Abstract:
A novel line-current protection circuit for detecting excessive line current from a telephone line and protecting line interface circuitry from the excessive line current is presented. The protection circuit is adapted to detect when an excessive current is present on the tip and ring lines indicative of the misconnection of the modem or other line interface device with a telephone network that presents an incompatible interface standard. The preferred embodiment appreciates the multimode hold current specifications throughout the world and evaluates the connection of the line interface circuit with a telephone line to determine the compatibility therewith. When excessive line current is detected, the line-protection circuit switches the vulnerable line interface out of the circuit. When compatible line current levels are detected, the present invention reconfigures the configurable hold circuit into its original operating (e.g., non-testing) state for call processing.

Description:
BACKGROUND OF THE INVENTION 
     1. The Field of the Invention 
     This invention relates to modem communication systems that are used by computer equipment to communicate data across public or private telephone lines. More particularly, the invention relates to a line-current protection circuit that protects modem circuitry from being damaged by an excessively high line current that can be supplied via the telephone line in the event of misconnection. 
     2. Background Art 
     Increasingly, users of personal computers and related computer equipment rely on the ability to utilize public and private telephone lines to transmit and receive data. This capability dramatically increases the value of the computer, and the productivity of the computer user. As the number of remotely accessible public and private networks, databases, computer-based bulletin boards and related computer services continues to expand, the need for this communication capability will continue to grow. 
     As is well known, computer equipment, such as personal computers, communicate across telephone lines via a piece of equipment known as a “modem.” The term modem is an acronym derived from the phrase “modulator-demodulator,” which is descriptive of the basic function performed by the modem. A modem permits the personal computer, which is a digital device, to be interfaced with telephone lines by modulating the outgoing digital data so that it is compatible with telephone networks, which are designed to handle analog signals (e.g., speech traffic). Similarly, the modem reverses that process by demodulating incoming analog data from the telephone line so that it is in a digital form that is useable by the computer. 
     The manner in which telephone equipment, such as a modem, must electrically interface with the public telephone system has also been standardized so as to provide a compatible interface. Generally, modem manufacturers must provide an electrical line interface that moderates all signals or energy being input by the modem into the telephone line. The Federal Communications Commission (FCC) and the various telephone companies require this standard interface. The line interface protects the telephone lines and central telephone systems from being damaged, and thereby insures the integrity and quality of transmissions over the telephone lines. 
     This line interface circuitry is referred to as the Data Access Arrangement (DAA) circuit. The DAA line interface circuit provides an impedance match between the telephone equipment and the telephone line, and it also isolates and protects the telephone equipment from transient signals and other electrical disturbances that may be present on the telephone line. The DAA line interface circuit also protects the telephone line from any disabling electrical influences that may be generated by the telephone equipment (such as a modem). For example, the public telephone system could be damaged if, instead of transmitting frequency signals, the modem were to inadvertently inject DC power into the telephone line. This situation is avoided because the modem must incorporate the FCC mandated DAA line interface circuit, thereby ensuring that such a damaging signal is not inadvertently transmitted onto the public telephone lines. 
     As already noted, modems are increasingly being used to interconnect computers via public telephone lines. Initially, modems were largely configured as external accessory units, housed in their own cases, and attached to a port-connector located on the personal computer via a cable (usually a RS-232 port and cable). The external modem is then electrically connected to the telephone line via a standard RJ-type connection scheme, as discussed above. Further, an external modem would be equipped with the standard DAA line interface circuitry, as also described above. 
     External modems are, and will continue to be, widely used. However, with the increased popularity of smaller portable computers (referred to variously as laptop, notebook, sub-notebook or palm-top computers), external modems are less desirable because of their cumbersome size, their need for an additional cable and computer port and their consequent lack of portability. As such, the external modem is simply not conducive to the needs of a portable computer user. In response to this drawback, smaller modems have been developed that are formed as an integral component within the portable computer. This type of internal modem is located within the housing of the portable computer such that it may be directly interfaced with the telephone line via an RJ-11 physical/electrical media connector and associated cable. The internal modem is also necessarily equipped with the required DAA line interface circuitry. 
     Although such internal modems are more conducive to the portability requirements of a portable computer, they have several drawbacks. For instance, as an integral component, such an internal modem cannot be easily interchanged with other computers. Further, these types of internal modems also take up limited physical space within the portable computer, often at the expense of other computer components, such as an internal disk drive. Further, as computer housings have continued to be downsized—even to the extent of being hand-held—internal spatial restrictions have necessitated that modems be even further reduced in size, without giving up functionality. 
