Browse Prior Art Database

Telephone Subscriber's Loop Current Detector

IP.com Disclosure Number: IPCOM000073912D
Original Publication Date: 1971-Feb-01
Included in the Prior Art Database: 2005-Feb-23
Document File: 2 page(s) / 33K

Publishing Venue

IBM

Related People

Hernandez, P: AUTHOR

Abstract

This detector is a logic circuit having two inputs and one output, which fulfils the following functions: when low, the output becomes high only when both inputs become high; when high, the output becomes low only when both inputs become low.

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Telephone Subscriber's Loop Current Detector

This detector is a logic circuit having two inputs and one output, which fulfils the following functions: when low, the output becomes high only when both inputs become high; when high, the output becomes low only when both inputs become low.

The detector output signals the closed or open status of the telephone subscriber's loop, and is independent from the longitudinal noises induced into the line, whether the line is open or closed. Logic circuit LC inputs are provided by the outputs of two current detectors DA and DB detecting the current flowing through legs A and B of the subscriber's loop circuit, respectively. At rest, transistors T1 and T2 of detectors DA and DB are blocked, as well as transistors T3, T4, T5 and T6 of logic circuit JC. Hence, to have both T1 and T2 saturated, activation of T3 is needed. As soon as T1 and T2 are saturated, T4, then T5 and T6 are made conducting. Transistor T3, which acted previously as an AND circuit, will now act as an OR circuit for DA and Db outputs. As the longitudinal noises are in phase on both conductors A and B with respect to ground, they are not detected. As a matter of fact: if the loop circuit is open, both T1 and T2 cannot saturate under action of noise signals having the same phase; if the loop circuit is closed, both T1 and T2 cannot be clocked under action of noise signals having the same phase, especially in presence of the DC current flowing through the loop c...