FANUC A860-2020-T301 Pulse Coder — Beta i Series BiA128 Absolute Encoder for CNC Servo Motors | New

Every FANUC Beta i series servo motor depends on one small component to function at all: the pulse coder attached to its non-drive end. Without it, the servo amplifier cannot close the position and velocity loop, and the axis goes nowhere. The FANUC A860-2020-T301 — designated the BiA128 — is the factory-standard absolute pulse coder fitted to FANUC Beta i series AC servo motors across a broad range of CNC machine tools. Hundreds of thousands of these encoders are running in production machines worldwide. This listing is for a new unit, ready for direct installation.

Confirmed Specifications

What the BiA128 Actually Does

The A860-2020-T301 is not a standalone device — it lives inside the servo motor housing, mounted at the rear shaft end, rotating with the motor shaft. Its job is to continuously report the shaft's angular position to the connected servo amplifier via a high-speed serial data link.

Being an absolute encoder means the BiA128 provides a unique position value for every angular position of the shaft — all 128,000 of them per revolution. This is fundamentally different from an incremental encoder, which only reports movement from a reference point. With absolute feedback, the servo amplifier always knows exactly where the motor shaft is, including through power interruptions. When the CNC powers on, every axis equipped with a functional A860-2020-T301 and a charged backup battery reports its current position immediately. No homing sequence, no reference return, no waiting — the machine is ready.

128,000 ppr — Position Density That Drives Axis Accuracy

The 128K resolution of the BiA128 provides 128,000 distinct position counts per full shaft revolution. The servo amplifier uses this data continuously to compute velocity, detect position error, and issue corrective current commands to the motor — all within the microsecond-scale control cycle that FANUC's servo system operates on.

At high interpolation settings within the servo drive, the effective counts per revolution at the control level can exceed even the native encoder resolution. In practice, the 128K ppr resolution means the servo can detect and correct positional errors smaller than one arc-second — positioning precision that translates directly into finished part dimensional accuracy on the machine tool.

The Motors That Use This Encoder

The B128iA encoder fits on FANUC Bi series servo motors, from series model βi2 onward — through βi4, βi8, βi12, βi22, and other variations of Beta i servo motors. These Beta i motors are the workhorses of smaller CNC applications: ATC arm drives, turret indexing, 4th axis drives, and smaller feed axes on lathes and machining centers.

The A860-2020-T301 is the encoder specified for these motors when they are part of the Alpha i and Beta i series drive systems using the 61xx series amplifiers (A06B-6114, A06B-6130, etc.). This BiA128 absolute pulse coder is fitted on hundreds of CNC machine tools, providing absolute feedback for FANUC servo motors across the ai and βi motor range.

A Self-Contained, Sealed Assembly

The A860-2020-T301 is a sealed optical assembly mounted inside the motor. Because of how FANUC manufactures these encoders — with optical elements, signal processing electronics, and shaft coupling sealed together as a single precision unit — replacement is always at the complete assembly level.

Due to this construction method, the encoder is essentially non-repairable at the component level. This is actually a practical benefit from a maintenance planning standpoint: the failure mode is clear, the replacement is a known, priced part, and the installation is straightforward once the motor is properly disassembled and the shaft coupling accessed.

Common Failure Modes and How to Identify Them

When an A860-2020-T301 fails, the symptoms are recognizable. Typical alarm codes on the FANUC CNC or servo amplifier include encoder communication errors, pulsecoder disconnect alarms, and position feedback abnormality alarms. On the servo amplifier module itself, alarm codes indicating serial feedback failure point directly to the encoder or its cabling as the first area to investigate.

Before condemning the encoder, it is worth verifying the encoder cable — the serial feedback cable between motor and amplifier — as cable damage is a common cause of encoder-related alarms that can be mistaken for encoder failure. If the cable checks out, the encoder itself is the most likely cause.

Where This Encoder Is Found in Production

The A860-2020-T301 is essentially universal across the FANUC Beta i motor installed base. Machine types where this encoder is standard equipment include:

CNC Machining Centers — ATC carousel, tool changer arm, and 4th axis rotary table drives on vertical and horizontal machining centers running FANUC 0i, 16i, 18i, or 21i controls with βi amplifier systems

CNC Turning Centers — turret indexing motors and auxiliary axis drives on CNC lathes where Beta i series motors provide the servo positioning function

FANUC-Equipped Robots — joint axes on FANUC robot systems that use βi series servo motors for lighter-load arm segments

EDM and Specialized Machine Tools — axis drives on wire-cut EDM machines, drill-tap centers, and other specialized CNC equipment where Beta i motors provide the feed axis drive

T301 vs. T321 — The Sealing Difference

The A860-2020-T301 and its close relative A860-2020-T321 share the same BiA128 encoder designation and 128K absolute resolution. The distinction is environmental protection: the T321 is IP67 rated for better resistance to liquid ingress, making it the correct choice for motors in coolant-exposed positions. The T301 is the standard version suited for typical machine tool environments. Both variants provide identical electrical and feedback performance. Selecting the right variant depends on the motor's location on the machine and its exposure to coolant or cutting fluid.

FAQ

Q1: Which FANUC servo motors are compatible with the A860-2020-T301?

The A860-2020-T301 BiA128 encoder is fitted to FANUC Beta i series AC servo motors, spanning models from βi2 through βi22 and related variants. It is also used on certain Alpha i series small-capacity motors (ai4 through ai50 range) when installed in systems using the 61xx series amplifiers. The motor's nameplate or FANUC's motor maintenance documentation identifies the correct encoder part number for each specific motor model.

Q2: What is the difference between absolute and incremental pulse coders on FANUC systems?

An absolute pulse coder like the A860-2020-T301 stores position data and provides a unique position value for every shaft angle — 128,000 positions per revolution. Position is retained through power-off cycles via a backup battery in the servo amplifier. At power-up, the axis position is immediately known. An incremental encoder only counts movement from a reference point; after any power interruption, a homing sequence is required before the axis can be used. Absolute systems eliminate that homing step, saving time on every machine startup.

Q3: The CNC is showing a pulsecoder alarm on the axis using this encoder — is it definitely the encoder that's failed?

Not necessarily. Pulsecoder alarms can be caused by a failed encoder, a damaged serial feedback cable, a loose or corroded connector, or in some cases a failed servo amplifier. Before replacing the encoder, inspect the cable for damage along its full length and verify all connectors are fully seated. If the cable and connectors are good and the alarm persists, the encoder itself is the likely cause. Testing the motor on a known-good amplifier with a verified cable can confirm or rule out the encoder as the fault source.

Q4: Is a backup battery required for the A860-2020-T301 to function?

The encoder operates without a battery — it outputs absolute position data whenever the servo amplifier provides power. The battery (typically installed in the servo amplifier, not the encoder) is required to retain multi-turn position data through machine power-off cycles. Without battery backup, the single-revolution absolute position is still correct at power-up, but the multi-turn counter resets — requiring a reference return. For machines that need to power up without a homing sequence, maintaining the amplifier battery is essential.

Q5: Can the A860-2020-T301 be repaired, or must it always be replaced as a complete unit?

The A860-2020-T301 is a sealed optical assembly — component-level repair is not a viable option for end users and is generally not recommended even by specialist repair facilities. The encoder contains precision optical elements and signal processing electronics sealed together at the factory. When an A860-2020-T301 fails, complete unit replacement is the correct and practical course of action. This is why maintaining a spare unit in the parts inventory is the standard recommendation for any CNC machine that carries these encoders.