     Such requirements have resulted in the establishment of standards for the internal accessories of the computer. One set of standards applicable to memory cards has been developed by the Personal Computer Memory Card International Association (PCMCIA). This organization is comprised of hundreds of manufacturers of memory cards and related peripheral equipment. The PCMCIA has published specifications setting forth the spatial standard for all memory cards used in down-sized computers as being restricted to a rectangular space approximately 55 millimeters in width, 85 millimeters in length, and 5 millimeters in depth (commonly referred to as the PCMCIA Type II standard). 
     In keeping with the PCMCIA standards for memory cards, internal modem manufacturers have adopted the same spatial standards for use with their down-sized modem cards. By complying with the standards established by PCMCIA for memory cards, modem card manufacturers have assured themselves of compatibility and spatial conformity with computers utilizing and complying with the PCMCIA specifications. 
     The constraints imposed by the PCMCIA specification have resulted in the development of “credit card” sized modem cards. Thus, most of the components formerly housed in an external or integral modem are now contained within a credit-card sized, PCMCIA card. As with any PCMCIA component, a PCMCIA modem card is merely inserted into any computer equipped with a PCMCIA compliant socket. In this way, a PCMCIA modem is interchangeable with other computers, and requires only a minimal amount of physical space. 
     Although much reduced in size, a PCMCIA modem is interfaced with an external telephone subscriber line in the same manner as an external or internal modem. Thus, a PCMCIA modem can be connected to the telephone line via a standard, RJ-type connector interface and cable. Further, the PCMCIA modem card also includes the standard DAA line interface circuitry previously discussed. 
     Because the PCMCIA modem card is equipped with a standard RJ-type connector, portable computer users can connect to a telephone line and communicate from almost anywhere in the world. Although this is an important advantage of a PCMCIA modem equipped portable computer, the capability also raises an important problem. 
     Many offices, hotels, schools and similar buildings are wired such that the telephone equipment contained within the building is not connected directly to the public telephone system. Rather, buildings are increasingly equipped with “Private Branch Exchanges” (PBX). A PBX is an automatic switching system that is used to interconnect terminal equipment, such as telephone sets, within a building or campus of buildings. When communication is required outside of the building, the PBX will act as a gateway to the public telephone network. 
     Typically, telephone equipment is connected to a PBX installation with a standard RJ-type connector, as described above. However, although the physical connection between the PBX and the telephone equipment is the same, some PBX equipment utilizes the electrical wires, or leads, contained within the RJ-11 connection in a manner different from the public telephone network. For example, some PBX systems utilize the two center leads of an RJ-11 connector, commonly referred to as the “tip” and the “ring” leads, in a manner that is significantly different from a public telephone company. More specifically, these PBXs utilize the tip and ring leads to supply power to the specifically designed PBX telephone sets. This power is usually supplied as a differential DC voltage—typically in the range of 12 to 90 volts—that is applied across the tip and ring leads. However, no resistance is supplied to limit the current supplied. In contrast, the public telephone company places approximately 48 volts across the tip and ring leads, but the current is limited by a resistance of 400 Ohms to 1750 Ohms placed in series with the 48 volt source (thereby resulting a line-current that is usually less than around 120 mA.). 
     Thus, problems can arise when a piece of telephone equipment, such as a PCMCIA modem, is plugged into a RJ-type telephone jack connected to a PBX. If the PBX utilizes the tip and ring leads in the manner described above, the higher supply voltage of the PBX is applied across the DAA line interface circuitry. Since the resulting current drawn by the DAA circuitry is not limited, and thereby excessively high (i.e., greater than the maximum acceptable level of approximately 120 mA.), the DAA circuitry can be destroyed, and the PCMCIA modem thereby rendered inoperative. This problem is exacerbated by the fact that a portable computer user is usually unaware as to whether a particular RJ-11 (or similar RJ-type connector) telephone jack is connected to such a PBX. Consequently, the user may inadvertently plug the PCMCIA modem into such a “standard looking” jack, and thereby render the modem inoperative. 
     Solutions to this particular problem have heretofore not been entirely satisfactory. One solution is to protect the DAA line interface circuitry with a fuse type device. If the PCMCIA modem is taken “off-hook” while connected to a PBX that supplies an excessively high current, the fuse or fusible resistor will open, and thereby protect the DAA line interface circuitry. However, the fuse must then be physically replaced before the modem is again operative. Although this approach may work in an external type modem, wherein the fuse is relatively easy to access and replace, it is not practical in a PCMCIA modem card, wherein the electrical components are miniaturized and disposed within a sealed enclosure. Typically, the PCMCIA card must be returned to the manufacturer to have the fuse replaced—a costly and time consuming process. 
     Further, as is well known, the reaction time of a fuse is relatively slow and often unpredictable. Since fuses are generally slow to react to an excessive current, the DAA line interface circuitry must utilize components that are rated higher (and are thus physically larger) to compensate for the additional wattage consumed before the fuse reacts. This is not practical in a PCMCIA card environment, wherein physical space is extremely limited. 
