# US 4,396,809 A · Maintenance termination unit

> Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt

- Number: US 4,396,809 A
- Document ID: US4396809A
- Status: Granted
- Kind: A
- Application number: 06/289,325
- Filed: 1981-08-03
- Granted: 1983-08-02
- Earliest claimed: 1981-08-03
- Assignees: Bell Telephone Laboratories, Incorporated
- Inventors: James E. Brunssen
- Web page: https://patentgrep.com/patent/US4396809A

## CPC

- H04B 3/46: Monitoring; Testing
- H04M 3/301: {Circuit arrangements at the subscriber's side of the line}
- H04B: TRANSMISSION
- H04M: TELEPHONIC COMMUNICATION (circuits for controlling other apparatus via a telephone cable and not involving telephone switching apparatus G08)

## Abstract

A maintenance termination unit (10) comprises a voltage sensitive switch (30,50) connected permanently in series with each loop conductor (11,13) and an impedance termination unit (18) connected permanently across the loop conductors. Each voltage sensitive switch (30 or 50) comprises a triac (32) connected in series with a loop conductor (11 or 13), a capacitor (34) and resistor (36) connected across the terminals of the triac, and a bilateral switch (60) connected between one of the terminals and the gate electrode of the triac. In response to a signal having a voltage greater than the threshold voltage of the switch, the capacitor is charged to the threshold voltage, thereupon the capacitor discharges insuring that the triac is gated to the ON state.

## Claims

### Claim 1

A maintenance termination unit (10) connected in series with a telephone subscriber loop comprising tip conductor (11) and ring conductor (13) for determining whether a fault in said subscriber loop is located on the telephone set (12) side or the central office side of said maintenance termination unit, said maintenance termination unit comprising
an impedance termination network connected across said tip and ring conductors,
a first voltage sensitive switch (30) responsive to the magnitude of signals on and connected in series with said tip conductor,
a second voltage sensitive switch (50) responsive to the magnitude of signals on and connected in series with said ring conductor,
said first and second voltage sensitive switches being substantially identical, each of said voltage sensitive switches comprising
a triac (32) comprising first and second terminals and a gate lead (47) said terminals being connected in series with said tip or ring conductor,
a bilateral switch (60) connected between one of said terminals and said gate lead of said triac, and
a capacitor (34) and a resistor (36) connected across said first and second terminals of said triac.

### Claim 2 (depends on 1)

The maintenance termination unit according to claim 1
wherein
said bilateral switch (60) comprises a unilateral switch connected within a bridge comprising first (40), second (58), third (56) and fourth (46) steering diodes for permitting current to flow through said bilateral switch in either one of two directions.

### Claim 3 (depends on 2)

The maintenance termination unit according to claim 2
wherein
said unilateral switch comprises a first (42) and second (52) transistors so that the base of said first transistor is connected to the collector of the second transistor and the collector of said first transistor is connected to the base of said second transistor,
first resistor (48) connected across the base and emitter electrodes of said first transistor,
second resistor (54) connected across the base and emitter electrodes of said second transistor, and
a zener diode (44) connected across the collector and emitter electrodes of said second transistor.

### Claim 4 (depends on 3)

The maintenance termination unit according to claim 3
wherein
said first steering diode (40) is connected between either said first or second terminals of said triac and the emitter electrode of said first transistor (42),
said second steering diode (58) is connected between said first steering diode and the emitter electrode of said second transistor,
said third steering diode (56) is connected between the emitter electrode of said first transistor and said gate lead of said triac, and
said fourth steering diode (46) is connected between the emitter electrode of said second transistor and said gate lead of said triac.

### Claim 5 (depends on 4)

The maintenance termination unit according to claim 4
wherein
said capacitor (34) has a capacity for being charged up to the threshold voltage value of said voltage sensitive switch said capacity being
(a) the sum of the values of the forward biased voltage drops across said first and fourth steering diodes, the emitter-base junction of said first transistor, the gate junction of said triac and the zener voltage of said zener diode, or
(b) the sum of the values of the forward biased voltage drops across said third and second steering diodes, the emitter-base junction of said first transistor, the gate junction of said triac and the zener voltage of said zener diode.

### Claim 6 (depends on 5)

The maintenance termination unit according to claim 5
comprising
means (42,44,52) for discharging said capacitor (34) when said threshold voltage level is reached thereby providing a negative resistance characteristic and insuring that said voltage sensitive switch will switch to the ON state.