Current mode ring trip detector
AssigneeHarris Corporation
InventorGerald Michael Cotreau
Life of this patent
Estimating the term…
Abstract
A circuit and method for sensing in current mode when a telephone has been answered in a telephone system in which an answer is indicated when a dc component of a signal across a resistor exceeds a predetermined reference current. Currents sensed at the terminals of the resistor are combined to provide a third current that includes the dc component, without attenuation of the dc component. A filter separates the dc component from the third current so that the dc component can be compared to the reference current.
Claims16
A circuit for sensing when a telephone has been answered in a telephone system in which an answer is indicated when a dc component of a ring signal across a resistor exceeds a reference current, the circuit comprising:
- combining means for combining first and second currents sensed at first and second terminals of the resistor to provide a third current that includes the dc component;
- filter means for separating the dc component from said third current so that the dc component can be compared to the reference current;
- and
- means for clamping one of said first and second currents to ground when the ring signal is present and a telephone is not being rung.
A circuit for determining when a telephone has been answered in a telephone system in which a ring voltage is provided across a resistor when a telephone is to be rung, an answer being indicated when a dc component across the resistor exceeds a reference, the circuit comprising:
- a first voltage-to-current converter for providing a first current proportional to a first voltage at an input to the resistor;
- a second voltage-to-current converter for providing a second current proportional to a second voltage at an output from the resistor;
- means for combining said first and second currents to provide a third current proportional to the difference between said first and second voltages;
- and
- means for clamping a voltage output from said second voltage-to-current converter so that the voltage output cannot go higher or lower,
- whereby said third current may be used to determine whether the telephone has been answered.
A circuit for indicating when a telephone has been answered in a telephone system in which a ring voltage is provided across a resistor when the telephone is rung, the answer being indicated when a dc signal across the resistor exceeds a reference current, the circuit comprising:
- a first voltage-to-current converter for providing a first current proportional to a first voltage at a first terminal of the resistor;
- a first switch for providing said first current to one current mirror in a first pair of current mirrors responsive to a polarity of said first current,
- said first pair of current mirrors providing said first current to a first node in the circuit;
- a second voltage-to-current converter for providing a second current proportional to a second voltage at a second terminal of the resistor, said second current being provided to said first node and combined with said first current to form a third current;
- and
- a second switch for providing said third current to one current mirror in a second pair of current mirrors responsive to a polarity of said second current, said second pair of current mirrors providing said third current to means for comparing a dc component of said third current to the reference current.
A clamping circuit for a telephone system in which a ring voltage is provided across a resistor when a telephone is being rung, the clamping circuit comprising:
- a connection to current at a terminal of the resistor,
- a first transistor switch for selectively clamping said connection to ground when a ring voltage is provided and a telephone is not being rung;
- and
- a first current source connected to a base of said first transistor switch for operating said first transistor switch.
A method of indicating when a telephone has been answered in a telephone system in which a ringing generator voltage is provided across a resistor at a telephone exchange when the exchange rings the telephone, the answer being indicated when a dc component of a current across the resistor exceeds a reference current, the method comprising the steps of:
- (a) converting a first voltage at an input to the resistor to a first current proportional to the first voltage;
- (b) converting a second voltage at an output from the resistor to a second current proportional to the second voltage;
- (c) combining the first and second currents to provide a third current proportional to the difference between the first and second voltages by
- providing the first current to a first circuit node with a pair of current mirrors so that the first current has a polarity related to the polarity of the first voltage,
- providing the second current to the first circuit node with a pair of current mirrors so that the second current has a polarity related to the polarity of the second voltage;
- and
- (d) comparing the dc component of the third current to the reference current to determine whether the telephone has been answered.
An improved method of indicating when a telephone has been answered in a telephone system in which a ringing voltage is provided across a resistor when the telephone is to be rung, the answer being indicated when a dc component across the resistor exceeds a reference current, the improvement comprising the steps of:
- (a) adding currents sensed on either side of the resistor to provide a third current that includes the dc component;
- (b) separating the dc component from the third current so that the dc component can be compared to the reference current;
- and
- (c) selectively clamping a terminal of the resistor to ground when a ring voltage is provided and a telephone is not being rung.
Dependencies are inferred from the claim text.