circuit is shown in FIG.  3 . When the modem is on-hook  41 , the controller is waiting for a user command  43  telling it to go off-hook  45 . Upon going off-hook, the controller resets  47  the protection counter and reads the Vn voltage at node  38  to determine the Vtr tip to ring voltage according to equation [1]. The controller obtains the interface current by using equation [2]. This current is multiplied by the Vtr (tip to ring) voltage to obtain the power  63  through the system. This calculated power is compared  65  to a maximum power rating for the interface circuit. If the calculated power exceeds this maximum power rating, the protection counter is incremented  55 . The controller monitors  57  the contents of the protection counter to determine if it has reached its maximum count. If it has, the controller notifies  59  the modem of a fault condition, and instructs it to go on-hook. 
     The power method is particularly suited for protection of the modem line interface circuit. However, if the voltage of DPBX is reasonably low and the modem resistance is high, the power method would fail to detect the presence of a DPBX. 
     For example, if the DPBX voltage is 20V and the modem resistance is 400 ohms, the current through the modem would be 20V/400 ohms=50 mA and the power dissipation would be 20V×50 mA=1 W. This power is well within the maximum rating of the line interface circuit, and therefore the controller would not detect the presence of the DPBX. 
     The flow chart of FIG. 4 describes a loop resistance method  65  that overcomes this problem. When the modem is on-hook  41 , the controller calculates the voltage Vtr  67  using equation [1], which represents the battery voltage, Vbatt, at the central office (CO), as explained above. When the modem goes off-hook  45 , the controller determines  69  line voltage Vtr and line current Itr using equations [1] and [2], respectively. Assuming the simple DC circuit model for the PSTN shown in FIG. 1, the following equation will be true: 
     
       
           V batt− Rs×Itr=Vtr  solved for  Rs  yields 
       
     
     
       
           Rs= ( V batt− Vtr )/ Itr   [3] 
       
     
     Using equation [3], the controller calculates  71  the equivalent loop resistance Rs of the PSTN. If Rs is less than an expected minimum value Rs (min)  73 , the controller concludes that a DPBX or other unusual network is present, sets the modem on-hook  59  and notifies the user of the condition. 
     The controller can also determine the loop resistance while off-hook, without necessarily reading the on-hook line voltage Vtr. Since equation [3] is true for any values of Vtr and Itr, the controller goes off-hook and takes two readings  69  of Vtr and Itr, at time t( 1 ) and time t( 2 ). The following system of two equations can be written: 
     
       
           V batt− Rs×Itr ( 1 )= Vtr ( 1 ) 
       
     
     
       
           V batt− Rs×Vtr ( 2 )= Vtr ( 2 )  [4] 
       
     
     Solving equations [4] for the unknown variables Rs and Vbatt, the controller calculates  71  the value of Rs and determines if a DPBX is present, as above. 
     The controller can also determine the CO battery voltage Vbatt and the loop resistance Rs while off-hook, without necessarily reading the on-hook line voltage Vtr. Since equation [3] is true for any values of Vtr and Itr, the controller can take two independent readings of Vtr and Itr at state (1) and state (2). These two states can differ from each other by a different setting of the DAC, for example, or by having switch S 1  enabled or disabled, respectively. The following system of two equations in two unknowns, Vbatt and Rloop, can then be written: 
     
       
           V batt− R loop× Itr ( 1 )= Vtr ( 1 ) 
       
     
     
       
           V batt− R loop× Itr ( 2 )= Vtr ( 2 )  [5] 
       
     
     and the controller can then calculate the values of Vbatt and Rloop and determines if a DPBX is present, as above. 
     Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiments can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described herein.