Digital data-dependent ac impedance termination of telephone line

Abstract

The potential impedance mismatch of digital terminal equipment and a line test unit that has been equipped to conform with the Bellcore standard of terminating the line with a 600-900 ohm impedance during data reception is addressed by modifying the craftsman's test unit to include a controllably disabled AC impedance, which is operative to normally bridge the tip and ring leads of the telephone circuit under test with the required (600-900 ohm) impedance during data reception and, under direct control by the craftsman's keypad or software control resident in the test set's microcontroller, to selectively remove the AC matching impedance that would otherwise bridge the line. The AC impedance is comprised of a D.C. blocking capacitor and a resistor coupled in series with a controllable solid state relay which is normally rendered conductive during data reception to bridge the tip and ring leads. By direct manual control from the craftsman's keypad, or software control resident in the test set's microcontroller, the normally enabling control signal supplied by the microcontroller to the solid state relay may be removed, so as to selectively decouple the AC matching impedance that would otherwise bridge tip and ring leads of the telephone line during data reception, so as to make the test unit compatible with customer equipment, whether or not that equipment contains an AC impedance termination that satisfies the Bellcore requirement.