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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A20B-8100-0661 |
Part Number: A20B-8100-0661
Manufacturer: FANUC Corporation (Japan)
Product Category: PCB — Main CPU Board (CNC Control Motherboard)
Series: A20B-8100 — FANUC 16i-B / 18i-B / 21i-B main board family
The FANUC A20B-8100-0661 is the main CPU board for FANUC Series 18i-B CNC controllers. Whether the machine is an 18i-MB (machining centre), 18i-TB (lathe), or another 18i-B variant, this is the board at the centre of the control system — the single most consequential hardware component in the 18i-B controller.
The 18i-B sits in the middle of FANUC's second-generation i-series hierarchy.
It provides more axis capacity and advanced machining functions than the 21i-B, while sharing the same core architecture — FSSB digital servo communication, serial spindle control, and a modular plug-in board design — as the entire 16i-B/18i-B/21i-B generation.
This positioning made the 18i-B a popular choice for complex lathes (particularly multi-turret and sub-spindle machines), 5-axis machining centres, and other advanced machine tools needing more than the 21i-B's axis count but not requiring the full capability of the 16i-B.
The board mounts on the rear face of the LCD/MDI unit. Plug-in modules install onto its front face. Machine systems built around this board are identified by control unit designations beginning A02B-0283-Bxxx.
This board has been in service across a vast installed base of 18i-B machines and remains the primary maintenance item for this CNC generation.
| Parameter | Value |
|---|---|
| Compatible CNC | FANUC Series 18i-B (18i-MB, 18i-TB, and variants) |
| Mounting | LCD-mounted (rear of LCD/MDI operator panel) |
| CPU Module | A20B-3300-03xx series (Pentium-class) |
| Machine System | A02B-0283-Bxxx series |
| Compatible DRAM | A20B-3300-0310 (16MB), A20B-3300-0311 (32MB) |
| Power Supply | A20B-8100-0720 |
| Servo Interface | FSSB (Fibre-optic Serial Servo Bus) |
| Spindle Interface | Serial spindle (JA7A / JA7B) |
| SRAM Backup Required | Yes — mandatory before replacement |
| Status | Available — refurbished, tested |
| Origin | Japan |
The three main boards in the A20B-8100-066x generation serve three distinct CNC designations. Understanding the differences matters directly for maintenance:
18i-B vs 21i-B. The 18i-B supports more controlled axes and more sophisticated machining functions than the 21i-B.
It uses higher-specification CPU modules (A20B-3300-03xx) compared to the lower-powered CPUs in the 21i-B.
The two main boards are not interchangeable — substituting a 21i-B board into an 18i-B machine is not supported.
18i-B vs 16i-B. The 16i-B is the highest-specification board in this generation.
The 18i-B and 16i-B share the same CPU module family (A20B-3300-03xx) and the same DRAM module family (A20B-3300-0310/0311). Their main boards differ in the maximum axis counts and function sets defined by their respective CNC software. The boards themselves are not cross-compatible.
The A20B-8100-0661 hosts the following plug-in modules:
CPU module (A20B-3300-03xx). The Pentium-class processor card that runs the 18i-B system software.
A significant performance step up from the 486DX processors used in the model A generation, providing the processing power needed for complex multi-axis interpolation and high-speed machining functions.
FSSB servo card. The axis control module managing optical fibre communication with all servo amplifiers.
For the 18i-B's multi-axis configurations — lathes with multiple turrets and sub-spindles, machining centres with rotary axes — the FSSB card handles more simultaneous axis communications than a 21i-B servo card would manage.
Graphics card. Drives the LCD panel. Supports colour display on the 10.4-inch and 8.4-inch colour LCD units commonly fitted to 18i-B machines.
FROM/SRAM module. System software and machine data. SRAM is battery-backed — the backup battery must be functional to preserve data during power outages.
DRAM modules. Working memory. The 18i-B supports 16MB (A20B-3300-0310) or 32MB (A20B-3300-0311) configurations.
Power supply module (A20B-8100-0720). Mounts directly to the main board.
Q1: The 18i-B controller is completely dead — no display, no fan, no LED activity. The PSU module is confirmed functional. Is the main board at fault?
With a confirmed functional PSU and no signs of life, the main board becomes the primary suspect.
Before concluding this, verify the battery is not deeply discharged and the LCD unit's power connector is secure.
If these are confirmed and the board still shows no activity, the main board's primary power management circuitry has likely failed.
Board replacement or component-level repair is required.
Q2: Can an A20B-8100-0661 from one 18i-B machine replace one in a different 18i-B machine directly?
The main board type is compatible. However, the modules on the board — FROM/SRAM, CPU, servo card, DRAM — belong to the donor machine.
After transplanting the board with its modules, the receiving machine will have the donor's parameters, PMC ladder, and data, not its own.
The receiving machine's data must be restored from its own SRAM backup before the machine can return to service.
Always treat main board replacement as a data-critical procedure requiring a current backup.
Q3: The 18i-B shows alarm 360 (FSSB: error on initialising) on all axes at every startup, with servo amplifiers powered. What should be checked?
Alarm 360 on multiple axes with powered amplifiers indicates FSSB chain failure. Check the FSSB fibre connections at the COP10A port on the main board's FSSB servo card first — this is the most common failure point.
The fibre must be clean, undamaged, and fully latched. Inspect the fibre for sharp bends or damage. Also check the connection at the first amplifier in the FSSB chain.
If both are confirmed good and the alarm persists, the FSSB servo card on the main board may have failed — it can be replaced independently of the main board.
Q4: The backup battery was found dead when the replacement board was installed. Has the SRAM data already been lost?
SRAM data is lost during power-off periods without a functional battery, not at the moment of replacement. If the battery was dead before the board was powered down for maintenance, SRAM data may already have been lost before the replacement work began.
This is why SRAM backup to an external medium must always be performed before any maintenance work on an i-series CNC, regardless of whether the battery appears good. If data was lost, it must be re-entered from the machine builder's original parameter sheets and programme files.
Q5: After replacing the A20B-8100-0661 and restoring all data, the spindle does not function and shows alarm SP9001. What should be investigated?
SP9001 is a serial spindle communication alarm — the 18i-B is not receiving valid communication from the spindle amplifier. After a main board replacement, this most commonly results from a loose or disconnected JA7A cable between the main board and the SPM amplifier.
Check the JA7A connector at both ends. If the connection is secure, verify the spindle parameters — particularly the serial spindle configuration parameters — were correctly restored.
An incorrect spindle parameter after a restore can produce this alarm even with a good physical connection.
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