Absolute Fanuc Motor Encoder Board A20B-9000-0380

FANUC A20B-9000-0380 | Spindle Motor Encoder Signal Converter PCB — 12-Pin Interface, Quadrature Output

Part Number: A20B-9000-0380

Also Referenced As: A20B90000380

Manufacturer: FANUC Corporation (Japan)

Product Type: Spindle Motor Encoder / Signal Converter PCB

Series: A20B-9000 

What This Board Does

The A20B-9000-0380 sits inside the spindle motor housing, between the optical pickup element and the CNC control system. The pickup reads the coded disc on the rotor shaft and generates raw analogue sin/cos waveforms — position data, but not in a form the digital controller can directly use. This PCB converts those waveforms into two outputs:

Quadrature A/B pulse signals — square-wave output from which the CNC calculates actual spindle speed and rotation direction in real time.

Zero-pulse (one-revolution marker) — a single reference pulse generated once per full shaft revolution, approximately 120 µs wide, that establishes an absolute angular reference for spindle orientation and homing.

Without both outputs functioning correctly, speed feedback collapses and any operation that requires precise spindle-to-feed coordination — rigid tapping, C-axis positioning, spindle orientation for tool change — either alarms out or produces inaccurate results.

Failure Symptoms and Before-You-Replace Checks

The A20B-9000-0380 is passive with no moving parts, so it typically outlasts other spindle components. When it does fail, the usual causes are oil mist ingress, vibration-induced solder joint fatigue, or optical component degradation. Common CNC alarms:

• SP9001 / SP9002 — Spindle motor speed feedback error
• SP9003 — Spindle position coder signal abnormality
• Alarm 434 / 435 (varies by CNC generation) — Spindle sensor disconnection

Before replacing the board, check the 12-pin connector first. Corrosion or a loose pin on that connector produces exactly the same alarm presentation as a failed PCB — and a connector fix takes five minutes.

Key Specifications

FAQ

Q1: Which FANUC spindle motors use the A20B-9000-0380?

Early-generation FANUC AC spindle motors with an internal optical pickup sensor and a 12-pin feedback interface. The -0300 is the most commonly paired variant. Confirm compatibility by matching the part number on the installed board rather than relying on motor frame size — FANUC does not publish a comprehensive public cross-reference for this series.

Q2: Can another A20B-9000 variant substitute if -0380 is unavailable?

Not without verification. Different suffixes carry different pinouts and pulse counts. A physically similar board from the wrong suffix produces speed calculation errors or immediate alarms. If the exact -0380 is unavailable, have the motor specifications reviewed by a FANUC-qualified technician before any substitution.

Q3: What CNC systems is this board compatible with?

FANUC Series 0, Series 10/11, Series 15, and related control hardware from the corresponding production period. Later FANUC spindles (αi, βi families) use different encoder technologies with different boards and connectors — the A20B-9000-0380 does not apply to those.

Q4: Is this board the same as an A860-series spindle position coder?

No — they serve different roles. The A20B-9000-0380 is inside the spindle motor, converting the internal optical pickup output to digital feedback for speed regulation. A860-series position coders are external sensors used for high-resolution spindle orientation and C-axis control. Many machines use both simultaneously.

Q5: How should a used unit be evaluated before installation?

Check that the unit was electrically tested (not just visually inspected), confirm the 12-pin connector and solder joints are intact, and verify the optical pickup surface is free of oil and contamination. Handle with ESD precautions and keep the optical surface clean during storage and installation.