Fanuc Board A20B-3300-0260 A20B33000260 A2OB-33OO-O26O New And Original And Used

FANUC A20B-3300-0260 | MMX CPU Module — For FANUC Series 16i-A / 18i-A / 21i-A / 160i CNC, 486DX Processor Card, A20B-3300 Series, Japan Origin

Part Number: A20B-3300-0260

Manufacturer: FANUC Corporation (Japan)

Product Category: PCB — CPU Module (Processor Plug-in Card)

Series: A20B-3300

Overview

The FANUC A20B-3300-0260 is the original processor module for FANUC's 16i-A, 18i-A, and 21i-A CNC generation — the first generation of FANUC's high-capability "i-series" controllers that brought FSSB digital servo communication, high-resolution interpolation, and advanced PMC functions to production machine tools in the late 1990s. The module also sees use in FANUC Series 160i CNC systems.

It carries the 486DX processor with MMX instruction set extension in a compact plug-in format that installs directly onto the main model A motherboard.

The 16i-A/18i-A/21i-A generation marked a significant architectural shift in FANUC's CNC design. Earlier Series 16/18 controllers used entirely different hardware.

The i-series introduced FSSB fibre-optic servo communication, a modular plug-in board architecture where separate CPU module, FSSB servo card, graphics module, and memory modules all install into one host mother PCB. This architecture remains in production service on tens of thousands of machine tools worldwide today.

The A20B-3300-0260 is the model A generation processor — for the first revision of the 16i/18i/21i hardware. A subsequent model B generation followed with faster Pentium-class processors in different module formats.

The model A and model B generations are not interchangeable.

Key Specifications

The 16i-A / 18i-A Modular Main Board Architecture

Understanding the A20B-3300-0260's role requires understanding the model A main board system. The main board (A20B-8100-013x) is a large host PCB that provides mounting positions and connectors for all functional modules.

The A20B-3300-0260 CPU module is one of several boards that typically install on the main board simultaneously:

CPU module (A20B-3300-0260): The processor, running the CNC system software loaded from the FROM module.

FROM/SRAM module (A20B-3900-xxxx): Non-volatile flash memory containing the CNC system software; SRAM holds machine parameters and PMC data.

FSSB servo control card (A20B-3300-0032 or similar): The axis control module managing FSSB communication with servo amplifiers.

Graphics module: Drives the CNC's display output.

DRAM module (A20B-3300-0310 or similar): Working dynamic RAM for programme execution.

Each of these modules installs directly onto the main board as an SMD-style plug-in card. Together they form the complete intelligence of the 16i-A/18i-A controller.

The CPU module is the central processor around which all other modules operate — it is the only component that cannot simply be swapped with a variant from a different CNC generation.

486DX + MMX — Why This Processor Still Matters

The 486DX architecture may appear outdated by today's standards, but its relevance in CNC maintenance is straightforward: the FANUC 16i-A/18i-A/21i-A system software was written for this processor.

It runs on 486DX hardware exclusively. There is no upgrade path to a modern processor — the software is not ported, not emulated, and not designed to run on anything else.

The MMX instruction set extension added SIMD processing capability, accelerating certain numerical computation patterns relevant to motion control and signal processing.

For the 16i-A/18i-A CNC applications, this enhancement contributed to the processor's adequacy for real-time CNC operation at expected cycle times.

Within the model A generation, the A20B-3300-0260 is described as the base-speed variant — slower clock compared to later i-series Pentium-class processors, but fully correct and sufficient for the 16i-A/18i-A/21i-A software.

CPU Module Failure — How to Diagnose

The A20B-3300-0260 failure is best confirmed through process of elimination within the model A CNC system.

Symptom pattern: The CNC does not complete its startup sequence.

The main board's diagnostic LEDs stop at an early code during boot, before the display initialises. No alarm appears on the screen because the processor has not reached the point of initialising the display.

Isolation approach: If a known-good CPU module from an identical machine is available for temporary substitution, this is the fastest test.

A specialist repair shop can test the module by inserting it into a reference 16i-A controller and verifying the startup sequence completes.

Important note: Testing the A20B-3300-0260 in isolation is not practical — the module requires the full 16i-A/18i-A system context (main board, FROM module, and power supply) to demonstrate correct operation.

FAQ

Q1: The 16i-A CNC shows "LOADING BASIC TO DRAM" on the boot display and then the screen goes dark. Is the CPU module faulty?

"LOADING BASIC TO DRAM" is an early stage in the 16i-A/18i-A boot sequence where the boot ROM copies system software from FROM into DRAM. If the screen goes dark immediately after this message, the most common cause is an incompatibility between the software edition in the FROM module and the CPU module hardware revision — not necessarily CPU module failure.

Verify the FROM module contains software of the correct edition for this specific hardware combination.

The FANUC 16i-A CPU card modification additional manual documents the software edition requirements for each hardware revision.

Q2: Can the A20B-3300-0260 be replaced with a later Pentium-class CPU module from the 16i-B / 18i-B generation?

No. The model A (16i-A/18i-A) and model B (16i-B/18i-B) architectures use different main boards with physically different CPU module connectors and bus architectures.

Cross-generation substitution is not supported and is physically incompatible. 

Always use a model A CPU module for model A hardware.

Q3: After installing a replacement A20B-3300-0260, the controller boot sequence completes but several servo axes show alarm 401 (VRDY). What should be checked?

Alarm 401 after a CPU module replacement, with the boot sequence otherwise completing, typically points to the FSSB fibre-optic servo bus rather than the CPU module itself.

During the module replacement work, FSSB fibre cables connected to the FSSB servo card on the main board may have been disturbed. 

Check all fibre connections at both the main board's COP connectors and at each servo amplifier. Confirm each fibre end is clean, fully seated, and latched.

Q4: Can the A20B-3300-0260 module be repaired at component level after failure?

Component-level repair of the processor IC is generally not economical — a failed 486DX processor means replacing the entire module.

Peripheral components such as oscillators, voltage regulators, and decoupling capacitors are individually replaceable if they are the fault source. 

A qualified specialist can diagnose whether the failure is in the processor itself or in peripheral circuitry, which determines whether repair is viable.

Q5: The A20B-3300-0260 is listed at different prices by different suppliers. What explains the variation?

Price variation reflects differences in condition (refurbished versus tested good-pull versus untested surplus), verification rigor (functionally tested in a live 16i-A system versus visually inspected only), warranty coverage, and market scarcity.

The 16i-A/18i-A generation is old enough that supply is limited to the secondary market. 

For production-critical machines, a functionally tested unit with documented warranty is worthwhile over an untested unit at a lower price — the cost of a second failure far outweighs the initial price saving.