Switching circuit for preventing lost calls

Claim 2

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operatIve states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, means for connecting said switching means to said central office terminals, to said PBX terminals and to a source of voltage, means for establishing the second operative state of said switching means when the central office open circuits said first and second office terminals at a time when a telephone set is connected across said PBX terminals, and means for establishing the first operative state of said switching means under other voltage and current conditions at said PBX and central office terminals.

Claim 3

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, first sensing means for establishing and maintaining said first operative state when the central office applies a first potential to one of said office terminals, second sensing means for maintaining said first operative state when the PBX applies a second potential to one of said PBX terminals, third sensing means for maintaining said first operative state when current flows between the central office and PBX through the other of said office terminals, fourth sensing means for establishing and maintaining said first operative state when the central office applies the second potential to the other of said office terminals, fifth sensing means for maintaining said first operative state when current flows between the central office and the PBX through said one of said office terminals, sixth sensing means for establishing said first operative state when the central office applies the second potential to said one of said office terminals, and means for establishing said second operative state when one of said first through sixth sensing means establishes said first operative state.

Claim 4

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, first sensing means for establishing and maintaining said first operative state when the central office applies a first potential to one of said office terminals, second sensing means for maintaining said first operative state when the PBX applies a second potential to one of said PBX terminals, third sensing means for maintaining said first operative state when current flows between the central office and PBX through the other of said office terminals, fourth sensing means for establishing and maintaining said first operative state when the central office applies the second potential to the other of said office terminals, fifth sensing means for maintaining said first operative state when current flows between the central office and the PBX through said one of said office terminals, sixth sensing means for establishing said first operative state when the central office applies the second potential to said one of said office terminals, and means for establishing said second operative state when one of said first through sixth sensing means establishes said first operative state.

Claim 5

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, means for connecting said switching means to said office terminals, to said PBX terminals and to a source of voltage, means for establishing the first operative state of said switching means during ringing from the central office, means for establishing the first operative state of said switching means during dialing from the PBX, means for establishing the first operative state of said switching means during ring-trip, means for establishing said first operative state before, during and after seizure by the central office or the PBX, means for establishing said first operative state of said switching means during the transmission of voice signals between the central office and PBX, and means for establishing the second operative state of said switching means when a telephone set is connected across said PBX terminals and an open circuit appears at said central office terminals.

Claim 6

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, means for connecting said switching means to said office terminals, to said PBX terminals and to a source of voltage, means for establishing the first operative state of said switching means during ringing from the central office, means for establishing the first operative state of said switching means during dialing from the PBX, means for establishing the first operative state of said switching means during ring-trip, means for establishing said first operative state before, during and after seizure by the central office or the PBX, means for establishing said first operative state of said switching means during the transmission of voice signals between the central office and PBX, and means for establishing the second operative state of said switching means when a telephone set is connected across said PBX terminals and an open circuit appears at said central office terminals.

Claim 7

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an Open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to directly connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, means for connecting said switching means to said central office terminals, to said PBX terminals, and to a source of voltage, control means having a control input, said control means serving as means for establishing the first operative state of said switching means when the voltage at said control input exceeds a predetermined value and for establishing a second operative state of said switching means when the voltage at said control input is less than said predetermined value, ''''OR'''' circuit means for applying to said control input a voltage in excess of said predetermined value when said second office terminal is connected to ground or when said second office terminal is connected to the central office battery, or when said first office terminal is connected to central office battery or when current flows through said first or second office terminal, or when said second PBX terminal is at a potential having the same polarity as the potential of the central office battery, said ''''OR'''' circuit means being arranged to apply to said control input a voltage less than said predetermined value when none of said terminal voltage and current conditions exist.

