One New FANUC PLC Encoder A860-2020-T361 One year warranty A8602020T361 A860-2020-T361

Fanuc A860-2020-T361 | BiA128 Absolute Pulse Coder — Beta i Servo Motors, 128K ppr Serial Absolute, Flat Connector Version

Overview

The Fanuc A860-2020-T361 is the BiA128 absolute pulse coder with male flat connector, fitted to the rear shaft end of Fanuc Beta i series AC servo motors from the βi2 through βi22 motor class.

The BiA128 delivers 128,000 pulses per revolution of absolute serial position data to the connected Beta i servo amplifier, providing the CNC with a complete, battery-backed axis position reference that persists through machine power cycles.

The T361 suffix identifies this as the flat (male plug) connector variant of the BiA128, distinguishing it from the T301 and T321 variants which use different connector styles. Importantly, Fanuc has confirmed that the new flat version is backward-compatible with older encoder installations — a T361 replacement will physically and electrically interface correctly with existing motor and cable installations that previously used earlier BiA128 variants.

This is a significant practical benefit: service providers maintaining mixed-generation fleets can hold T361 stock as a single exchange unit for multiple BiA128 variants.

The BiA128's 128,000 ppr absolute resolution is appropriate for the Beta i motor's application context.

Beta i motors are Fanuc's cost-effective servo motor line, used extensively on feed axes of small-to-medium CNC machine tools, peripheral positioning equipment, and lighter automation applications where the premium performance of alpha i motors is not required.

At 128K ppr, the position resolution is fine enough for all standard CNC contour and positioning work within the Beta i motor's rated speed and torque envelope.

As a serial absolute encoder, the BiA128 communicates position data to the amplifier via a high-speed serial link rather than discrete quadrature pulse channels.

This serial protocol carries not just position counts but also encoder status flags, motor ID data, and fault diagnostics — information the amplifier uses for protective functions and motor matching verification.

The amplifier refuses to power the motor if the encoder's reported motor ID does not match the parameter-stored motor specification, a safety feature that prevents encoder mismatch from causing unexpected axis behaviour.

Key Specifications

T361 Connector — Flat Version vs Earlier BiA128 Variants

The suffix structure in Fanuc encoder part numbers carries functional information. For BiA128 encoders, the T301 suffix is the earliest version, T321 added an IP67 moisture protection rating, and T361 introduced the flat (male plug) connector design. Each successive version has been made backward-compatible by Fanuc, meaning the T361 can replace a T301 or T321 without any adapter or cable modification — the cable on the motor end accepts the flat connector of the T361 directly.

This backward compatibility is deliberate.

It allows service providers to stock a single BiA128 variant (T361) as the current production version and use it as a universal replacement across machines equipped with any BiA128 generation — simplifying spare parts inventory management significantly for facilities operating multiple machine tools from different production vintages.

Absolute Position and Battery Management

The "absolute" characteristic of the BiA128 means the encoder can report its current angular position within one revolution at any time, and the amplifier maintains the accumulated multi-turn count through battery backup.

Together, these provide the full absolute machine axis position without any reference return cycle. The axis position displayed on the CNC at power-up is the same as it was at power-down, provided the battery has remained healthy.

Battery condition is monitored by the CNC.

When battery voltage falls to the alarm threshold, the CNC generates a low battery warning (typically displayed as BAT alarm) and a log entry. At this stage, the battery should be replaced before the machine's next scheduled power-off — if battery power drops to zero while the machine is off, the absolute position memory in the amplifier is lost, requiring a manual reference return to re-establish the axis datum. 

The battery replacement procedure does not require removing the encoder or disturbing any mechanical components; it is performed at the amplifier's battery connector.

FAQ

Q1: What Beta i motor models specifically use the A860-2020-T361 BiA128 encoder?

The BiA128 fits Fanuc Beta i series motors from the βi2 through βi22 torque class: βi2/3000, βi4/3000, βi8/3000, βi12/3000, and βi22/3000, along with their speed variants and B-body versions (where the T361 applies rather than the T371).

The specific motor part number's last digit in the B-segment typically corresponds to the encoder type suffix — confirming the motor order specification document gives a definitive answer for any particular motor.

When the motor nameplate is legible, the encoder listed in the motor's configuration table in the Fanuc β series servo motor manual is the authoritative reference.

Q2: The T361 is described as the "new flat version" — will it physically fit in the same motor body as an older BiA128?

Yes. Fanuc designed the T361 to be a drop-in mechanical replacement for T301 and T321 encoders on the same motor family.

The mounting flange dimensions, shaft coupling, and cable connector at the motor end are compatible. 

The flat male plug on the T361 mates correctly with the encoder feedback cable's female connector that was installed for earlier BiA128 variants. No bracket adaptation, coupling modification, or cable rework is required.

Q3: What is the serial communication protocol between the BiA128 and the servo amplifier?

Fanuc uses a proprietary serial protocol between the encoder and the amplifier — it is not a standard industrial protocol. The amplifier initiates the serial polling cycle at each servo loop period, and the encoder responds with its absolute position count, status flags, and motor data.

This communication runs at high speed (typically several megabaud) on two dedicated encoder feedback wires within the motor cable.

The CNC does not directly process this serial stream — the amplifier decodes it and presents position data to the CNC over the FSSB ring in the standard axis feedback format.

Q4: How can I tell if the BiA128 encoder is faulty rather than the feedback cable or amplifier?

Swap the feedback cable first (use a known-good cable from another axis). If the alarm moves to the other axis, the cable is faulty. If the alarm remains on the original axis, disconnect the encoder connector at the motor end and inspect it for contamination, bent pins, or coolant ingress.

Clean and reseat the connector and test again. If the alarm persists through these checks, the encoder itself or the amplifier's encoder input circuit is at fault.

The amplifier can be tested by temporarily swapping it with a working amplifier from another axis — if the alarm follows the amplifier, the encoder input circuit has failed; if the alarm stays with the original axis, the encoder requires replacement.

Q5: Is the absolute position in the BiA128 truly retained through encoder replacement?

No — encoder replacement always requires a new reference return. When a BiA128 is replaced, the new encoder starts from an arbitrary angular position within one revolution. Even though the encoder is absolute within one turn, the amplifier's multi-turn accumulated count was associated with the old encoder's mechanical installation.

After fitting the replacement encoder, the axis must be physically moved to its reference point and a reference return executed to re-establish the absolute position datum in the amplifier's memory.

This is a standard procedure after any encoder change and should be included in the service documentation for the machine.