     Finally, as noted above, the portable computer user is often unaware that a particular telephone jack is connected to a PBX system that could cause damage to the PCMCIA modem&#39;s DAA circuitry. Thus, even if a PCMCIA modem card is equipped with a fuse, and the modem is inadvertently connected to such a PBX, the modem will no longer be operable because the fuse remains open. The user typically will not know why the PCMCIA modem card has failed and is no longer operative. Diagnosis of the problem is very difficult—again resulting in a costly and a time-consuming process. 
     Similar “fuse” like devices have the same drawbacks. For instance, circuit breaking or current limiting devices are also generally slow to react. Further, these types of devices are relatively large, and again cannot physically fit within the limited space of a PCMCIA card. 
     A further problem arises when a multiplicity of hold current standards are employed. For example, it is known that the United States embraced an “off-hook” or hold current profile having a linear voltage and current outline with respect to the voltage and current presented to the modem. Such profile represents boundaries within which corresponding equipment should operate in order to properly signal to the central office on the telephone line that the modem or other device is in an off-hook state. While this linear hold current standard has been adopted by other countries, it is known that a significant number of countries have adopted various other “off-hook” profiles that take on various other voltage and current outlines. Most notably, and other prevailing standard that is commonly employed is known as the TBR21 standard. This standard exhibits a non-linear voltage and current profile. However, this non-linear profile more closely approximates the linear profile voltage and current specifications than the PBX specifications. Therefore, there exists a need to be able to accommodate a particular hold-current modem to which the modem has been previously configured while enabling the evaluation of the telephone line and its compatibility with the modem to which it has been connected. It should be reiterated that the evaluation all the voltage and current on the telephone line as presented to the modem or other device must be performed in a very timely manner in order to preclude, or at least minimize, damage to either the modem or other device is connected to the telephone line. 
     Therefore, there remains a need for a line-current protection circuit that will insure that the DAA line interface circuitry is not damaged when inadvertently subjected to an excessively high current. 
     OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
     The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art not heretofore fully or completely solved by line current protection schemes used in connection with modem cards. However, it is not intended that the apparatus of the present invention will necessarily be limited solely to modem cards, since it may also find useful application with other devices requiring protection from exposure to excessive line-current conditions. Thus, it is an overall object of the present invention to provide a line-current protection circuit that provides for cost effective and reliable protection of circuitry from an excessive line current condition. 
     Another object of the present invention is to provide a line-current protection circuit that utilizes electrical components that physically can be used within the limited space of a miniaturized or mobile architecture such as a PCMCIA-architecture card. 
     Yet another object of the present invention is to provide a line-current protection circuit that is capable of detecting and responding to an excessive line current condition fast enough so as to insure that standard DAA line interface components used within a PCMCIA modem card are not damaged. 
     Another object of the present invention is to provide a line-current protection circuit that is capable of detecting and not responding to a transient signal present on the telephone line. 
     Still another object of the present invention is to provide a line-current protection circuit which is automatically reset after the excessive line current condition is removed, so that there is no need to physically remove or otherwise replace circuit components. 
     A further object of the present invention is to provide a line current protection circuit capable of indicating the presence of an excessive line current condition allowing the PCMCIA modem user to be aware of the condition and the reason for the modem&#39;s inoperability. 
     A yet further object of the invention is to provide a line current protection circuit capable of operating in a plurality of hold-current modes and detecting when a telephone line, having characteristics outside of the parameters of the specific modes, is connected to a modem. 
     Additional objects and advantages of the present invention will become more fully apparent from the following detailed description taken in conjunction with the drawings and claims, or may be learned by the practice of the invention. 
     Briefly summarized, the foregoing and other objects are achieved with a novel line-current protection circuit. The protection circuit is particularly advantageous in that it may be used within small architecture modem configurations such as a PCMCIA-architecture modem card. The protection circuit is oriented for placement in series with the tip lead of a telephone subscriber loop once the PCMCIA modem is plugged into the telephone subscriber loop via a standard RJ-type, or similar, connection. When connected to the telephone line in this manner, a differential DC voltage is presented to the DAA line interface circuitry across the tip and the ring leads of the telephone line. The line protection circuit configures the DAA to operate in a linear mode for purposes of testing for the presence of excess current and voltage which could damage the DAA. If an excessive line current is presented to the DAA line interface circuitry, that condition will be detected by the protection circuit, which will then respond by automatically disconnecting the DAA circuitry from between the tip and ring leads. This automatic disconnection occurs rapidly, before any damage is caused to the DAA circuit electrical components. Further, the connection will not be permitted to be reestablished until such time as the excessive line current condition is removed. If excessive line current is not present, then the DAA is configured to operate in the mode as specified either by the user or manufacturer as originally configured. 
