Encoders: incremental and absolute
How incremental (A/B/Z) and absolute encoders report position, resolution and edge evaluation, interfaces (HTL/TTL, SSI, EnDat, fieldbus) and how to count them in a PLC.
Incremental encoders
Two channels A and B, 90° phase shifted (quadrature), and an index pulse Z once per revolution.
- Direction = which channel leads.
- Resolution: pulses per revolution (PPR). Evaluating all edges of A and B (×4) gives 4 × PPR counts per revolution: a 1024-PPR encoder → 4096 counts.
- After power-up the absolute position is unknown → homing to a reference switch and/or the Z pulse.
Signals: HTL (10–30 V, push-pull, robust over long cables) or TTL/RS-422 (5 V differential, high frequency). Use differential signals and shielded twisted pairs over longer distances.
Absolute encoders
Every position has a unique code — known immediately after power-up.
- Single-turn: position within one revolution (e.g. 13 bit = 8192 positions).
- Multi-turn: also counts revolutions (e.g. 12 bit = 4096 turns), mechanically (gears) or electronically (battery / energy harvesting).
- Interfaces: SSI (synchronous serial), EnDat, HIPERFACE, BiSS, or fieldbus (PROFINET, EtherCAT, CANopen CiA 406, IO-Link).
Counting in a PLC
| Frequency | Solution |
|---|---|
| < input filter frequency (a few hundred Hz) | Normal digital input + software counter — fragile |
| Up to ~100 kHz | High-speed counter (HSC) inputs or technology modules |
| Positioning with drives | Encoder connected to the drive; PLC gets position via fieldbus telegram |
Check maximum frequency: f = rpm / 60 × PPR — 3000 rpm with 2048 PPR = 102.4 kHz per channel.
Typical problems
Missing counts from noise (unshielded cables, no differential receivers), wrong direction (swap A/B or invert in configuration), overflow of 16-bit counters, and lost position of incremental encoders after power loss when no homing is done.
Connections
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Sources
Updated 2026-10-02