Checking Quadrature Encoder State Transitions Before Counting Position

An incremental encoder can produce clean looking individual pulses while the combined A and B sequence is interpreted incorrectly. Position counting depends on their order and timing. Review the state transitions before compensating a position error with a scale factor or periodic reset.

Write the expected sequence

Record the four A/B combinations and their order for each direction. The TI note on encoder outputs describes the four unique states of a quadrature signal and the timing margin provided by the phase relationship. The decoder must observe the sequence with sufficient resolution to distinguish direction.

For one convention, a forward sequence may be 00, 01, 11, 10, then 00. The reverse order indicates the other direction. Confirm the convention used by the actual wiring and decoder instead of assuming this example defines the product’s positive direction.

Identify invalid observations

If both bits appear to change between consecutive observations, an intermediate state may have been missed or the inputs may have been corrupted. Log such events separately from ordinary position increments. A decoder that silently assigns a direction to every observed jump can accumulate plausible but incorrect counts.

Capture A and B together near the receiving pins. Check threshold levels, edge shape, and timing separation. A long cable or unequal filtering can change the relationship even when both signals individually cross valid logic levels.

Budget maximum edge rate

Use encoder cycles per revolution, shaft speed, and the selected counting multiplier. A hypothetical encoder with 1000 A-channel cycles per revolution at 3000 revolutions per minute produces 50,000 cycles per second. A decoder counting all four quadrature transitions then processes 200,000 transitions per second.

Check peripheral capability, input qualification, software service limits, and counter width. A debounce filter suitable for a hand operated switch may suppress valid edges at motor speed. Evaluate the filter’s timing against the minimum valid state duration.

Verify position and direction

Move through a known number of revolutions in both directions and compare counts. Include slow reversals, maximum speed, and relevant acceleration. Inspect any index signal separately; it can provide a reference without proving that all intervening transitions were counted correctly.

Save waveform captures, counting mode, input filtering, direction convention, and invalid transition statistics. A reliable encoder interface demonstrates correct state interpretation over the required speed range. Matching the final count after one convenient motion is useful evidence, but it does not establish immunity to missed edges or direction ambiguity during every operating transition.