     The line-current protection circuit is comprised of a configurable hold circuit means for configuring the line interface circuit to operate between at least a first linear mode and a second nonlinear mode. The configurable hold circuit means is controlled by a controller means which is capable of reconfiguring the configurable hold circuit means and activating/deactivating a relay means, which is connected between the DAA interface circuitry and the tip lead of the telephone line, and which selectively provides for either an open or a closed connection between the DAA circuitry and the tip and ring leads of the telephone line. The protection circuit is further comprised of a sensing or detection means, which is capable of detecting the magnitude of the line current that is presented to the DAA line interface circuitry. The results of the line current evaluation in the sensing means is conveyed to the controller means for evaluation which then causes the relay means to either open or close, depending on the magnitude of the line current. Thus, if the line current exceeds a predetermined maximum, such as approximately 120 mA, the current sensing or detection means will sense that condition and therefore cause control circuit means to direct the relay means to open the circuit connection between tip and ring leads. In this manner, excessive current does not reach the DAA line interface circuitry. When the detected current is not excessive, the controller means reconfigures the configurable hold circuit means to operate in the originally configured or operating mode. 
     In a preferred embodiment of the protection circuit of the present invention, the detection means is comprised of a current sensing means which is for providing an excessive line current signal when the magnitude of the line current presented to the DAA circuitry exceeds a predetermined magnitude. 
     Also in the preferred embodiment of the protection circuit of the present invention, the controller circuit means is comprised of executable instructions that perform steps of configuring the configurable hold circuit means into a test mode, which in the preferred embodiment is the linear mode, when the configurable hold circuit means is not already configured to the linear mode; detecting excessive line current from a sensing means; disconnecting the line interface circuit from the telephone line when excessive line current is detected; and when excessive line current is not detected, configuring the configurable hold circuit means into an operating mode, either linear or non-linear, as originally selected by the user or programmed by the manufacturer or other party. 
     The protection circuit may further include a backup fuse means, also connected between the DAA interface circuitry and the tip lead of the telephone line, which is for opening the connection between the DAA interface circuitry and the tip and ring leads if the line current were to exceed a second predetermined maximum magnitude. This fuse means acts as a backup protection in the event that the components making up the relay means or detector means were to fail. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to more fully understand the manner in which the above-recited and other advantages and objects of the invention are obtained, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope, the invention in its presently understood best mode for making and using the same will be described with additional specificity and detail through the use of the accompanying drawings in which: 
     FIG. 1 is a perspective view of one environment in which the protection circuit of the present invention may be used; 
     FIG. 2 is an exploded perspective view, illustrating a partial cross-sectional view of a PCMCIA modem card, the circuit components contained thereon, and the manner in which the card typically is connected to an external telephone subscriber loop, including the incoming tip and ring leads of an RJ-11 6-position connection; 
     FIG. 3 is a functional block diagram which schematically illustrates the primary components of one presently preferred line-current protection circuit of the present invention; 
     FIG. 4 is a detailed electrical schematic diagram which illustrates, as an example, the presently preferred embodiment and presently understood best mode for implementing the line-current protection circuit of the present invention; and 
     FIG. 5 is a flowchart depicting the processing steps for configuring the circuit for detecting excessive line current and reacting accordingly and returning to an operational configuration when appropriate, in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
     Referring now to FIG. 1, illustrated is a typical environment in which the line-current protection circuit of the present invention can be used. A portable computer, designated generally at  10 , is shown as having a PCMCIA socket  12 . The PCMCIA socket  12  is designed to be capable of receiving any PCMCIA-architecture compliant card, as for instance the PCMCIA modem card illustrated and designated generally at  14 . As is shown, modem card  14  is a standard PCMCIA card, having a plastic type enclosure  16  in which is disposed a printed circuit board  18  (shown in FIG. 2) onto which is placed the modem&#39;s various electrical components  20  (also shown in FIG.  2 ). Formed along one edge of the PCMCIA modem card  14  is an edge connector  22 , which is designed to be detachably received within a corresponding connector slot (not shown) disposed within the PCMCIA socket  12 . In this way, the PCMCIA modem card  14  is electrically interfaced with the portable computer  10 . 
     Formed on the opposite edge of the PCMCIA modem card  14  is a standard RJ-11 socket-connector  24 , which is designed to detachably receive a standard RJ-11 plug  26  and telephone wire  28  assembly. The opposite end of the wire  28  is also equipped with a RJ-11 plug  26 ′, which is then detachably received within a RJ-11 telephone-jack  30 , i.e., as would commonly be found on the wall of a office, etc. The telephone-jack  30  is in turn electrically connected to the public telephone lines, either directly or via a Private Branch Exchange (not shown). In this way, the PCMCIA modem card  14  is electrically connected to a telephone subscriber loop. 
