Method and circuit for failsafe stitch-hook detection during fault in a telephone system

Abstract: A circuit and method for maintaining switch-hook detection (SHD) in the presence of a fault in a telephone system to include a comparison of one of a tip current and a ring current to a reference current. A switch-hook detection indicates that a request for service has been made. The reference current may be a conventional switch-hook detection threshold current, ISHD, times a factor (G0+1)-1, where G0 is the predetermined operating gain of the amplifier in the telephone subscriber line interface circuit.

Description: The present invention relates to switch-hook detection (SHD) and fault detection circuits for telephone systems, and is particular to a circuit and method for maintaining switch-hook detection in the presence of a fault in a telephone system. Figure 1 shows a telephone system that includes a subscriber telephone 12 connected to a central office through a telephone subscriber line interface circuit (SLIC) 14. The SLIC connects a balanced two wire transmission path (the path to and from the subscriber telephone) with an unbalanced four wire transmission path (the path to and from the distribution circuitry in the telephone central station). SLICs perform various functions, including battery feed, overvoltage protection, ringing, signaling, hybrid, and timing. SLIC operation is known and need not be considered in detail, and only portions of the SLIC 14 are shown in Figure 1. Telephone systems operate in potentially harsh environments and the telephone may be miles from the central office where the SLIC is typically located, thereby exposing portions of the system to faults, such as short circuits from accidental grounding, power lines, lightning, etc. The overvoltage function of the SLIC is designed to protect the SLIC from faults. If the SLIC does not detect the short circuit, the SLIC may attempt to drive the short and overheat. When a fault is detected, the SLIC protects itself by limiting or turning off amplifiers 16 in the SLIC that are used to drive the line current to the telephone. Another function of a SLIC is to supervise telephone operation by detecting when a request for service is made at a telephone. The request for service is made by lifting the handset from the cradle, thereby activating the telephone's hook switch. As indicated in Figure 1, the request for service is made by closing switch 20. When switch 20 is closed, a dc load is applied between nodes VT and VR. The closing of switch 20 is referred to as a switch-hook and is detected by the SLIC. Switch-hook detection occurs when the dc current through the telephone line 22 (the line current) exceeds a predetermined magnitude, referred to a threshold current, ISHD. As indicated in Figure 1, the line current comprises a current IT that is proportional to the tip voltage plus the amplifier gain G times IT. Typically, a voltage across a resistor RSHD in series with the telephone is measured and compared to reference voltage in a comparator 24, where RSHD and the reference voltage have been selected so that a switch-hook detection is indicated when the line current exceeds ISHD. When the measured voltage exceeds the reference voltage, the comparator 24 sends a SHD signal to the SLIC to indicate a request for service. The request for service indication is maintained until the measured voltage no longer exceeds the reference voltage. When the measured voltage no longer exceeds the reference voltage the telephone is disconnected. Thus, if a fault has been detected and the amplifier 16 is limited or turned off, the line current will be reduced. When the line current is reduced, the SHD measured voltage will no longer exceed the reference voltage and the telephone will be disconnected. The telephone user must redial and attempt to reconnect in order to complete a disconnected call. When the fault is only transitory, lasting only a few seconds or less, it is desirable to avoid disconnection due to SHD interruption. However, the conflicting, and overriding, need to protect the SLIC by reducing line current has heretofore made it impossible to avoid disconnect due to loss of SHD. US-A-5 335 271 describes the principle of switch-hook detection in a telephone system having a telephone connected through a telephone line to a SLIC including an OFF-hook detection circuit. An object of the present invention is to provide a circuit and method for switch-hook detection that obviates the problems of the prior art, and for maintaining a SHD indication in the presence of a fault. Another object is to provide a circuit and method for switch-hook detection in which a tip or ring current is compared to a reference current to determine whether a request for service has been made, and to a method for switch-hook detection in which a copy of one of a tip or ring current is compared to a reference current that is ISHD times a factor that includes G0, the gain of the SLIC amplifier, to determine whether a request for service has been made. A further object is to provide a circuit and method for maintaining a SHD indication in the presence of a fault in which one of a tip and a ring voltage that has been converted to a current is compared to a reference current to determine whether a request for service has been made, and in which a line current is reduced in response to the fault without affecting the SHD comparison. The present invention includes a circuit for indicating switch-hook operation in a telephone system, the telephone system having a telephone connected through a telephone line to a first circuit for carrying a first current, I1, and a first current amplifier for amplifying I1 by G, and