Telephonic communications – Subscriber line or transmission line interface – Protective circuit
Reexamination Certificate
2000-03-14
2004-08-17
Tsang, Fan (Department: 2645)
Telephonic communications
Subscriber line or transmission line interface
Protective circuit
C379S377000, C379S393000, C379S399010, C361S119000
Reexamination Certificate
active
06778664
ABSTRACT:
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
Price Tim Urry
Rollins Randy
Rupp Scott W.
3Com Corporation
Escalante Ovidio
Tsang Fan
Workman Nydegger
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