A practical wiring guide for incremental (A/B/Z) encoders to PLC HSC inputs—covering NPN/PNP (open-collector), push-pull, and RS-422 line-driver outputs, with shielding/grounding tips, max-frequency notes, and brand specifics (Siemens / Mitsubishi / Omron).

Who this is for & what you’ll get

• For controls engineers and technicians wiring encoders to PLCs on packaging, printing, and conveying lines.

• You’ll get: standard wiring patterns, a comparison table, a ready-to-use checklist, and vendor references.

HSC + ABZ refresher (60 seconds)

A high-speed counter (HSC) captures fast pulses from incremental encoders using two quadrature channels A/B (90° apart) for direction and optional Z (index) for reference. x1/x2/x4 decoding multiplies effective resolution

ABZ timing 

Use this timing chart to explain A-leads-B direction and the single Z pulse per revolution.

*Reasoning: line-driver outputs source and sink current actively, driving longer cables with cleaner edges; open-collector relies on pull-ups and has slower transitions.

Wiring patterns by output type (step-by-step)

*Always confirm your exact PLC/HSC terminal names and the encoder datasheet before wiring.

A) Open-collector NPN/PNP → PLC HSC (single-ended)

1. Power & common: Encoder V+ → 24 V (or specified 5/12/24 V). Tie 0 V to PLC 0 V.

2. Signals: A → HSC A, B → HSC B, Z → HSC Z (if used).

3. Polarity & input mode:

   • NPN (sinking to 0 V) → use PLC source-type input or add pull-up to V+.

   • PNP (sourcing to V+) → use PLC sink-type input or add pull-down to 0 V.

4. Shielding: Terminate cable shield at PLC side only (single-point).

5. Digital filtering: Set HSC input filter appropriate to target max frequency. (Siemens S7-1200 docs describe HSC current value mapping and configuration.)

Common symptom: missed counts at higher RPM (slow edges). Consider push-pull or line-driver.

B) Push-pull (Totem-pole) → PLC HSC (single-ended)

1. Power & common as above.

2. Signals to A/B/Z inputs directly.

3. Verify input voltage/current limits on the PLC card/CPU. (Vendor manuals list input types and limits.)

Why better than open-collector? Actively driven high/low edges reduce rise/fall time issues.

C) Line-driver (RS-422) → PLC HSC (differential) — recommended for high speed/long runs

1. Power as specified (5 V or 24 V variants exist—match your encoder).

2. Pairs: A+ → HSC A+, A- → HSC A-; B+ → HSC B+, B- → HSC B-; Z+/Z- likewise.

3. Cable: twisted pairs per channel, overall shield to PLC ground (one-end).

4. Check module max freq; e.g., Mitsubishi FX5-2HC supports differential encoders up to MHz-class inputs.

Siemens provides dedicated wiring pages for TTL/RS-422 on S7-1500/ET200MP technology modules (e.g., TM PosInput). Use those diagrams if that’s your hardware

Brand-specific notes (quick refs)

• Siemens (S7-1200 / S7-1500 / TM modules)
  Configure HSC in TIA Portal; check input filter times and current value addresses; S7-1500 supports connecting external encoders via HSC/technology objects (Motion Control).

• Mitsubishi (iQ-F / FX5-2HC)
  FX5-2HC high-speed counter module explicitly supports differential line-driver encoder inputs for high-resolution signals (see module datasheet/manual).

• Omron (CJ-series CT units)
  Manuals show ABZ to counter inputs and mode examples—handy for reference wiring and commissioning

Commissioning checklist (copy/paste for the panel door)

• Polarity: NPN/PNP matched to sink/source input mode.

• Power: Encoder supply voltage correct; commons tied; shield terminated one end.

• Pairs: For differential, A+/A-, B+/B-, Z+/Z- not crossed.

• Filters: HSC input filter time set for target max frequency (too long ⇒ missed counts).

• Direction: Verify A leads B in forward; swap A/B or invert direction bit if needed.

• Z index: Confirm once-per-rev zeroing/homing logic.

• Log: Capture raw counts at idle vs. motion to spot noise.

Troubleshooting quick guide