An incremental encoder lets a controller track movement by detecting successive signal changes. In a common square-wave rotary encoder, these changes form pulse trains. Count them to obtain position change, relate them to time to calculate speed, and decode the appropriate channel pair to determine direction.

Incremental describes how movement is reported, rather than how it is sensed. Both optical and magnetic designs are available. The A/B/Z discussion below concerns square-wave outputs; other incremental interfaces, including sin/cos, require different receiving electronics.

Two channels make direction visible

Channels A and B are approximately 90 electrical degrees apart. The receiver determines direction from which channel changes first. Channel A alone can supply pulses for counting or speed measurement, but cannot independently establish direction. Electrical phase must not be confused with mechanical shaft angle.

One possible forward sequence is 00 → 10 → 11 → 01 → 00. Reverse movement runs through the sequence in the opposite order. Verify which sequence your system treats as positive: the viewing direction, wiring and counter configuration all matter.

An index is a reference, not a turn number

Many rotary models add a Z or index channel with one reference pulse per revolution. Its width and relationship to A/B depend on the design. The same index reappears each turn, so Z alone cannot identify the revolution number. A machine may combine an external home sensor, the index and a controlled homing sequence to establish its datum.

An absolute encoder reports a position value within its specified range. An incremental system builds position from counts relative to a known starting point. Whether that reference survives a power interruption depends on the complete system. Saving the last count does not capture movement while counting is unavailable. This matters in motion control and servo feedback systems.

From 1000 PPR to 4000 counts

Take a 1000 PPR encoder, where the specification means 1000 complete A-channel cycles per revolution. ×1 decoding produces one count per cycle, ×2 produces two, and ×4 uses every edge of A and B to produce four. The resulting scales are 1000, 2000 and 4000 counts per revolution. Check the manufacturer's definitions of PPR and CPR before setting the scale.

With ×4 decoding, the theoretical angular step is 360° / 4000 = 0.09°. A net change of 500 counts represents 45° of movement. This is the counting increment, rather than an accuracy guarantee. The scaling in the PLC or encoder readout must match the selected decoding mode.

Check the counting rate at maximum speed

A-channel frequency = PPR × rpm / 60. At 3000 rpm, the example produces 50,000 Hz on A and 200,000 counts per second with ×4 decoding. Compare both the encoder's output limit and the receiver's counting-frequency specification. Establish whether a quoted limit means channel cycles or decoded counts.

Reading fast pulses through an ordinary digital input once per program scan can miss transitions. Use a suitable high-speed counter or encoder interface, and check input filtering, minimum pulse width and counter rollover. A wrong pulses-per-revolution setting also gives a wrong speed on a counter or tachometer. Keep channel frequency and the ×4 count rate distinct when converting either to rpm.

Measuring length with a rotary encoder

A known mechanical relationship turns rotation into distance. Suppose a measuring wheel has an effective circumference of 200 mm and drives the same 1000 PPR encoder. At ×4 decoding, the nominal step is 200 / 4000 = 0.05 mm; 10,000 net counts represent 500 mm of travel. Wheel slip, wear, backlash and transmission ratio affect the actual result.

Measure at the point that matters. Motor rotation does not always establish load displacement. Depending on the mechanism, a linear encoder or suitable draw-wire encoder may provide a more direct measurement. Where the feedback is taken can matter more than increasing the pulse count.

Wiring and mounting: match the complete interface

There is no universal wire count or pinout. A/B/Z models need the appropriate supply and signal reference; differential versions may add A+, A−, B+, B− and Z+, Z−. Shielding and protective earth must not be treated as interchangeable with signal 0 V. Use the exact model's wiring diagram, rather than assuming cable colors identify the functions.

TTL, HTL, push-pull and other output circuits need compatible inputs. A line-driver output using RS-422 differential signaling requires the correct receiver and pair connections. A pulse-and-direction command interface is a different signal arrangement from A/B quadrature feedback; conversion requires suitable hardware where needed.

Also check hollow-shaft mounting, couplings and accessories, maximum speed, temperature and IP protection. A resolver is a different angle sensor with excitation and decoding requirements. There is no universal rule that encoders are always more accurate or resolvers always cheaper. Compare the sensor and receiver together, and assess encoder pricing against the complete installation requirements.