in which a line current in the telephone line is the sum of I1 and I1 times G, and in which switch-hook operation is indicated by the line current exceeding a predetermined switch-hook detection threshold current, ISHD, comprising: a current source for providing to a comparison circuit a reference current that is ISHD times a factor that includes G0, where G0 is a predetermined operating gain of the current amplifier; and a first current mirror for providing a copy of I1 to said comparison circuit; whereby switch-hook operation is indicated when said copy of I1 exceeds said reference current. The present invention also includes a method of maintaining switch-hook detection in the presence of a fault in a telephone system comprising the steps of comparing one of a tip and a ring current to a reference current to determine whether a request for service is being made, and reducing a line current in the telephone system in response to detection of a fault without affecting the determination whether a request for service is being made. The invention will now be described by way of example, with reference to the accompanying drawings in which; Figure 1 is schematic diagram of a portion of a prior art telephone system illustrating switch-hook detection and fault detection circuits. Figure 2 is schematic diagram of an embodiment of a switch-hook detection circuit of the present invention. Figure 2a shows an alternative with a separate current comparator. Figure 3 is schematic diagram of an embodiment of switch-hook detection and fault/thermal detection circuits of the present invention. Figures 2-2a, in which like numerical designations for like elements have maintained to facilitate an understanding of the invention, illustrate an embodiment of the present invention for indicating switch-hook detection which includes a current mirror 30 for providing a copy of IT for comparison to a reference current, IREF, from a current source 32, and in which a switch-hook detection may be indicated when IT exceeds the reference current. The SHD indication is not affected by changes in line current. Instead of using a copy of IT, a copy of IR from current mirror 34 may be used. In a further alternative, an average of IT and IR may be used. Note that the embodiment of Figure 2 includes current mirrors 30 and 34 and, with appropriate selection of current mirror values, is able to provide an average of the two currents for comparison to the reference current, although this is not required for the present invention. An additional current mirror 36 may be needed to set the polarity of IT (or IR). The combination of the copied current and the reference current may be provided to the SLIC to indicate SHD. Alternatively, as shown in Figure 2a, the copied current and the reference current may be compared in a current comparator 38 that provides a SHD signal to the SLIC. The reference current from current source 32 may be selected so that a comparison to either IT or IR (or their average) can indicate whether a SHD threshold has been exceeded. For example, looking at node VT, the line current, ILine, is the sum of IT and the current from the amplifier 16, IT times G. That is, ILine = IT x (G+1) The line current will have exceeded a threshold current, ISHD, when, IT > ISHD (G0+1)-1 where G0 is the predetermined operating gain of the amplifier ( e . g ., 400 in an embodiment of the present invention), and where G is G0 if there is no fault. A switch-hook may be detected by comparing either IT or IR (or their average) to a reference current that is a scaled multiple of ISHD. That is, ISHD may be scaled by multiplying times a factor that includes G0. The factor may be (G0+1)-1. Thus, the comparison to a reference to detect a switch-hook is made free of any changes in line current that may result from detection of a fault, or from a problem that affects the output of amplifier 16, such as overheating. Figure 3 illustrates another embodiement that includes a fault detector 40 and amplifier shut-down circuit 42. In this embodiment, when a fault is detected while the switch 20 is closed (that is, the telephone is connected), SHD is not changed and the telephone remains connected. When a fault is detected and a shut-down signal provided from shut-down circuit 42, the gain G of amplifiers 16 is reduced, thereby leaving the line current across sense resistors RS1 and RS2 and telephone load RL that are in series between battery and ground. RS1 and RS2 are at least an order of magnitude larger than RL so that when the amplifiers are shut down and G is reduced, nodes VT and VR are pulled together and currents IT and IR increase so as to continue to exceed the reference current, thereby maintaining a SHD indication. That is, as seen in equation (1), IT increases as G decreases, and as seen in equation (2), as IT increases ISHD also increases because G0 is fixed. The SHD indication, in effect, becomes harder. When the fault goes away, the caller may resume conversation without having to redial to reconnect the telephone. The amplifier shut-down circuit 42 may also be responsive to a an overheat in the system (such as an amplifier overheat) that is detected and indicated by a thermal detector 44. A circuit and method for maintaining switch-hook detection (SHD) in the presence of a fault in a telephone system to include a comparison of one of a tip current and a ring current to a reference current. A switch-hook detection indicates that a request for service has been made. The reference current may be a conventional switch-hook detection threshold current, ISHD, times a factor (G0+1)-1, where G0 is the predetermined operating gain of the amplifier in the telephone subscriber line interface circuit.