Claim 8

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an Open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to directly connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, means for connecting said switching means to said central office terminals, to said PBX terminals, and to a source of voltage, control means having a control input, said control means serving as means for establishing the first operative state of said switching means when the voltage at said control input exceeds a predetermined value and for establishing a second operative state of said switching means when the voltage at said control input is less than said predetermined value, ''''OR'''' circuit means for applying to said control input a voltage in excess of said predetermined value when said second office terminal is connected to ground or when said second office terminal is connected to the central office battery, or when said first office terminal is connected to central office battery or when current flows through said first or second office terminal, or when said second PBX terminal is at a potential having the same polarity as the potential of the central office battery, said ''''OR'''' circuit means being arranged to apply to said control input a voltage less than said predetermined value when none of said terminal voltage and current conditions exist.

Claim 9

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to directly connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when

Claim 10

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to directly connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when

Claim 11

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state, said switching means being adapted to apply a predetermined voltage to said PBX terminals and to disconnect said central office terminals when said switching means is in a second operative state, means for connecting said switching means to said central office terminals, to said PBX terminals and to a source of voltage, means for establishing the second operative state of said switching means each time that the central office open circuits said first and second office terminals at a time when a telephone set is connected across said PBX terminals, and means for establishing the first operative state of said switching means under other voltage and current conditions at said PBX and central office terminals.

Claim 12

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state, said switching means being adapted to apply a predetermined voltage to said PBX terminals and to disconnect said central office terminals when said switching means is in a second operative state, means for connecting said switching means to said central office terminals, to said PBX terminals and to a source of voltage, means for establishing the second operative state of said switching means each time that the central office open circuits said first and second office terminals at a time when a telephone set is connected across said PBX terminals, and means for establishing the first operative state of said switching means under other voltage and current conditions at said PBX and central office terminals.

Claim 13

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state, said switching means being adapted to apply a predetermined voltage to said PBX terminals and to disconnect said central office terminals when said switching means is in a second operative state, means for connecting Said switching means to said office terminals, to said PBX terminals and to a source of voltage, means for establishing the first operative state of said switching means during ringing from the central office, means for establishing the first operative state of said switching means during dialing from the PBX, means for establishing the first operative state of said switching means during ring-trip, means for establishing said first operative state during seizure by the central office or the PBX, means for establishing said first operative state of said switching means during the transmission of voice signals between the central office and PBX, and means for establishing the second operative state of said switching means when a telephone set is connected across said PBX terminals and an open circuit appears at said central office terminals.

Claim 14

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state, said switching means being adapted to apply a predetermined voltage to said PBX terminals and to disconnect said central office terminals when said switching means is in a second operative state, means for connecting Said switching means to said office terminals, to said PBX terminals and to a source of voltage, means for establishing the first operative state of said switching means during ringing from the central office, means for establishing the first operative state of said switching means during dialing from the PBX, means for establishing the first operative state of said switching means during ring-trip, means for establishing said first operative state during seizure by the central office or the PBX, means for establishing said first operative state of said switching means during the transmission of voice signals between the central office and PBX, and means for establishing the second operative state of said switching means when a telephone set is connected across said PBX terminals and an open circuit appears at said central office terminals.

Claim 15

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, means for operatively connecting said switching means to said office terminals, to said PBX terminals and to a source of voltage, means responsive to ringing voltage or current for establishing the first operative state of said switching means during ringing, means operatively connecting said ringing responsive means in said circuit to sense the presence of the ringing condition, means operatively connecting said ringing responsive means to said switching means to control the operative state thereof, means responsive to dialing voltage or current for establishing the first operative state of said switching means during dialing, means operatively connecting said dialing responsive means in said circuit to sense the presence of the dialing condition, means operatively connecting said dialing responsive means to said switching means to control the operative state thereof, means responsive to trip current for establishing the first operative state of said switching means during ring-trip, means operatively connecting said ring-trip responsive means in said circuit to sense the presence of the ring-trip condition, means operatively connecting said ring-trip responsive means to said switching means to control the operative state thereof, means responsive to seizure voltages or currents for establishing said first operative state during seizure, means operatively connecting said seizure responsive means in said circuit to sense the presence of the seizure condition, means operatively connecting said seizure responsive means to said switching means to control the operative state thereof, means responsive to d-c talking voltage or current for establishing said first operative state of said switching means during the transmission of voice signals between the central office and PBX, means operatively connecting said talking responsive means in said circuit to sense the presence of the talking condition, means operatively connecting said talking responsive means to said switching means to control the operative state thereof, and means for establishing the second operative state of said switching means when a telephone set is connected across said PBX terminals and an open circuit appears at said central office terminals.

