Absolute is a position function, not one code format
An absolute reader identifies position from the currently observed code. A conventional multi-track n-bit layout can represent 2^n code words, but modern designs may combine a coarse address with interpolation or decode a sequence within a reading window. Track count therefore need not equal the final output bit count. Gray, natural binary and pseudo-random arrangements are alternatives tied to particular readers.
Multi-track words and sequence windows
A multi-track reader samples the states of several tracks at one angular location. A sequence reader observes a span of symbols along one or more tracks and matches that window to a code map. Specify window length, point order, reading direction and the usable range. A sequence is useful only if each required window identifies a unique permitted address, including the end or full-circle wrap region.
Why transition behaviour differs
Natural binary may change several bits at a boundary. In a conventional reflected Gray sequence, adjacent valid words change one bit. This reduces multi-bit ambiguity but does not remove optical threshold errors, registration errors or noise. A delayed changing bit may indicate one of the neighbouring words; valid decoding and boundary handling remain necessary. Natural binary can be used with a reader designed to coordinate sampling and handle its transitions.
| Representation | Position 7 | Position 8 | Changed bits |
|---|---|---|---|
| Natural binary | 0111 | 1000 | 4 |
| Reflected Gray code | 0100 | 1100 | 1 |
Release the full interpretation of the pattern
The code map needs bit significance, track order, clear/opaque states, observed side, increasing-angle direction and zero datum. For reflected binary Gray code, G = B XOR (B >> 1); conversion back to binary uses successive XOR operations. A custom map may follow a different convention. Verify the actual word at zero and at selected transitions instead of relying on a product photograph.
Single-turn angle and multi-turn position
A single-turn disc identifies position modulo one revolution. It can reveal the current within-turn angle at power-up when the reader is operating; the glass itself stores neither turn history nor an electronic count. Multi-turn identification uses additional mechanisms such as geared tracks, retained counting or energy-harvesting counting. Serial protocols and electrical outputs belong to the complete encoder, not the glass code pattern.
Validate boundaries and coarse/fine alignment
Test the approved code words and boundary behaviour with the installed sensing window in both directions. For a coarse absolute track plus a fine periodic track, define how fine phase is assigned at coarse transitions and how wrap-around is handled. Track registration, reader offset, thresholds and sensing gap can all shift the practical transition. Record invalid-word handling and the coordinate convention for the reported result.