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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A20B-8100-0669 |
Part Number: A20B-8100-0669
Manufacturer: FANUC Corporation (Japan)
Product Category: PCB — Main CPU Board (Motherboard)
Series: A20B-8100 — FANUC i-Series CNC main CPU board family
The FANUC A20B-8100-0669 is a main CPU printed circuit board belonging to FANUC's A20B-8100 series — the family of LCD-mounted master boards at the centre of FANUC's i-series CNC controller generation.
These boards serve as the central hardware platform for FANUC's 16i-B, 18i-B, and 21i-B family of CNC controllers, the product generation that defined FANUC's mid-range and high-capability CNC offering through the 2000s and into the 2010s.
Every i-series CNC controller of this generation is built around one of these main boards.
The board hosts the plug-in CPU module, FROM/SRAM memory module, axis control card, and display/communication circuits. It connects physically to the LCD panel, making it an integrated LCD-mounted assembly — the controller's electronics and display are a single self-contained unit.
The A20B-8100 main board family covers multiple CNC variants through different part numbers. Each number in the series corresponds to a specific CNC model with a defined set of PMC capabilities, Ethernet options, and axis configurations.
The A20B-8100-0669 occupies this family and serves its designated CNC configuration with the same physical architecture and plug-in module system shared across the i-series B-generation boards.
| Parameter | Value |
|---|---|
| Board Type | LCD-mounted main CPU PCB |
| Series | A20B-8100 |
| Controller Generation | FANUC i-Series (B-generation) |
| Servo Interface | FSSB (Fibre-optic Serial Servo Bus) |
| Spindle Interface | Serial spindle |
| I/O Bus | I/O Link |
| Power Supply Module | A20B-8100-0720 (shared across 16i/18i/21i-B) |
| SRAM Backup | Required before any replacement |
| Status | Available — refurbished, tested |
| Origin | Japan |
FANUC's i-series B-generation — covering the 16i-B, 18i-B, and 21i-B controllers — brought a revised hardware platform compared to the earlier Model A generation.
The B-generation main boards share a common physical architecture while each variant carries the hardware configuration appropriate for its CNC software capabilities.
The main board is the anchor of the entire controller assembly. It provides:
FSSB servo bus communication. All servo amplifiers connect to the main board via FSSB optical fibre cables at the COP connectors.
The FSSB link carries position commands and feedback data in real time, replacing the older analogue and pulse-train servo interfaces of earlier FANUC generations.
Serial spindle interface. The spindle amplifier connects through the JA7A serial spindle port. Rigid tapping synchronisation, Cs-axis control, and spindle orientation all depend on this link.
I/O Link for machine I/O. The distributed machine I/O — limit switches, solenoids, door interlocks, tool change signals — connects through the I/O Link bus terminated at JD51A.
Plug-in module slots. The FROM/SRAM module, CPU processor module, and axis control card all install directly onto the main board.
These are replaceable as individual units during maintenance.
Power supply connection. The A20B-8100-0720 power supply module plugs directly into the main board and provides the regulated voltages the entire controller needs.
Replacing an A20B-8100-0669 correctly requires following FANUC's established procedure for i-series main board replacement. Skipping any step creates risk of data loss or a non-starting controller.
SRAM backup first. The SRAM module on the main board holds machine parameters, PMC ladder data, tool offsets, work coordinates, and stored part programmes.
Always execute a full SRAM backup before touching the board. Back up to a FANUC memory card, PC card, or RS-232C transfer to a PC. Confirm the backup is complete and readable before proceeding.
Module transfer. The FROM/SRAM module, CPU processor card, and axis control card from the original board transfer to the replacement.
These modules carry the machine-specific software and must not be swapped with modules from a different machine without verifying full software compatibility.
Post-installation check. After reassembly and SRAM restore, boot to the parameter check screen and verify all parameter pages are present.
Check the PMC ladder status. Jog each axis and confirm servo ready status. Test the spindle in JOG mode before returning the machine to production.
Q1: The controller loses power and will not restart after the main board was replaced. The display remains blank. What is the most likely cause?
A blank display immediately after main board replacement points to the FROM/SRAM module not being properly seated.
The module is the first thing the boot ROM looks for — if it cannot find the module or reads it incorrectly, the boot sequence cannot progress and the display stays dark.
Power down, reseat the FROM/SRAM module firmly, and retry. Also verify the A20B-8100-0720 power supply module is fully seated and its status LED is green.
Q2: After installing the A20B-8100-0669, all servo axes show alarm 401 (VRDY off). The servo amplifiers appear powered. What should be checked?
Alarm 401 immediately after a main board swap almost always traces to a disturbed FSSB fibre connection.
During board handling, the optical fibres at the COP10A connector(s) on the main board may have been pulled slightly loose.
Check each fibre end — it must be fully inserted and latched. Clean the fibre end faces with a dry lint-free swab if there is any visible contamination. Reconnect and cycle power.
Q3: Is a SRAM backup taken from a 16i-B or 18i-B compatible machine transferable to the A20B-8100-0669's host controller?
No. SRAM data is machine-specific. The parameter set, PMC ladder, and stored programmes are tied to the exact CNC model, software edition, and machine configuration of the original machine. SRAM from a different CNC variant will cause system alarms or incorrect machine behaviour.
Always restore the SRAM backup taken from the same machine the replacement board is going into.
Q4: Can the A20B-8100-0669 main board be repaired at component level?
Component-level repair is possible for peripheral circuits — discrete components, connectors, voltage regulators, clock circuits — but is not practical for the main processor or ASIC devices on the board. Most specialist FANUC repair facilities test the board in a live controller system and replace the board as a unit if the fault is in a non-repairable device.
Component repair is cost-effective only when the defective component is an identifiable discrete or a connector, not an ASIC.
Q5: How is the correct replacement main board identified if the machine's original part number is not known?
The existing main board's part number is printed on the board's identification label — a white or silver sticker typically located on one edge of the PCB. Photograph this label before removing the board.
If the label is damaged, the machine's system unit drawing number (beginning with A02B-) can be cross-referenced against FANUC's hardware configuration documentation to identify the correct main board part number for that controller system.
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