The ENCODER300 is a 3-axis quadrature encoder interface board for IBM PC/AT bus-compatible devices. Phase 0, phase 90 and index pulse inputs are provided for each encoder. Inputs may be single ended (A, B or C) or differential (A+, A-, B+, B-, C+ or C-). Power and ground connections are also provided for use by the encoder if needed. Inputs are conditioned by a four-stage digital filter, and the maximum input rate in quadrature decode mode is 1 MHz. The conditioned inputs are applied to a 16-bit counter, which may be used for quadrature decoding, pules or direction-input counting, or as a pulse input up/down counter.