Telephonic communications – Telephone line or system combined with diverse electrical... – Having transmission of a digital message signal over a...
Reexamination Certificate
1999-01-06
2001-08-28
Smith, Creighton (Department: 2742)
Telephonic communications
Telephone line or system combined with diverse electrical...
Having transmission of a digital message signal over a...
C379S002000, C379S027010, C379S222000
Reexamination Certificate
active
06282271
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to computer modems and communication devices and, more specifically, to a device that couples between a modem and a telephone line to prevent damage to the modem as a result of dangerous line conditions.
2. Description of the Related Art
A modem is a device that modulates data received from a computer so that it can be transmitted via a telephone line and demodulates data received from a telephone line so that it can be provided to a computer. A modem may be integrated internally into a computer or may be external in a separate enclosure and connected to a computer by a cable. To connect the modem to the telephone line, a user typically plugs one end of a cable into the modem and the other end into a telephone wall jack. Telephone wall jacks are commonly provided in homes, businesses, hotels and other buildings to provide access to the public telephone network. In the United States, wall jacks conform to the standard known as RJ-11. A RJ-11 jack has four pins that mate with four pins of a RJ-11 plug on the cable.
Plugging a modem into a wall jack can, under certain circumstances, cause damage to the modem. Many wall jacks used in businesses are not directly coupled to the public switched telephone network (PSTN), but rather are coupled to a private branch exchange (PBX) system, which is in turn coupled to the PSTN. A PBX system is owned by the business and provides telephone service to many extension lines in various locations, intercom service between extensions, and myriad other communication features. All modern PBX systems are digital. That is, some of the signals they provide at the wall jack corresponding to each extension are digital. The digital signals are used for controlling the operation of the digital telephone that is plugged into the wall jack. If a user plugs an ordinary modem directly into the wall jack of a PBX extension, the modem can be damaged by the digital signals, which typically provide current in excess of 500 milliamps (mA). Modems are designed to receive only the signals provided at the wall jack coupled directly to the PSTN, and those signals typically provide only tens of milliamps of loop current.
A modem can be damaged by other causes as well. Plugging a modem designed in accordance with the telephone line standards of one country into a wall jack in another country could damage it if the voltage, current or other parameters of the two countries'standard telephone line signals are incompatible. Modems can also be damaged by power surges that sometimes occur as a result of lightning striking a telephone line or other events.
Commercially available products exist that attempt to address some of the above-described problems. One such product is a line tester having warning indicator lights that illuminate if a user plugs it into a wall jack that provides current in excess of about 90 mA or that provides signals of polarity opposite that of the modem. Although most modems will operate regardless of the signal polarity of the telephone line, the operation of certain modems may be degraded if the modem polarity is opposite the telephone line polarity. Another such product is similar in that it has warning indicator lights that illuminate if a user plugs it into a wall jack that provides excessive current or that provides signals of polarity opposite that required by modems. In addition, it has another jack into which the user can plug the modem cable if the indicator light indicates safe conditions by remaining un-illuminated. It also has a switch that a user can toggle to reverse the polarity of the wall jack with respect to the polarity of the modem jack. It further has a surge-protection circuit that limits the current to the modem. Still another such product has a surge-protection circuit that maintains a constant loop current, a reversed-polarity indicator light, and a switch for reversing the polarity.
The above-mentioned products are inconvenient to use and require the user to perform a number of steps. Observing an indicator light and then plugging in a modem only if it remains un-illuminated requires some degree of understanding by the user of the excessive current problem and involves multiple steps. Similarly, observing an indicator light and then toggling a switch to reverse polarity requires some degree of understanding by the user of the polarity problem and involves multiple steps. Furthermore, the surge-protection circuits of these products may provide inadequate protection for the modem under certain circumstances. It would be desirable to provide a modem protection device that provides a higher level of protection and is easy to use.
These problems and deficiencies are clearly felt in the art and are satisfied by the present invention in the manner described below.
SUMMARY OF THE INVENTION
The present invention relates to an electronic device and method for protecting a modem against harmful telephone line signals or reversed polarity telephone line signals. The device includes a telephone line interface and a modem interface that include suitable connectors. The telephone line interface is coupled to a telephone line in a suitable manner, such as by a user connecting them with a cable. The modem interface is similarly coupled to the communication port of a modem in a suitable manner, such as by a user connecting them with a cable or by an integral, hard-wired connection. The device is connected to both the telephone line and the modem and couples them in a manner that protects the modem against potentially harmful telephone line conditions. Operation of the device as described below begins automatically in response to a user connecting the device to the telephone line. A user need not press any buttons or perform other actions to use the device.
In embodiments of the invention, the apparatus may include a measurement circuit that determines whether a value of a predetermined electrical parameter of a signal provided by the telephone line is within predetermined limits. For example, it may determine whether the loop current is less than a predetermined maximum that is harmful to most modems. Such an apparatus may further include an indicator circuit that indicates to the user whether the value of that parameter is within those predetermined limits. For example, the indicator circuit may cause a light to illuminate if the current exceeds the predetermined maximum.
In embodiments of the invention, the apparatus may include a measurement circuit of the type described above and an isolation circuit that electrically isolates the modem from the telephone line if the value of the parameter is not within the predetermined limits. For example, the isolation circuit may isolate the modem from the telephone line if the current exceeds the predetermined maximum.
In embodiments of the invention, the apparatus may include a polarity detection circuit that determines the polarity of the telephone line with respect to the polarity of the modem communication port. If the polarity of the telephone line is opposite the polarity of the modem communication port, modem performance could be degraded, and a polarity reversal circuit reverses the polarity of the telephone line with respect to the polarity of the modem communication port, thereby improving the potential performance of the modem.
The above-described features of embodiments of the present invention may be included in various combinations with each other or in combination with other features. Such other features may include, for example, an off-hook circuit that automatically initiates operation of the measurement and indication circuits when an off-hook condition is detected at the modem communication port. Such other features may also include a power control circuit that disconnects the measurement circuit from the telephone line when such operation is completed, thereby reducing the load on the telephone line.
The foregoing, together with other features and advantages of the present invention,
Gutzmer Alan A.
Wasemiller George J.
Brown Martin Haller & McClain
Gutzmer Enterprises, Ltd.
Smith Creighton
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