     Referring now to FIG. 2, illustrated is an enlarged perspective view of the PCMCIA modem card  14  of FIG.  1 . As generally discussed, modem card  14  is equipped with a standard RJ-11 socket connector  24 , designed to detachably receive a twisted-pair telephone wire  28  via a RJ-11 plug  26  assembly. It will be appreciated that this RJ-11 connection scheme, although described as an RJ-11 connector, could be any type of RJ-type connector. For instance, some PBX systems utilize an RJ-45 connector. 
     Typically, the telephone wire  28  will be comprised of six (6) separate wire leads. Although the number of wire leads may vary (for instance, there may be eight or four leads), there are generally a minimum of two center lead wires, referred to as a tip lead  32  and a ring lead  34 . As is further shown, the tip  32  and ring  34  leads are electrically interfaced with the circuit components  20  contained on the printed circuit board  18  contained within the PCMCIA enclosure  16 . As discussed above, a portion of the circuit components  20  comprise a portion of line interface circuitry, commonly referred to as a Data Access Arrangement circuit, which acts as an electrical interface between the telephone line and the rest of the modem circuitry. 
     Referring now to FIG. 3, a functional block diagram illustrating one presently preferred embodiment of the current invention is shown. The protection circuit, designated generally at  36 , comprises, by way of example, relay means for selectively providing an open or a closed connection between the DAA Line Interface Circuitry  38  and the tip and the ring leads  32 ,  34 ; and a current sensing or detection means, which detects the magnitude of any line current that is being supplied to the DAA line interface circuitry  38 . The control circuit means causes the relay means to open or close, depending on the magnitude of that line current. In one preferred embodiment, the detection means comprises, for example, a current sensing means for providing an “Excessive Line Current” signal when the magnitude of the line current presented to the DAA circuitry  38  via the tip lead  32  exceeds a predetermined magnitude, such as approximately 120 mA. A controller means receives the Excessive Line Current signal from the sensing means and causes the relay means to be either opened or closed depending on the status of the Excessive Line Current signal. The control means also configures the hold circuit means to operate either in linear or nonlinear modes. 
     With continued reference to FIG. 3, when the PCMCIA card  14  is detachably connected to a telephone subscriber loop, the tip  32  and the ring  34  leads are electrically connected across the DAA circuitry  38 , and a DC voltage potential V IN , indicated at  40 , will be present across the tip and ring leads  32 ,  34 . The magnitude of that voltage will vary, depending on the nature of the telephone subscriber loop to which the modem card  14  is connected. For example, if the subscriber loop is connected directly to the public telephone system, the voltage will be approximately 48 volts, limited by a series resistance of approximately 400 Ohms to 1750 Ohms. In that case, the magnitude of the resulting current I IN , shown as  42 , is limited to a maximum of approximately 120 mA. 
     If the subscriber loop is connected directly to the public telephone system in a country subscribing to the TBR21 line standard, the voltage will be approximately −48 volts, limited by a series resistance of approximately 400 Ohms to 2400 Ohms. In that case, the magnitude of the resulting current I IN , shown as  42 , is limited to a maximum of approximately 120 mA. Those skilled in the art appreciate that the voltage and current profile of the TBR21 standard defines a current wall (i.e., the origin of its description as the “non-linear” mode standard) at approximately 60 mA. Because of the resulting discontinuous non-linear voltage and current profile, the linear profile is selected for testing for an overvoltage condition due to the fact that the voltage and current for both linear and non-linear modes can generally be circumscribed by the single linear standard. Hence, during testing, only the linear mode is used and the originally configured or operating mode (i.e., linear of non-linear) is restored upon successful completion of the excessive line current test. 
     In contrast, if the modem card  14  is connected to the subscriber loop of a PBX system (not shown), the voltage V IN  can be anywhere in the range of 12 to 90 volts—without any resistance to the resulting current I IN . In that case, the magnitude of current I IN  that is supplied to the DAA circuitry  38  can exceed the desired maximum magnitude of approximately 120 mA, and thereby destroy the DAA circuit components, rendering the modem card  14  inoperative. 
     As is shown in FIG. 3, in one presently preferred embodiment, detection means is comprised of, the functional components contained within the dotted box  44 . In this particular embodiment, detection means is comprised of a current sensing means, as for example current sensing circuitry  46 , and controller or control circuit means, as for example programmable digital processor  48 . 
     With continued reference to FIG. 3, the tip lead  32  of the incoming telephone line  28  is connected in series with the current sensing circuitry  46 , as is schematically shown at line  50 . The current sensing circuitry  46  is thereby positioned to detect the magnitude of line current I IN  being supplied to the DAA circuitry  38 . If the magnitude of that current exceeds a predetermined maximum, which in the preferred embodiment is approximately 120 mA, the current sensing circuitry  46  will generate an Excessive Line Current signal, as is schematically shown at line  52 . The Excessive Line Current signal is then electrically connected, via line  52 , to an input of the programmable digital processor  48 . 