Claims: A circuit for indicating switch-hook operation in a telephone system, the telephone system having a telephone (12) connected through a telephone line (22) to a first circuit for carrying a first current, I1, and a first current amplifier (16) for amplifying I1 by G, and in which a line current in the telephone line is the sum of I1 and I1 times G, and in which switch-hook (20) operation is indicated by the line current exceeding a predetermined switch-hook detection threshold current, ISHD, characterized by : a current source (32) for providing to a comparison circuit (38) a reference current (I REF ) that is ISHD times a factor that includes G0, where G0 is a predetermined operating gain of the current amplifier; and a first current mirror (30) for providing a copy of I1 to said comparison circuit (38); whereby switch-hook (20) operation is indicated when said copy of I1 exceeds said reference current (I REF ). A circuit as claimed in claim 1 characterized by a fault detector (40) for providing a signal to reduce an output from the first amplifier (16), wherein a relationship between said copy of I1 and said reference current (I REF ) is not affected by a change of output from said first amplifier (16), in which said fault detector (40) is adapted to detect when a short circuit is detected in the telephone line (22). A circuit as claimed in claims 1 or 2 wherein said fault detector (40) is adapted to detect when a temperature of the circuit or the first amplifier (16) exceeds a predetermined value, and the comparison circuit (38) comprises a comparator for indicating when said copy of I1 is greater than said reference current, and in which said factor includes (G0+1)-1. A circuit as claimed in claim 1 characterized by the telephone system having a telephone (12) connected through a telephone line (22) to a tip circuit for carrying a current, IT, that is proportional to tip voltage and to a ring circuit for carrying a current, IR, that is proportional to a ring voltage, and current amplifiers (16) for amplifying IT and IR by G, and in which a line current in the telephone line is the sum of IT and IT times G, and in which switch-hook (20) operation is indicated by the line current exceeding a predetermined switch-hook detection threshold current, ISHD, the circuit comprising: a current source (32) for providing to a detection circuit a reference current that is ISHD times (GO + 1)-1, where GO is a predetermined operating gain of the current amplifier; and a first current mirror (30) for providing a copy of IT to said detection circuit; a second current mirror (34) for providing a copy of IR to said detection circuit; a comparator (38) in said detection circuit for comparing an average of the copies of IR and IT to said reference current; whereby switch-hook (20) is detected when said average exceeds said reference current. A circuit as claimed in claim 4 characterized in that said comparator (38) provides a switch-hook detection signal when said average exceeds said reference current (I REF ). A circuit as claimed in claim 4 or 5 characterized by a fault detector (40) for providing a signal to reduce an output from the amplifiers when a short circuit is detected in the telephone line (22), in which a relationship between said average and said reference current is not affected by a change of output from said amplifiers (16), A circuit as claimed in any one of claims 1 to 6, for maintaining switch-hook detection in the presence of a short circuit in a telephone system, characterized by: a current source (32) for providing to a comparison circuit (38) a reference current (I REF ) that is ISHD times a factor that includes G0; a first current mirror (30) for providing a copy of I1 to said comparison circuit (38); and a fault detector (40) for providing a signal to reduce an output from the first amplifier (16) when a short circuit is detected in the telephone line (22), a relationship between said copy of I1 and said reference current not being affected by a change of output from said first amplifier (16), and said comparison circuit (38) comprises a comparator for indicating which one of said copy of I1 and said reference current (I REF ) is greater. A method of maintaining switch-hook detection in the presence of a fault in a telephone system characterized by the steps of comparing one of a tip and a ring current to a reference current to determine whether a request for service is being made, and reducing a line current in the telephone system in response to detection of a fault without affecting the determination whether a request for service is being made. A method of indicating switch-hook operation in a telephone system, the telephone system having a telephone connected through a telephone line to a first circuit for carrying a first current, I1, and a first current amplifier for amplifying I1 by G, and in which a line current in the telephone line is the sum of I1 and I1 times G, and in which switch-hook operation is indicated by the line current exceeding a predetermined switch-hook detection threshold current, ISHD, the method characterized by the steps of: (a) providing to a comparison circuit a reference current that is ISHD times a factor that includes G0; and (b) copying I1 to the comparison circuit; whereby switch-hook operation is indicated when the copy of I1 exceeds the reference current, and preferably the factor includes (G0+1)-1, and the step of comparing the copy of I1 and the reference current indicating when the copy of I1 is larger than the reference current. A method as claimed in claim 9 characterized by the step of providing a signal to reduce an output from the first amplifier when a fault is detected, wherein a relationship between the copy of I1 and the reference current is not affected by a change of output from the first amplifier, so that the fault detected is a temperature of the circuit or the first amplifier that exceeds a predetermined value. A method as claimed in claim 8, the telephone system having a telephone connected through a telephone line (22) to a first circuit for carrying a first current, I1, and a first current amplifier (16) for amplifying I1 by G, and in which a line current in the telephone line is the sum of I1 and I1 times G, and in which switch-hook (20) detection is indicated by the line current exceeding a predetermined switch-hook detection threshold current, ISHD, the method comprising the steps of: (a) providing to a comparison circuit a reference current that is ISHD times (G0+1)-1; (b) providing a copy of I1 to the comparison circuit; and (c) providing a signal to reduce an output from the first amplifier when a short circuit is detected in the telephone line, a relationship between said copy of I1 and said reference current not being affected by a change of output from said first amplifier, including the further steps of comparing the copy of I1 and the reference current, and indicating when the copy of I1 is larger than the reference current.

Inventors: COTREAU GERALD M, WHITNEY DONALD K JR, COTREAU, GERALD M., WHITNEY, DONALD K., JR.
Applicants: HARRIS CORP, HARRIS CORPORATION
Application year: 1996
Main classification: H04M3
IPC codes: H04M19, H04M3, H04M1