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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A20B-2101-0870 |
The FANUC A20B-2101-0870 is the control PCB for the Alpha i-B spindle amplifier generation — the A06B-6220 series SPM modules that represented FANUC's high-performance spindle drive platform for i-series CNC controllers.
The Alpha i-B spindle amplifier brought significant improvements to the previous Alpha i generation: enhanced thermal performance through improved power module design, better energy efficiency, and updated serial communication firmware that provided faster spindle speed response and improved spindle synchronisation accuracy.
The A20B-2101 series is the control board family specifically associated with the Alpha i-B spindle (the "B" designation distinguishing it from the earlier Alpha i-A).
This board handles the entire digital control side of the spindle drive — every speed command, every orientation request, every rigid tapping synchronisation signal, and every diagnostic readout passes through this board's processor.
The power section (IGBTs, rectifier, DC bus) handles the high-current motor phase switching separately, but without the A20B-2101-0870 directing those power electronics, the amplifier cannot operate.
The 6220 series designation in the official description identifies which specific amplifier family this board serves — the A06B-6220 is FANUC's factory ordering root for the Alpha i-B SPM. This precision matters because FANUC's spindle amplifier control boards are not universally interchangeable across families: using the wrong control board for a specific amplifier chassis and power section produces incorrect gate drive timing, wrong current limit scaling, and potentially unsafe operation.
| Parameter | Value |
|---|---|
| Function | Spindle amplifier control PCB |
| Compatible Amplifier | A06B-6220-Hxxx (Alpha i-B SPM) |
| Spindle Interface | Serial spindle (JA7A/JA7B) |
| Feedback Supported | Position coder, magnetic sensor |
| Series | A20B-2101 |
| Status | Discontinued — spare available |
| Origin | Japan |
Inside the A06B-6220 Alpha i-B spindle amplifier, the A20B-2101-0870 control board and the power module (IPM) work in a strict master-servant relationship. The control board is the master — it makes all decisions about what currents to apply to the motor, when to switch the transistors, and whether to allow or inhibit spindle operation.
The power module is the servant — it switches high currents according to the gate drive commands it receives from the control board.
This means most spindle intelligence lives in the control board:
Speed control: The control board receives the commanded spindle speed from the CNC via the serial interface (JA7A connector), compares it against the actual speed derived from position coder or motor sensor feedback, and calculates the torque command needed to minimise the speed error.
It translates this torque command into phase current references and gate drive signals.
Orientation control: Spindle orientation (stopping the spindle at a precise angular position for tool changing) requires the control board to switch from speed control to position control mode, using the position coder signal to determine the spindle's angular position and drive it to the target angle with controlled deceleration.
Rigid tapping: Synchronising the spindle rotation with Z-axis feed for rigid tapping requires the control board to output precise real-time position data to the CNC via the serial link, allowing the CNC to close the synchronisation loop.
The control board's serial communication latency directly affects rigid tapping accuracy.
C-axis control: When the spindle is used as a CNC-controlled C-axis for turning operations, the control board manages the transition from spindle mode to servo mode and maintains position control throughout the C-axis motion.
The Alpha i-B SPM communicates with the CNC controller exclusively through the serial spindle link (typically JA7A cable for the first spindle, JA7B for the second).
This is a high-speed serial protocol that FANUC developed specifically for spindle-to-CNC communication — faster and more capable than the older analogue ±10V velocity command interface it replaced.
When the A20B-2101-0870 control board's serial communication circuitry fails, the CNC immediately loses contact with the spindle.
The characteristic symptom is a spindle alarm at CNC power-up, typically one of the "A" series alarm codes, appearing before any spindle operation is attempted.
The CNC cannot verify that the spindle amplifier responded to its initialisation handshake, and without that confirmation, it generates the spindle alarm and prevents spindle operation.
Importantly, this same alarm presentation can also come from a faulty serial spindle cable, a disconnected JA7A connector, or a failure in the CNC main board's serial spindle transmitter circuit — not only from the control board.
Systematic fault isolation (checking cable continuity, connector seating, CNC-side serial spindle circuits) before concluding the A20B-2101-0870 has failed saves unnecessary parts replacement.
The A20B-2101-0870's discontinued status from FANUC means it is no longer available through official FANUC channels as new old stock.