     Using programming techniques that are well known in the art, the digital processor  48  programmably monitors the status of the Excessive Line Current signal present on line  52 . If the line current I IN    42  exceeds the maximum allowable level of 120 mA, the current sensing circuitry  46  will indicate the fault condition by asserting the Excessive Line Current signal input to the digital processor  48 . The digital processor  48  is programmed to respond to the fault condition by asserting a Relay Control Signal, as is indicated at line  54 . 
     A configuration path or control line(s)  62  provide a signaling path between the control circuit means and the line interface circuitry, and more particularly, the configurable hold circuit  66 . Line(s)  62  facilitate the configuration of configurable hold circuit  66  into a plurality of modes, most notably a linear mode and a nonlinear (e.g., TBR21) mode. While FIG. 3 depicts line  62  as being a single control line, it is contemplated that the actual configuration of configurable hold circuit may take the form of the transferring of a command or other data set up provision from the control circuit or processor means and the configurable hold circuit means. 
     FIG. 3 further illustrates how protection circuit  36  is comprised with a relay means, as for example relay circuitry  56 . Relay circuitry  56  is also connected in series with the tip lead  32 , as is schematically illustrated at line  58 . Relay circuitry  56  can be operated so as to selectively open or close the circuit connection between the DAA circuitry  38  and the tip and the ring leads  32 ,  34 . The relay circuitry  56  will be opened or closed depending on the status of the Relay Control Signal generated by the digital processor  48 , as is shown at line  54 . Thus, in this particular embodiment, if Relay Control Signal is asserted, thereby indicating an excessive line current fault condition (i.e., line current I IN    42  exceeds 120 mA), relay circuitry  56  will open, and prevent the line current I IN  from damaging the DAA circuitry  38 . This condition is otherwise referred to as an “On-Hook” state, wherein the PCMCIA modem card  14  is effectively disconnected from the telephone line. When the line-current is below the maximum allowable amount (i.e., the modem card  14  is plugged in to a telephone socket having a standard voltage present across the tip and the ring leads  32 ,  34 ), the Relay Control Signal will not be asserted by digital processor  48 , and the relay circuit  56  will close, thereby completing the circuit connection between the DAA circuit  38  and the tip and ring leads  32 ,  34 , as is schematically shown at line  60 . This condition is otherwise known as an “Off-Hook” state, wherein the modem card  14  can send and receive data via the telephone line. 
     An important advantage of the protection circuit  36  is its capability to automatically and dynamically open or close the circuit connection depending on the testing configuration and resulting magnitude of the line-current I IN . Advantageously, when the fault condition is removed (i.e., the user plugs the modem card into a telephone-jack not having an excessive line-current I IN ), the modem will function correctly. There is no need to have components, such as burned fuses, replaced. Further, in the preferred embodiment, the digital processor  48  is the processor which is used to perform the PCMCIA modem&#39;s general command processing functions and the host (e.g., the portable computer utilizing the PCMCIA modem card) interface functions. Thus, when a fault-condition does occur, and the digital processor causes the relay circuitry  56  to open, the digital processor  48  will also be programmed to cause a text string signal to be displayed on the screen of the host computer. This text screen will inform the user that an excessive line-current condition exists at the telephone-jack in which the PCMCIA-modem is connected. In this way, the user is alerted as to why the modem is not functioning. 
     Referring now to FIG. 4, an electrical schematic of one presently preferred embodiment of the protection circuit  36  is shown. The current sensing circuitry ( 46  in FIG. 3) is comprised of, for example, a sensing resistor R 1  placed in series with the tip lead  32  so that line-current I IN  passes therethrough. It will be appreciated that the sensing resistor R 1  could alternatively be placed in series with the ring lead  34 , and still have line-current I IN  pass therethrough. Connected across the sensing resistor R 1  is an optoelectronic device, such as photo-coupler U 2 . Thus, line-current I IN  will generate a voltage across sensing resistor R 1 , the magnitude of which will vary in proportion with the magnitude of I IN  in compliance with Ohm&#39;s law. As long as I IN  is sufficiently low (i.e., below approximately 120 mA), the photo-responsive transistor portion of U 2  will not conduct, and the Excessive Line Current signal output will stay grounded through pull-down resistor R 2 . Once the magnitude of line-current I IN  exceeds an allowable maximum, as for example 120 mA, the resulting voltage across R 1  will cause the corresponding photo-emitting diode of device U 2  to turn on. The photo-responsive transistor portion of U 2  will respond by conducting, and thereby asserting the Excessive Line Current Signal, by pulling it high. 