Claim 16

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, switching means having first and second operative states, said switching means being adapted to connect respective office and PBX terminals when said switching means is in a first operative state and to apply a predetermined voltage to said PBX terminals when said switching means is in a second operative state, means for operatively connecting said switching means to said office terminals, to said PBX terminals and to a source of voltage, means responsive to ringing voltage or current for establishing the first operative state of said switching means during ringing, means operatively connecting said ringing responsive means in said circuit to sense the presence of the ringing condition, means operatively connecting said ringing responsive means to said switching means to control the operative state thereof, means responsive to dialing voltage or current for establishing the first operative state of said switching means during dialing, means operatively connecting said dialing responsive means in said circuit to sense the presence of the dialing condition, means operatively connecting said dialing responsive means to said switching means to control the operative state thereof, means responsive to trip current for establishing the first operative state of said switching means during ring-trip, means operatively connecting said ring-trip responsive means in said circuit to sense the presence of the ring-trip condition, means operatively connecting said ring-trip responsive means to said switching means to control the operative state thereof, means responsive to seizure voltages or currents for establishing said first operative state during seizure, means operatively connecting said seizure responsive means in said circuit to sense the presence of the seizure condition, means operatively connecting said seizure responsive means to said switching means to control the operative state thereof, means responsive to d-c talking voltage or current for establishing said first operative state of said switching means during the transmission of voice signals between the central office and PBX, means operatively connecting said talking responsive means in said circuit to sense the presence of the talking condition, means operatively connecting said talking responsive means to said switching means to control the operative state thereof, and means for establishing the second operative state of said switching means when a telephone set is connected across said PBX terminals and an open circuit appears at said central office terminals.

Claim 17

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, a control relay having a coil and first and second contact sets, means for connecting said first contAct set to said first office terminal, to said first PBX terminal and to a source of voltage, means for connecting said second contact set to said second office terminal, to said second PBX terminal and to said source of voltage, said first and second contact sets serving to connect said first office and PBX terminals and said second office and PBX terminals, respectively, when said coil is in a first state of energization, said first and second contact sets serving to connect said first and second PBX terminals to said source of voltage when said coil is in a second state of energization, a first transistor having a power circuit and a control circuit, said first transistor serving as means for controlling the state of energization of said coil, a second transistor having a power circuit and a control circuit, said second transistor serving as means for controlling the state of energization of said coil, means for connecting the control circuit of said first transistor to said first office terminal and to ground, means for connecting the control circuit of said second transistor to said second office terminal and to ground, means for connecting the power circuits of said first and second transistors in state controlling relationship to said coil, first and second unidirectional conducting means for bypassing current around the control circuits of said first and second transistors, respectively, third unidirectional conducting means for connecting said second office terminal in state controlling relationship to said coil, and means for connecting said second PBX terminal in state controlling relationship to said coil.

Claim 18

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, a control relay having a coil and first and second contact sets, means for connecting said first contAct set to said first office terminal, to said first PBX terminal and to a source of voltage, means for connecting said second contact set to said second office terminal, to said second PBX terminal and to said source of voltage, said first and second contact sets serving to connect said first office and PBX terminals and said second office and PBX terminals, respectively, when said coil is in a first state of energization, said first and second contact sets serving to connect said first and second PBX terminals to said source of voltage when said coil is in a second state of energization, a first transistor having a power circuit and a control circuit, said first transistor serving as means for controlling the state of energization of said coil, a second transistor having a power circuit and a control circuit, said second transistor serving as means for controlling the state of energization of said coil, means for connecting the control circuit of said first transistor to said first office terminal and to ground, means for connecting the control circuit of said second transistor to said second office terminal and to ground, means for connecting the power circuits of said first and second transistors in state controlling relationship to said coil, first and second unidirectional conducting means for bypassing current around the control circuits of said first and second transistors, respectively, third unidirectional conducting means for connecting said second office terminal in state controlling relationship to said coil, and means for connecting said second PBX terminal in state controlling relationship to said coil.

