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

FANUC A20B-3300-0260 | 486DX MMX CPU Module — Series 16i-A / 18i-A / 21i-A / 160i CNC

Processor Module for the First i-Series Generation

The A20B-3300-0260 is the original CPU module for FANUC's 16i-A, 18i-A, 21i-A, and 160i CNC controllers — the first i-series generation that introduced FSSB fibre-optic servo communication and modular plug-in board architecture to FANUC's CNC platform. The 486DX processor with MMX instruction set extension runs the CNC system software for this controller generation.

The 16i-A/18i-A modular architecture places this CPU module on the main host PCB (A20B-8100-013x) alongside other plug-in cards:

• FROM/SRAM module — non-volatile system software + machine parameters
• FSSB servo control card — manages fibre-optic servo bus communication
• DRAM module — working memory for programme execution
• Graphics module — drives the display output

The CPU module is the central processor around which all other modules function. It is not interchangeable across CNC generations.

Key Specifications

486DX + MMX — The Right Processor for This Software

The 486DX architecture is not outdated in this context — the 16i-A/18i-A system software was written for this processor. It runs on 486DX hardware exclusively: not emulated, not ported, not designed to run on anything else. The MMX instruction set extension accelerates certain numerical computation patterns relevant to real-time CNC operation at the expected cycle times for this controller generation.

Model A vs Model B — Not Interchangeable

A subsequent 16i-B/18i-B generation followed with Pentium-class processors in a different module format. The model A (A20B-3300-0260) and model B processors use different main boards with physically different CPU module connectors. Cross-generation substitution is physically incompatible. Confirm the installed controller is 16i-A, 18i-A, or 21i-A — not 16i-B or 18i-B — before ordering.

Diagnosing a Failed CPU Module

The CPU module failure presents as a failed boot sequence: the main board's diagnostic LEDs stop at an early code before the display initialises. The screen stays dark because the processor has not reached the display initialisation stage.

Before concluding the CPU module is faulty, check: control power supply voltages at the board (a failing supply mimics CPU failure), FROM/SRAM module seating, and DRAM module seating. A boot that proceeds to "LOADING BASIC TO DRAM" and then goes dark is more likely a FROM software edition mismatch than a CPU module failure — verify the FROM module contains software compatible with the specific hardware revision.

FAQ

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

This specific symptom — boot proceeds to DRAM loading then stops — is more commonly a software edition mismatch between the FROM module and the CPU hardware revision than a CPU module failure. Verify the FROM module contains the correct software edition for this hardware combination before replacing the CPU module.

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

No. Model A (16i-A/18i-A) and model B (16i-B/18i-B) use different main boards with different CPU module connectors and bus architectures. Cross-generation substitution is physically incompatible — always use a model A CPU module for model A hardware.

Q3: After installing a replacement, the boot completes but servo axes show alarm 401 (VRDY).

Alarm 401 after a CPU module replacement points to the FSSB fibre-optic connections disturbed during the work — not the CPU module itself. Check all fibre cable connections at the main board COP connectors and at each servo amplifier. Confirm each fibre end is clean, fully seated, and latched.

Q4: Is component-level repair of this CPU module possible?

A failed 486DX processor IC makes the module uneconomical to repair at component level. Peripheral components — oscillators, voltage regulators, decoupling capacitors — are individually replaceable if they are confirmed to be the fault source. A specialist can determine whether the failure is in the processor or in peripheral circuitry before deciding whether repair is viable.

Q5: Why does price vary significantly between suppliers for this module?

Condition, verification rigor, and warranty coverage drive the variation. Refurbished units tested in a live 16i-A system carry more confidence than visually inspected surplus stock. For a production-critical machine, a functionally tested unit with documented warranty is worthwhile — the cost of a second failure outweighs the initial price saving.