Circuits and methods for improved network interface circuit protection

A method of protecting a network interface circuit which operates in a constant-resistance mode or a constant-current mode within a communications network which provides a communications network voltage to the network interface circuit, the method comprising the steps of:

• setting an off-hook/on-hook circuit to an off-hook state wherein the 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;
• generating at least one feedback voltage which is proportional to the off-hook voltage;
• generating at least one status signal based on the feedback voltage;
• and
• controlling the state of the off-hook/on-hook circuit based on the status signals and the operating mode.

A method of protecting a network interface circuit connected to a communications network which provides a communications network voltage to the network interface circuit comprising the steps of:

• setting an off-hook/on-hook circuit to an off-hook state wherein the 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;
• generating an input voltage which is proportional to the off-hook voltage;
• generating an over-voltage which is proportional to the off-hook voltage;
• clamping the input voltage to a first predetermined value if the input voltage exceeds the first predetermined value;
• setting the off-hook/on-hook circuit in the on-hook state if a constant-resistance mode is selected and the input voltage is clamped to the first predetermined value;
• clamping the input voltage to a second predetermined value if the input voltage exceeds the second predetermined value;
• and
• setting the off-hook/on-hook circuit in the on-hook state if a constant-current mode is selected and the over-voltage exceeds a third predetermined value.

Apparatus for protecting a network interface circuit which operates in a constant-resistance or a constant-current operating mode within a communications network which provides a communications network voltage to the subscriber-end device, the means comprising:

• means for setting an off-hook/on-hook circuit to an off-hook state wherein the 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;
• means for generating at least one feedback voltage which is proportional to the off-hook voltage;
• means for generating at least one status signal based on the feedback voltage;
• and
• means for controlling the state of the off-hook/on-hook circuit based on the at least one status signal and the operating mode.

Apparatus for protecting a network interface circuit connected to a communications network which provides a communications network voltage to the network interface circuit comprising:

• means for setting an off-hook/on-hook circuit to an off-hook state wherein the 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;
• means for generating an input voltage which is proportional to the off-hook voltage;
• means for generating an over-voltage which is proportional to the off-hook voltage;
• means for clamping the input voltage to a first predetermined value if the input voltage exceeds the first predetermined value;
• means for setting the off-hook/on-hook circuit in the on-hook state if a constant-resistance mode is selected and the input voltage is clamped to the first predetermined value;
• means for clamping the input voltage to a second predetermined value if the input voltage exceeds the second predetermined value;
• and
• means for setting the off-hook/on-hook circuit in the on-hook state if a constant-current mode is selected and the over-voltage exceeds a third predetermined value.