1 PC New Fanuc A20B-2902-0060 Memory Module A20B29020060 A2OB-29O2-OO6O

FANUC A20B-2902-0060 | 2-Axis Digital Servo Control Module — Series 16MB / 16TB / 18MB / 18TB SIF PCB

Part Number: A20B-2902-0060

Manufacturer: FANUC Ltd. (Japan)

Product Type: Servo Control Module — 2-Axis Digital Servo SIF (Serial Interface) PCB 

Overview

The A20B-2902-0060 is the two-axis digital servo control module that adds 3rd and 4th axis servo capability to FANUC Series 16 and 18 CNC systems. It plugs into slot 7 of the main CPU board — or onto a compatible option board — and uses FANUC's serial servo interface (SIF) to command the connected servo amplifier axes.

Series 16 and 18 were FANUC's high-performance CNC platforms for the 1990s and early 2000s, installed widely in multi-axis machining centres, turning centres, and grinding machines. Many of these machines remain in production. The A20B-2902-0060 enables the 3rd and 4th controlled axes that most multi-axis FANUC 16 and 18 installations depend on — a failed module stops those axes immediately, typically triggering servo alarms on the CNC display.

The SMD construction of the A20B-2902-0060 — surface-mounted components on both sides — was standard for FANUC's high-density servo modules of this generation. It delivers the processing density needed for the module's axis control calculations in a compact form factor.

Key Specifications

Servo Alarms Associated with A20B-2902-0060 Failure

FANUC Series 16 and 18 CNC systems generate specific servo alarms when the A20B-2902-0060 module cannot communicate with the servo amplifiers or loses its internal processing functions. The alarms most commonly associated with this module are in the 300-series and 900-series ranges:

Alarm 301 to 305 — Servo alarm, axis-specific communication or control errors on the 3rd and/or 4th axes. These typically appear on the CNC display as "301 SERVO ALARM: 3RD AX. SERVO" or equivalent, identifying the specific axis and fault type.

Alarm 920 to 924 — RAM parity error alarms indicating a hardware memory fault within the servo module itself. These alarms typically mean the module has failed internally and requires replacement.

When any of these alarms appear consistently on the 3rd or 4th axis after clearing — particularly if other axes remain unaffected — the A20B-2902-0060 is the primary suspect.

Installation and Slot Position

The A20B-2902-0060 plugs into slot 7 of the FANUC main CPU board or into the designated slot on the A16B-2202-0400/0401 option board. Slot 7 is the standardised position for 3rd/4th axis servo modules on Series 16 and 18 Model B main CPU boards.

Before inserting the replacement module, power the CNC off completely and confirm the DC bus has discharged. Handle the board only with ESD protection — all FANUC SMD modules contain ESD-sensitive components. Insert firmly and verify the board is seated fully in the connector before applying power.

FAQ

Q1: The CNC shows Alarm 301 and 302 simultaneously on the 3rd and 4th axes after power-on. The 1st and 2nd axes are normal. Is the A20B-2902-0060 the cause?

Alarms 301 and 302 appearing together on both the 3rd and 4th axes immediately at power-on — with the 1st and 2nd axes unaffected — is the classic presentation of a failed A20B-2902-0060 module.

The 1st and 2nd axes are typically managed by the main CPU's onboard servo circuits, not by this add-on module. Confirm by reseating the module connector first. If the alarms persist after reseating, replace the module.

Q2: A replacement A20B-2902-0060 has been installed and the alarms have cleared, but the 3rd axis runs erratically during positioning. What should be checked?

Erratic axis behaviour after a module replacement often indicates a fibre-optic cable issue in the serial servo link between the new module and the servo amplifier. Check all SIF fibre-optic cables for cleanliness and correct connection at both ends. 

Also verify the servo amplifier's parameters have not been disturbed during the maintenance. A servo gain imbalance or position loop gain mismatch can also produce erratic positioning after any board change.

Q3: Is the A20B-2902-0060 compatible with Series 16 Model A machines, or only Model B?

The A20B-2902-0060 with its SMD design is specifically designed for the Series 16 and 18 Model B (MB/TB) main CPU architecture. Model A machines use a different physical form factor and connector scheme.

Confirm the specific CNC model designation (MB or TB) from the machine's serial number plate before ordering — installing the wrong variant produces interface mismatches that prevent axis operation.

Q4: The failed A20B-2902-0060 shows Alarm 920 (RAM parity). Can this board be repaired at component level?

Alarm 920 (RAM parity) indicates a failed RAM component on the module itself. Component-level repair is technically feasible if the specific RAM IC is identifiable and replaceable. However, FANUC SMD modules use proprietary or now-legacy memory devices that may be difficult to source.

A tested exchange module from a specialist FANUC repair company is usually faster and more reliable than attempting RAM-level repair, particularly when production downtime is a factor.

Q5: Two machines use the same A20B-2902-0060 module type. Can the module be temporarily transferred from the working machine while a replacement is sourced?

Yes — if both machines use the same Series 16MB/16TB or 18MB/18TB main CPU type and the module is in the same slot position, physical transfer is feasible. The module itself stores no machine-specific parameters — axis parameters are stored in the CNC's SRAM on the main CPU.

The transferred module begins operating immediately with the receiving machine's parameters. This is a viable short-term measure, with the risk of both machines being down if the second machine also encounters a fault.