The supply comes entirely from the secondary market: refurbished units that have been recovered from functioning machines during upgrades, surplus units held by specialist distributors, and repaired units from specialist repair centres.
This supply pool is finite and depletes over time as more machines require maintenance.
For facilities with multiple machines running A06B-6220 Alpha i-B spindle amplifiers, holding a spare A20B-2101-0870 control board on site is sound practice.
The alternative — waiting to source a replacement after a spindle failure on a production machine — typically involves days of downtime as the part is located, tested, and shipped.
Against the daily production value of a functioning machine tool, the cost of a spare control board is justified many times over.
Specialist FANUC repair centres can also test and overhaul A20B-2101-0870 boards recovered from failed amplifiers — replacing deteriorated capacitors, cleaning contamination, and testing on a matched A06B-6220 spindle amplifier test rig.
A properly overhauled board can provide years of additional service life.
Q1: The spindle amplifier shows alarm "A" codes at power-up and the spindle cannot start at all. Is the A20B-2101-0870 the cause?
Alarm codes displayed on the SPM's two-digit LED beginning with certain patterns at power-up indicate communication or initialisation failures. Before assuming the control board has failed, check:
(1) The JA7A serial cable is fully connected at both ends and undamaged;
(2) The 24VDC power supply to the amplifier is within specification (24V ±10%);
(3) The CNC's serial spindle parameter is correctly set for the amplifier being used;
(4) The fuse on the SPM's control circuit power input has not blown. If all of these check out and the alarm persists, the A20B-2101-0870 is the next item to investigate.
Q2: After replacing the A20B-2101-0870, the spindle runs but rigid tapping is inaccurate. What parameter settings should be checked?
Rigid tapping accuracy depends on the correct setting of spindle-side parameters that affect the position feedback gain and synchronisation timing. After control board replacement, verify that the motor type code and position coder parameter are correctly set for the specific spindle motor connected.
The position coder pulse count per revolution must match the motor's actual encoder specification. If these were set correctly on the original board and were reset to defaults on the replacement board, they need to be re-entered.
Consult the FANUC Alpha i series parameter manual (B-65270EN or equivalent) for the specific parameters.
Q3: The control board was replaced but the spindle now runs at incorrect speed — faster or slower than commanded. What causes this?
Speed inaccuracy after control board replacement indicates the speed feedback scaling is incorrect. The motor speed sensing (whether from a position coder, magnetic sensor, or from the current feedback circuit) must be correctly configured on the control board.
Check: the motor type parameter setting (which determines the assumed encoder resolution and motor speed constant); the gear ratio parameter if a gearbox is between the motor and the spindle; and whether the speed command scaling parameter matches the CNC's output range.
These parameters are not stored on the control board itself but in the CNC's spindle parameter memory — they should not have changed with the board swap, but verify they are correct.
Q4: Can the A20B-2101-0870 be used in an Alpha i-A SPM (A06B-6112 series) if the A06B-6112's original control board is unavailable?
No. The Alpha i-A (A06B-6112 series) and Alpha i-B (A06B-6220 series) spindle amplifiers, while related in architecture, use different control board specifications.
The A20B-2101-0870 is specified for the 6220 series, and the A06B-6112 series uses its own control board variants.
Gate drive timing, current sensing calibration, and communication firmware may differ between generations. Installing the wrong control board in a spindle amplifier chassis risks incorrect gate drive signals being applied to the power module, which can damage the IGBT power section.
Q5: The Alpha i-B SPM cooling fan has failed and the amplifier is overheating. Could this have damaged the A20B-2101-0870 control board?
Sustained overheating is a significant risk for the control board. The control board's components — particularly the processor IC, power supply regulators, and electrolytic capacitors — have specific maximum operating temperature ratings.
Operating the amplifier with a failed cooling fan, even for relatively short periods, can cause cumulative thermal damage that shortens component life.
Signs of heat-related damage on the control board include swollen or leaking electrolytic capacitors, discoloured PCB substrate around heat-generating components, and intermittent failures that worsen as the amplifier warms up.
Replace the cooling fan immediately when it fails — the fan is a consumable item and its timely replacement protects the more expensive control and power boards.
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