Subscriber circuit provided with a circuit for monitoring a subscriber instrument

• US 5,568,547 A
• Filed: 06/15/1994
• Issued: 10/22/1996
• Est. Priority Date: 06/15/1993
• Status: Expired due to Term

• Current Assignee: Renesas Electronics Corporation
• Original Assignee: NEC Corporation
• Inventors: Nishimura, Kouichi
• Primary Examiner(s): Brown, Thomas W.
• Assistant Examiner(s): WEAVER, SCOTT LOUIS
• Application Number:
  US08/261,281
• Time in Patent Office: 860 Days
• Field of Search: 379/350, 379/377, 379/382, 379/385, 379/399, 379/407, 379/413, 379/27, 379/32, 379/33
• US Class Current: 379/382
• CPC Class Codes: H04M 3/005 Interface circuits for subscriber lines current supply H04M19/00 and subgroups; supervisory, monitoring or testing arrangements H04M3/22 and subgroups; in key telephone systems H04M9/006
• CPC Class Codes: H04M 3/2272 Subscriber line supervision circuits, e.g. call detection circuits
• CPC Class Codes: H04M 3/30 for subscriber's lines , for the local loop

Abstract

A subscriber circuit comprising a monitor circuit operating with a power supply voltage supplied to a subscriber instrument and monitoring an operation of the subscriber instrument for generating a monitor signal indicative of the result of monitor, and a power supply voltage abnormality detecting circuit receiving the power supply voltage supplied to the subscriber instrument, for generating an active abnormality detection signal when the power supply voltage deviates from a predetermined voltage range. In response to the active abnormality detection signal, the monitor circuit is inhibited from outputting the monitor signal

Claims

1. A subscriber circuit comprising:

a hook monitor circuit for connection to a pair of subscriber lines for operating with a power supply voltage supplied to a subscriber instrument, said hook monitor circuit detecting a current flowing through said pair of subscriber lines in the form of a voltage and comparing said voltage with a first reference voltage, such that if said voltage is higher than said first reference voltage, said hook monitor circuit generates a monitor signal;

a power supply voltage abnormality detecting circuit for receiving said power supply voltage supplied to said subscriber instrument and for generating an active abnormality detection signal; and

means for responding to said active abnormality detection signal for inhibiting said hook monitor circuit from outputting said monitor signal;

wherein said power supply voltage abnormality detecting circuit includes a voltage dividing circuit receiving said power supply voltage supplied to said subscriber instrument and outputting a voltage-divided voltage, a level shift circuit receiving said voltage-divided voltage and outputting a level-shifted voltage, and a comparator comparing said level-shifted voltage with a second reference voltage and generating said active abnormality detection signal when said level-shifted voltage is higher than said second reference voltage

wherein said voltage dividing circuit includes first, second and third resistors series-connected to each other in the named order, the series connection of said first, second, and third resistors being connected between a ground level and said power supply voltage supplied to said subscriber instrument, in the named order from said ground level, so that said voltage-divided voltage is produced from a connection node between said first and second resistors, and a voltage of one half of said power supply voltage is produced at a connection node between said second and third resistors, and

wherein said level shift circuit includes a first transistor with a base connected to the connection node between said first and second resistors and with a collector connected to said power supply voltage, a second transistor with a base connected to a third reference voltage and with a collector connected to a second power supply voltage, and voltage dividing circuitry connected between emitters of said first and second transistors for dividing a voltage between said emitters of said first and second transistors to supply a divided voltage as said level-shifted voltage.

2. A subscriber circuit comprising:

a hook monitor circuit for connection to a pair of subscriber lines for operating with a power supply voltage supplied to a subscriber instrument, said hook monitor circuit detecting a current flowing through said pair of subscriber lines in the form of a voltage and comparing said voltage with a first reference voltage, such that if said voltage is higher than said first reference voltage, said hook monitor circuit generates a monitor signal;

a power supply voltage abnormality detecting circuit for receiving said power supply voltage supplied to said subscriber instrument and for generating an active abnormality detection signal; and

means for responding to said active abnormality detection signal for inhibiting said hook monitor circuit from outputting said monitor signal;

wherein said power supply voltage abnormality detecting circuit includes a voltage division and level shift circuit receiving said power supply voltage supplied to said subscriber instrument and outputting a voltage-divided and level-shifted voltage, and a comparator comparing said voltage-divided and level-shifted voltage with a second reference voltage and generating said active abnormality detection signal when said voltage-divided and level-shifted voltage is higher than said second reference voltage;

wherein said voltage division and level shift circuit includes a first resistor having one end connected to a third reference voltage, a base-grounded first transistor with a collector connected to the other end of said first resistor, a second transistor with an emitter connected to said power supply voltage and with a base and a collector short-circuited, and second and third resistors series-connected to each other between an emitter of said first transistor and the collector of said second transistor, so that said voltage-divided and level-shifted voltage is obtained from a connection node between said first resistor and said collector of said first transistor and is supplied to one input of said comparator, and a voltage of one half of said power supply voltage is obtained from a connection node between said second and third resistors, and wherein said subscriber circuit further includes a reference voltage generation circuit composed of fourth and fifth resistors series-connected to each other between said third reference voltage and the ground level, so that said second reference voltage is produced at a connection node between said fourth and fifth resistors and is supplied to an input of said comparator.