Encoder Interfaces for PLC: How to Connect Them Correctly
The choice of encoder output interface is determined by the type of PLC or controller used and the wiring distance. This guide explains the available options and when to use them.
Push-Pull (HTL) — The Universal
Push-Pull output (also called HTL or totem-pole) switches between supply voltage and GND. It works as both NPN (sink) and PNP (source), making it compatible with any PLC. Typical supply: 10-30 Vdc.
When to use: standard applications, short distances (< 30 m), generic PLCs.
Line Driver (RS-422) — The Noise Fighter
Line Driver output (TTL/RS-422) transmits differential signals on wire pairs. Each channel has two lines (A and /A, B and /B) with complementary voltages. The differential receiver subtracts noise picked up on the cable, ensuring electromagnetic noise immunity.
When to use: long distances (up to 200 m), environments with strong EMC interference, high frequencies (> 100 kHz).
SSI (Synchronous Serial Interface)
SSI is a synchronous serial point-to-point protocol for absolute encoders. The master (PLC) generates the clock and the encoder responds with position in Gray or binary format. Typical clock speed: 100 kHz - 2 MHz. Distance: up to 50 m.
When to use: absolute encoders with Siemens, Beckhoff, Allen-Bradley PLCs.
Fieldbus (EtherCAT, PROFINET, Profibus)
Fieldbus protocols allow connecting multiple encoders on a single cable in daisy-chain topology. They offer integrated diagnostics, remote parameterization, and deterministic cycle times (< 1 ms for EtherCAT).
When to use: complex systems with many axes, diagnostics requirements, Industry 4.0 integration.
Summary Table
| Interface | Max Distance | Speed | Cost | Complexity |
|---|---|---|---|---|
| Push-Pull | 30 m | Medium | Low | Minimal |
| Line Driver | 200 m | High | Low | Low |
| SSI | 50 m | Medium | Medium | Medium |
| EtherCAT | 100 m | Very High | High | High |
| PROFINET | 100 m | High | High | High |
