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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A20B-2902-0290 |
Part Number: A20B-2902-0290
Manufacturer: FANUC Corporation (Japan)
Product Type: 2-Axis Digital Servo Control Module (SMD Daughter Board)
Board Series: A20B-2902
Compatible Systems: FANUC Series 15, 16-C, 18-C, FS-21 CNC and robot systems
Design: SMT (Surface-Mount Technology) plug-in module
The A20B-2902-0290 is a 2-axis digital servo control module — a surface-mount daughter board that plugs into the main CPU board of compatible FANUC CNC controllers and robot systems.
It provides the servo interface electronics for two axes: it processes position commands from the CNC's interpolation software, manages encoder feedback from the servo motors, and drives the digital servo communication that connects the controller to the servo amplifiers.
This module is part of the A20B-2902 series, a family of compact SMD plug-in boards used extensively across FANUC's Series 15, 16-C, 18-C, and FS-21 platforms.
These controller generations were among the most widely deployed FANUC systems during their production era, appearing in turning centres, machining centres, and industrial robot controllers across global manufacturing.
The A20B-2902-0290 was the servo control module for 3rd and 4th axis control in many of these configurations.
The board uses SMT construction throughout — surface-mount components on a compact board that mates with a socket on the main CPU board.
The physical design is small and efficient. The functional capability it provides — closed-loop digital servo control for two machining axes — is anything but small.
| Parameter | Value |
|---|---|
| Part Number | A20B-2902-0290 |
| Manufacturer | FANUC Corporation |
| Product Type | 2-Axis Digital Servo Control Module |
| Board Series | A20B-2902 |
| Axis Count | 2 |
| Compatible Systems | FANUC Series 15, 16-C, 18-C, FS-21 (CNC and robot) |
| Design | SMD / SMT plug-in daughter board |
| Origin | Japan |
| Operating Temperature | 0 – 55°C |
| Storage Temperature | −20 – 60°C |
| Humidity | 75% RH max (non-condensing) |
| Condition Available | New (surplus) / Refurbished / Repaired |
In FANUC's digital servo architecture, the servo control module is the board that executes the servo control algorithm in real time. It does not just pass signals — it actively computes.
At every servo interpolation cycle (typically 1 millisecond), it reads the current encoder position for each assigned axis, compares it to the commanded position from the CNC interpolator, calculates the position error, runs the position loop calculation to produce a velocity command, and sends that velocity command to the servo amplifier.
The A20B-2902-0290 does this for two axes simultaneously.
In a 4-axis machining centre, for example, the primary servo module might handle axes 1 and 2 (typically X and Z, or X and Y), while this module handles axes 3 and 4 (additional axes like the B-axis or a second linear axis).
The quality of the position loop calculation on this module determines the precision of the machine's axis positioning. The response time, the gain settings, the feedforward compensation — all of these work through the DSP processing on this module.
A degraded or failed module produces position errors on the two axes it controls, leaving the other axes unaffected.
The FANUC controller generations that use the A20B-2902-0290 represent a significant segment of the installed global CNC fleet. Series 15 was FANUC's high-end platform for complex multi-axis machining. Series 16-C and 18-C were the refined mid-range controllers that followed the earlier 16-A and 18-A platforms.
FS-21 was the more compact variant of the 21 series, used in smaller machines.
All of these platforms used the A20B-2902 family of servo modules to control their axes.
The plug-in architecture was consistent across these platforms. The main CPU board had socket positions for servo modules.
The modules plugged in and the axis assignments were defined by the board's socket position and the CNC software parameters.
The A20B-2902-0290 module has been successfully tested on Series 15 system configurations, confirming its compatibility across the intended platform range.
When the A20B-2902-0290 module fails, the two axes it controls show servo alarms. The other axes on the machine — controlled by other modules or by the main board's own servo interface — continue to function normally. This selective pattern is diagnostic: if axes 3 and 4 both alarm while axes 1 and 2 operate, and the axis 3/4 amplifiers and motors test as good, the servo module for axes 3 and 4 is the fault.
Intermittent position errors — occasional position overshoots or slow following errors only on the module's axes — can also indicate a degraded module before it fails completely. These intermittent errors may become more frequent as the module ages, providing some warning before full failure.
Q1: Axes 3 and 4 both show servo alarms but axes 1 and 2 are normal. The amplifiers and motors for axes 3 and 4 test as good. Is this the A20B-2902-0290?
This is the classic pattern for a servo module fault. When both axes controlled by the same module alarm simultaneously while other axes function correctly, the module is the highest-priority suspect.
Confirm by swapping the suspect module with a known-good module (if available) and observing whether the alarms follow the module or stay with the axes.
If they follow the module, the board is confirmed faulty.
Q2: After installing a replacement A20B-2902-0290, the axes now move but position accuracy on axis 3 is slightly off. Axis 4 is correct. What might explain this?
Axis-specific accuracy issues after a module replacement typically indicate a servo parameter setting problem for that axis.
The servo loop gains and compensation values for each axis are stored in the CNC's parameter memory — they are not on the module.
After module replacement, confirm all servo parameters for axis 3 are correctly set. If the parameters are correct, the axis's feedback system (encoder, cable) should be verified.
Q3: The module was removed during an unrelated repair. It was handled carefully and is now reinstalled, but an axis 3 alarm appeared that didn't exist before. What happened?
A new alarm after reinstalling a previously working module is almost always a connector issue.
The module's socket contacts may not be fully engaged after reinsertion, or an ESD event during handling may have damaged an input circuit.
Remove the module, inspect the socket contacts, discharge yourself with an anti-static strap, and reseat firmly.
If the alarm persists with confirmed full engagement, the module may have been damaged during handling.
Q4: Can this module be used in a robot controller in addition to CNC machine tools?
Yes. The A20B-2902-0290 is documented as compatible with FANUC robot systems using the FS-21 platform alongside CNC applications.
The servo interface architecture is the same in both application types.
Confirm the robot controller's main board is compatible with the A20B-2902-0290 socket specification before installation.
Q5: Is there a functional difference between the A20B-2902-0290 and the similar A20B-2902-0070?
Both are 2-axis digital servo modules in the A20B-2902 family, but they serve different controller generations and may have different interface specifications, servo algorithm versions, or axis count support details.
Part numbers within the same series are not interchangeable without verification.
Always confirm the exact part number of the installed module from its label and source the identical part number for replacement.
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