Abstract: A line card integrates subscriber line interface circuitry, A/D and D/A converters, and digital signal processing technology. The digital signal processing technology performs many line card tasks such as switch hook detection, ground key detection, DC feed control, polarity reversal, ringing tests, fault detection, power cross detection, and ring trip detection. A line card having a digital signal processor supports both integral and external ringing signal generators. During application of a ringing signal, the line card detects ring trip conditions while discriminating against short duration short circuits. The line card includes programmable features that facilitate adaptation to varying regional ring trip detection and short duration short circuit discrimination specifications and subscriber loop characteristics.