     False signals, such as transient spikes on the output of photo-coupler U 2 , are avoided by placing a capacitor C 1  in parallel with the pull-down resistor R 2 . Alternatively, capacitor C 1  could be placed in parallel with sensing resistor R 1 , on the photo-responsive transistor of U 2 . Also, C 1  can be eliminated completely and any false signals can instead be detected by the digital processor/microcontroller U 3 . In this embodiment, the microcontroller U 3  will be programmed to check for the presence of the Excessive Line Current Signal twice. If the signal is detected, the processor will wait for an amount of time, such as one millisecond, and then recheck the status of the signal. In this way, the microcontroller U 3  can insure that the first occurrence of the signal was not merely a transient, false signal, as may be caused by line noise or other conditions, or that may result when the modem is connected to certain analog PBXes. 
     The photo-coupler U 2  shown in FIG. 4 is available as a single component, mounted in a small outline package, which can easily be mounted within the PCMCIA card  14  enclosure. Further, the device U 2  provides the current sensing function, and yet has galvanic isolation from the telephone line, and thus has no need for an earth-ground voltage measurement reference and is largely impervious to large common-mode 60 Hz noise signals. One presently preferred photo-coupler U 2  is the device made by NEC Corporation and identified by the part number PS2705-1. 
     As discussed, the Excessive Line Current Signal is supplied to an input of a digital processor ( 48  in FIG.  3 ). In the preferred embodiment, the digital processor  48  is comprised of a microcontroller U 3 , which is used to perform the PCMCIA modem&#39;s general command processing functions and the host (e.g., the portable computer utilizing the PCMCIA modem card) interface functions. A preferred microcontroller U 3  is available as part of a chip-set manufactured and sold by Rockwell Corporation and known as the RC96ACL/RC144ACL integrated data/fax/voice modem device set family. This microcontroller U 3  is more particularly known as a Rockwell C29 or C39 microcomputer. It will be appreciated that various other digital microprocessors, which are well known in the art, could also be used. 
     Microcontroller U 3  is programmed, using techniques that are well known in the art, to continuously monitor the Excessive Line Current signal output by Opto-Coupler U 2 . When line-current I IN  exceeds approximately 120 mA, the signal is asserted, and the microcontroller responds by asserting the Relay Control signal, which is electrically connected to relay circuitry ( 56  in FIG.  3 ). The microcontroller is also programmed to execute the steps and instructions as detailed in FIG. 5, including the capability of reconfiguring the configurable hold circuit from an operational configuration to a testing configuration and back, responding to off-hook conditions (e.g., ring signal receipt and user invoked call initiation), and traditional call processing. 
     The relay circuitry is comprised of, for example, a solid-state relay device, which in the preferred embodiment is a photo-MOS relay U 1 , sold by Aromat and known by the part number AQV214S. The Relay Control Signal is input to the relay device, as is schematically shown at line  32 , so as to control the current flow through the LED portion of the photo-MOS relay U 1 . Thus, when a fault condition exists (i.e., excessive line current), microcontroller U 3  asserts Relay Control Signal, thereby causing a non-conduct state through the relay device and resulting in an open circuit, or “On-Hook” state. In this state, line-current I IN  does not reach the DAA line circuitry. In contrast, when Relay Control Signal is not asserted, the relay is in a closed position and the PCMCIA modem will be in an “Off-Hook” state, and function as intended. 
     Importantly, the protection circuit  36 , using the above-described components, is capable of detecting and responding to an excessively high line-current in the range of from 1 to 2 milliseconds. This insures that none of the DAA line interface circuit components are damaged by lingering high current. 
     FIG. 4 also illustrates the preferred embodiment as having a backup fuse means, such as fuse  64 , which is connected in series with the tip lead  32  and the DAA line interface circuitry. Fuse  64  is for providing backup protection to the DAA line interface circuitry, and will open up the circuit connection in the event that the other circuit components fail, and the line-current I IN  exceeds a second predetermined maximum magnitude, such as 250 mA. 
     FIG. 5 depicts a flowchart for performing the steps of testing the parameters of a telephone line that is connected to the line interface circuit in order to determine if the parameters of the telephone line are within the operating parameters of the line interface circuit. The present invention addresses the problem of detecting excessive wattage when a DAA is configured and designed for connection with a linear or a non-linear (e.g., TBR21) telephone line, but could be damaged by the erroneous connection to a telephone line exhibiting other electrical standards such as a PBX connection. 
     The means for executing the steps of the flowchart as depicted in FIG. 5 are suitable for a computing environment in which the invention may be implemented. Although not required, the invention, and the steps of the flowchart in general, are implemented as executable code or instructions for execution by a microcontroller. While a microcontroller is depicted above, those skilled in the art appreciate that executable code may reside in a form of memory. While no such memory is expressly shown, it is well known in the art that memory may be included within embedded microcontrollers or memory in the form of ROM and RAM may be external to the microcontroller and is not shown in the present invention in an attempt to not confuse the more inventive aspects of the present embodiments. 
