Encoder Signal Structures — Differential Output, TTL, and HTL

Encoder signal types such as single-ended and differential outputs, together with TTL and HTL level standards, are often confused. Below is a reader-friendly explanation of differential output principles, its relation to TTL/HTL, and practical connection tips that technicians can apply directly in the field.

What Is Differential Output?

Differential output uses two complementary signals per channel, transmitted simultaneously — usually labeled as A & A-, B & B- (and optionally Z & Z-). The receiver measures the voltage difference rather than the absolute level of these signals. This technique cancels out common noise and provides a clean and stable signal even over long cable distances or in electrically noisy environments.

Noise immunity: Common-mode noise affects both lines equally, so the receiver cancels it out when taking the difference — this minimizes EMI impact.
Signal integrity: Differential transmission maintains clean signal edges and prevents threshold errors during high-speed communication or long-distance runs.
Robustness: Encoders using differential outputs are more reliable in harsh industrial environments, reducing the risk of miscounts or false triggers.

Relationship Between Differential, TTL, and HTL

TTL operates at 5 V logic levels and, when used differentially, follows the RS-422 electrical standard. HTL operates between 10–30 V (commonly 24 V) and can be either push-pull or open-collector type. Remember this golden rule: “Differential or single-ended” defines the transmission method; “TTL or HTL” defines the voltage level standard.

+ TTL Differential (RS-422): 5 V level, perfect for long distance and high-speed accuracy.
+ HTL Differential: 24 V compatible with strong noise immunity — ideal for industrial environments.
+ TTL or HTL Single-Ended: Suitable for short distance and low/medium speed systems.

Practical Wiring and Installation Tips

+ Use the correct input card: Connect differential encoders only to RS-422 compatible or differential counter inputs on PLC/HSC units. Avoid mixing with single-ended inputs.
+ Termination: Add ≈120 Ω termination at the receiver side, as recommended by the encoder manufacturer.
+ Cable selection: Always use shielded twisted-pair cable — pair A/A- together, B/B- together. Ground the shield at one end only to prevent ground loops.
+ Noise suppression: If your environment contains strong switching noise, use zero-phase reactors and EMI filters for additional protection.
+ Speed vs distance: Higher encoder frequency increases signal sensitivity; differential drivers make long-distance operation reliable.

Incremental and Absolute Encoder Applications

Incremental encoders with differential A/B/Z signals minimize phase shift and ghost counting. Absolute encoders also benefit from differential drivers, especially in multi-bit parallel or serial interfaces such as SSI and BiSS.

Quick Summary

Differential = noise cancellation + long distance + high reliability. TTL and HTL represent signal voltage levels. If your environment is noisy or requires long cables, use differential. For 5 V systems, go with TTL; for 24 V systems, choose HTL. Always ensure your PLC input module supports true differential reception.

Related Links

+ Encoders Main Page
+ Encoder Output Types
+ TTL and HTL Output Signals
+ Encoder Counters
+ PLC Systems for Encoders
+ Encoder Price List

Pro tip: Route encoder cables separately from motor power lines, crossing them at 90° if necessary. Activate the high-speed counting function in your PLC for accurate pulse tracking.