Claim 19

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, first and second contact sets each including a movable contact, a first fixed contact and a second fixed contact, means for connecting the first fixed contacts of said first and second contact sets to said first and second central office terminals, respectively, means for connecting the movable contacts of said first and second contact sets to the first and second PBX terminals, respectively, means for connecting the second fixed contacts of said first and second contact sets to a source of voltage, a coil for closing said movable contact against said first fixed contact when said coil is de-energized and for closing said movable contact against said second fixed contact when said coil is energized, controllable switching means having a control input, said controllable switching means serving as means for maintaining said coil in a de-energized state when the control voltage and current at said control input exceed predetermined minimum values and for energizing said coil when the voltage and current at said control input are less than said predetermined values, first and second transistors each having a power circuit and a control circuit, means for connecting the control circuit of said first transistor to said first office terminal and to ground, means for connecting the control circuit of said second transistor to said second office terminal and to ground, first and second unidirectional conducting means for by-passing current flow around said first and second control circuits, a third transistor having a power circuit and a control circuit, means for connecting the power circuit of said third transistor between ground and said control input, means for connecting the control circuit of said third transistor between ground and the power circuits of said first and second transistors, means for connecting the control circuit of said third transistor between ground and said second PBX terminal and means for connecting said second office terminal and said control input. said switching means is in a second operative state, sensing means for maintaining the first operative state of said switching meAns when current flows between the central office and PBX, sensing means for establishing and maintaining the first operative state of said switching means when central office battery potential is applied to either central office terminal, sensing means for establishing and maintaining the first operative state of said switching means when ground potential is applied to one of said central office terminals, sensing means for maintaining the first operative state of said switching means when a potential having the same polarity as the central office battery potential is applied to one of said PBX terminals, and means for establishing the second operative state of said switching means when none of said sensing means are in condition to establish or maintain the first operative state thereof.

Claim 20

In a circuit for maintaining the connections between a telephone set and a PBX in the presence of an open circuit at the central office side of that PBX, the combination of first and second central office terminals, first and second PBX terminals, first and second contact sets each including a movable contact, a first fixed contact and a second fixed contact, means for connecting the first fixed contacts of said first and second contact sets to said first and second central office terminals, respectively, means for connecting the movable contacts of said first and second contact sets to the first and second PBX terminals, respectively, means for connecting the second fixed contacts of said first and second contact sets to a source of voltage, a coil for closing said movable contact against said first fixed contact when said coil is de-energized and for closing said movable contact against said second fixed contact when said coil is energized, controllable switching means having a control input, said controllable switching means serving as means for maintaining said coil in a de-energized state when the control voltage and current at said control input exceed predetermined minimum values and for energizing said coil when the voltage and current at said control input are less than said predetermined values, first and second transistors each having a power circuit and a control circuit, means for connecting the control circuit of said first transistor to said first office terminal and to ground, means for connecting the control circuit of said second transistor to said second office terminal and to ground, first and second unidirectional conducting means for by-passing current flow around said first and second control circuits, a third transistor having a power circuit and a control circuit, means for connecting the power circuit of said third transistor between ground and said control input, means for connecting the control circuit of said third transistor between ground and the power circuits of said first and second transistors, means for connecting the control circuit of said third transistor between ground and said second PBX terminal and means for connecting said second office terminal and said control input. said switching means is in a second operative state, sensing means for maintaining the first operative state of said switching meAns when current flows between the central office and PBX, sensing means for establishing and maintaining the first operative state of said switching means when central office battery potential is applied to either central office terminal, sensing means for establishing and maintaining the first operative state of said switching means when ground potential is applied to one of said central office terminals, sensing means for maintaining the first operative state of said switching means when a potential having the same polarity as the central office battery potential is applied to one of said PBX terminals, and means for establishing the second operative state of said switching means when none of said sensing means are in condition to establish or maintain the first operative state thereof.