     Those skilled in the art will appreciate that the invention may be practiced in network computing environments with many types of computer system configurations, including personal computers, hand-held devices, multi-processor systems, or programmable electronics. The microcontroller as shown above executes instructions for caring-out the steps of the flowchart of FIG.  5  and such computer-executable instructions may be stored on computer-readable media including ROM, RAM, other magnetic media such as disks, optical media such as optical disks, as well as other imaginable configurations. All such embodiments are considered within the scope of the present invention. 
     While the process flow is generally shown as process  70  in FIG. 5, it is important to remember that such processing, in the preferred embodiment, is performed by digital processor or microcontroller U 3 , as shown in FIG.  4 . It should also be appreciated that other implementations such as state machines could be implemented to carry out the steps as depicted in FIG.  5 . FIG. 5 depicts a process  72  in a modem which is originally configured by either the user or another party, such as a manufacturer, to initialize the line interface circuitry in a compatible manner for interaction with the target telephone line standard. In the preferred embodiment, the original or operational configuration takes the form of either a linear or a non-linear (e. g., TBR 21) hold current specification. Such original configuration information is preferably stored in non-volatile memory and is referenced by the digital processor or microcontroller U 3 , all of FIG.  4 . 
     In the preferred embodiment of the present invention, a linear configuration of a current sensing means is implemented. Therefore, in order to configure a hold circuit into a compatible test state, the configurable hold circuit of line interface circuitry  38  (FIG. 4) is programmed, in step  74 , to operate in the linear mode for the duration of the excessive line current testing process. Those skilled in the art, appreciate that the method for programming the configurable hold circuit may take the form of a single control line  62  (FIGS. 3-4) or may also be command driven over a command bus implementation. The actual implementation of choice is dependent upon the specific interface available between the line interface circuitry and the digital process or microcontroller. 
     Subsequent to configuration step  72  which may be performed by various entities including a user who self configures a modem or a manufacturer who preconfigures a modem destined for a particular country or region, two additional steps are performed prior to actually measuring the current and voltage as presented by the telephone line to the modem. FIG. 5 depicts a first step  74  which verifies that the hold circuit is in a linear mode or alternatively, when the hold circuit was originally configured into a non-linear mode, configures the hold circuit into a linear mode. This step is performed since the current sensing means of the preferred embodiment is configured to evaluate the current and voltage conditions of a linear telephone line interface. It should be pointed out that non-linear telephone line characteristics, for the purposes of evaluating in a time-efficient manner, exhibit current and voltage characteristics that can also be sufficiently evaluated using a linear hold current profile. 
     The subsequent step, step  76 , is largely implemented as a wait state that awaits the initiation from either an off-hook directive by a user, or the receipt of a ring signal from the telephone line. Such an indication is a directive to an off-Hook process, depicted as process  78 , which performs the requisite switching of the hold circuit into a conductive state allowing the voltage and current from the telephone line to pass to the hold circuit via the current sensing means. While steps  74  and  76  are depicted in FIG. 5 occurring in a specific sequence, a reverse sequencing of steps  74  and  76  also function equally well in that the off-hook directive could initiate the reversion of the hold circuit into the linear mode. 
     FIG. 5 depicts a query step  80  which performs the actual detection of excessive line current through the current sensing means. Furthermore, in step  80  when excessive line current is detected, processing passes to a step  82  which performs the process of returning the line interface circuitry into an on-hook state. Such a process is performed by the control circuit means, and in the preferred embodiment, is accomplished through the switching of a relay means into an open circuit state. 
     When query state  80  does not detect an excessive line current state, processing passes to a query state  84  which determines if the original country configuration, step  72 , was originally preconfigured into a configured linear mode, or whether the hold circuit needs to be reconfigured into a non-linear mode, (e.g.,TBR 21 mode) as depicted in step  86 . Following the accurate reconfiguration of the hold circuit into its original or operating mode, and in the absence of excessive line current, call processing resumes in step  88 . 
     In summary, the line-current protection circuit as described above addresses the previously unsolved problems in the prior art. The line-protection circuit, and method, insures that the line interface circuitry of a modem is not damaged when a modem is inadvertently connected to a telephone line having an excessive line current. Also, the protection circuit automatically resets when the fault conditions removed, and thus electrical components do not have to be replaced, or is any servicing required on the modem card. When the fault condition does exist, the protection circuit is capable of causing a message to be displayed, thereby notifying the user of the error condition. Finally, the protection circuit is able to distinguish between short duration, transient signals, and valid excessive line current conditions. 
     The process, as described in the present invention, finds application in multimode hold circuit configurations that are available in the form of global or configurable line interface circuits such as DAAs. The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restricted. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.