Engineering · Motors & Drives
Encoder Types and Selection
Incremental or absolute, TTL or HTL, SSI or fieldbus: which encoder fits the job?
An encoder turns shaft motion into electrical signals for a drive, PLC or motion controller. Choosing one means answering four questions in order: must it know true position after a power loss, which signal level or interface will carry the data over the cable you have, how fine must the resolution be, and how will it mount on the shaft. This page lets you turn a code disc, watch incremental and absolute encoders lose and keep position, test TTL and HTL signals over a long cable, build an SSI data word, calculate resolution and pulse frequency, and run a selection wizard.
An incremental encoder gives pulses; the controller counts them from a reference, so a turn of the shaft while the power is off is never counted. An absolute encoder reads a unique code for each angle, so the position is correct as soon as power returns. The Gray code changes one bit at a time between neighbouring positions, which avoids the large false readings that binary can give at the edges between sectors.
| Item | TTL (usually RS-422 differential) | HTL (push-pull) |
|---|
The received waveform is a simplified model: a first-order cable delay and added noise. It shows the trend, not a cable calculation. Always check the encoder datasheet, the cable and the receiving input for the real limits.
| Interface | How it works | Typical use |
|---|
| Type | Mounting | Good for | Watch for |
|---|
Incremental resolution is quoted in pulses per revolution (PPR). A controller that counts every edge of A and B (quadrature) gets four counts per pulse. Absolute resolution is the number of bits: each extra bit doubles the positions. A multi-turn encoder adds bits that count whole revolutions.
| Feature | Incremental | Absolute | TTL | HTL | SSI |
|---|
| Application requirement | Suggested encoder |
|---|
Limits for cable length, frequency and supply vary between encoder makers and receiving inputs. The values here are typical, to be checked against the datasheets. Switch off and lock out before wiring or fitting an encoder.