Circuits and methods for improved network interface circuit...

Details Circuits and methods for improved network interface circuit... Circuits and methods for improved network interface circuit...

1998-03-25

2001-08-14

Chan, Wing F. (Department: 2643)

Telephonic communications

Subscriber line or transmission line interface

Protective circuit

C379S399010, C379S093050

Reexamination Certificate

active

06275583

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the field of communications, and more particularly to the field of networked communications.
BACKGROUND OF THE INVENTION
Communications networks, such as a public switched telephone network (PSTN), may provide services to a subscriber-end device, such as a telephone or modem, by allowing the subscriber-end device to connect to the communications network. For example, a telephone may connect to the PSTN by entering an active state thereby enabling communication between the telephone and the PSTN. The active state may be referred to as off-hook and the inactive state may be referred to as on-hook.
The subscriber-end device may be required to exhibit certain electrical characteristics when connected to the communications network. Moreover, different communications networks may require different electrical characteristics and exhibit different failure modes. For example, the electrical termination characteristics of some countries may be met using a constant-resistance termination (constant-resistance mode) while other countries, such as France, may be met using a constant-current limiting termination (constant-current mode). Many subscriber-end devices, therefore, use a programmable network interface circuit (NIC) to provide the electrical characteristics for the particular communications network to which the subscriber-end device is connected.
FIG. 1
graphically illustrates the electrical characteristics of two exemplary communications networks. The line
101
shows the electrical characteristics, referred to as constant-resistance mode, of a telephone connected to a PSTN wherein the current is controlled by the NIC to present a constant resistive load to the PSTN in relation to the voltage provided by the communications network. The constant-resistance mode may present a low-resistance load of about 100 ohms or high-resistance load of about 300 ohms. The line
102
shows the electrical characteristics, referred to as constant-current mode, of a telephone connected to a PSTN wherein the current is kept constant over a range of voltages provided by the communications network.
The communications networks described above may not perform as specified. For example, a PSTN operating in constant-resistance mode may be specified to source a current not to exceed 120 milliamperes (ma) as shown by the dotted line in FIG.
1
. If, however, a communications network failure occurs, the source current may exceed 120 ma, causing excessive power dissipation and thereby possibly damaging the NIC. Similarly, a PSTN operating in constant current mode may be specified to provide a network voltage not to exceed 70 volts. If, however, the voltage provided by the communications network exceeds the specified maximum voltage, the NIC may be damaged due to excess power dissipation.
Alternatively, a user may inadvertently connect the NIC to a communications network which is incompatible with the electrical characteristics described herein. For example, Private Branch Exchange (PBX) networks may provide a relatively large current to connected subscriber-end devices. The physical connection to the PBX, however, is such that a subscriber-end device intended for a PSTN may be mistakenly connected to the PBX. Consequently, the current provided by the PBX may cause excessive power dissipation in the NIC, thereby possibly damaging the NIC.
As illustrated in
FIG. 2
, existing systems may use an in-line device
202
to protect the NIC
201
from the damage described above. The in-line device
202
may be a fuse or resettable device (such as a relay) which de-couples the communications network
203
from the NIC
201
before NIC
201
is damaged. Fuses may need to be replaced once the failure occurs. Resettable devices may need to be reset once the failure occurs. Fuses and resettable devices may also be packaged in such a way so as to be undesirable for some applications requiring relatively high levels of integration. For example, a fuse or relay may be undesirable for use in an integrated modem of a laptop computer due to space and power restrictions. Moreover, a fuse or resettable device may not readily provide an indication that a failure has or is about to occur. For example, if the PSTN sources too much current, the user may be unaware that the NIC may soon fail unless the NIC is de-coupled from the communications network. In view of the above, there is a need for an improved NIC protection.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide improved network interface circuit protection.
It is another object of the present invention to allow reduced reliance on user intervention to avoid damage to the network interface circuit.
These and other objects are provided by a network interface circuit which operates in a constant-resistance mode or a constant-current mode within a communications network that provides a communications network voltage to the network interface circuit. An off-hook/on hook circuit provides an off-hook voltage in the off-hook state that is substantially equal to the communications network voltage and wherein the off-hook/on-hook circuit provides an on-hook voltage in the on-hook state. At least one feedback voltage is generated that is proportional to the off-hook voltage. At least one status signal is generated based on the feedback voltage. The state of the off-hook/on-hook circuit is controlled using the status signals and the operating mode. The present invention can decouple the network interface circuit from the communications network when the status signals indicate that the network interface circuit may be subject to excessive power dissipation.
In one embodiment, a network interface circuit includes an off-hook/on-hook circuit, responsive to the communications network, that provides an off-hook voltage substantially equal to the communications network voltage in an off-hook state and that provides an on-hook voltage in an on-hook state. A voltage controlled current source provides a load current which is proportional to a communications network voltage provided by the communications network. A divider circuit, responsive to the off-hook/on-hook circuit, provides an input voltage and produces an over-voltage, wherein the input voltage is proportional to off-hook or on-hook voltage. A clamping circuit limits the input voltage to one of a first or second predetermined value based on the operating mode and generates a current limited voltage. A control circuit, responsive to the over-voltage and the current-limited voltage and the operating mode and responsive to the off-hook/on-hook circuit, sets the network interface circuit in the off-hook state or the on-hook state based on the input voltage, the current-limited voltage, and the operating mode.
The present invention can reduce reliance on user intervention to avoid damage to the network interface circuit by controlling the state of the off-hook/on-hook circuit based on the operating mode and the input voltage. The network interface circuit may therefore be de-coupled from the communications network under the control of the present invention. According to the prior art, the user may need to remove the network interface circuit from the communications network manually. Alternatively, the user may need to replace or reset an in-line device to re-initiate a connection. Moreover, the user may not receive any indication of a fault which may lead to a loss of service until the user determines that the in-line device requires attention.
The present invention also may be embodied in a integrated fashion thereby providing protection for a network interface circuit in situations for which conventional in-line devices may not be well suited. For example, the present invention may be utilized to provide convenient protection to a network interface circuit within a laptop computer. Conventional in-line devices may not be well suited for use in laptop computers due to power and space requirements. Moreover, a laptop computer modem may not be readi

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