1 PC Used Fanuc A20B-2100-0182 A20B21000182 A2OB-21OO-O182 PCB Board In Good Condition

FANUC A20B-2100-0182 | Servo Amplifier Control PCB — A20B-2100 Alpha Series Drive Control Board, CNC Industrial Automation Spare Part, Japan Origin

Overview

The FANUC A20B-2100-0182 belongs to one of the busiest board families in the FANUC spare parts ecosystem — the A20B-2100 series, which covers the control PCBs for a wide range of Alpha and Alpha i series servo amplifier modules.

These boards are the unsung workhorses of FANUC servo systems: they sit inside the amplifier module's housing, mounted on the face of the power section, accessible from the front of the drive rack.

Most of the service interaction with a servo amplifier — reading alarm codes, checking the LED display, connecting the encoder cable and FSSB fibre — happens at the control board's face.

The A20B-2100 family as a whole covers Alpha series SVM1 (single-axis), SVM2 (dual-axis), SVM3 (three-axis), and SPM (spindle) amplifier modules across multiple current ratings and FSSB/Type A/Type B interface variants.

Different part numbers within the family denote different control configurations: some boards are optimised for higher-current axes requiring different gate drive parameters, some add HRV3 (High Response Vector 3) servo control algorithm capability that improves high-speed position tracking accuracy, and some are specific to spindle modules rather than servo axis modules.

The A20B-2100-0182 occupies its specific position within this family, providing the control functions needed for its designated amplifier configuration.

Like all A20B-2100 series boards, it works in conjunction with a companion wiring/power board within the same amplifier module — the two boards together form the complete control and power electronics of the servo drive.

Key Specifications

A20B-2100 Series Architecture — Control Board and Power Board Partnership

Understanding the A20B-2100-0182's role requires understanding the two-layer architecture of FANUC Alpha series servo amplifier modules.

The power board (a wiring board from the A16B-2202-xxxx or A16B-2203-xxxx series) handles everything involving large currents: the IGBT transistors that switch the motor current, the current sensing resistors that measure actual motor current, the DC bus connection that draws power from the PSM (Power Supply Module), and the motor output terminals.

These components deal with voltages up to 300V DC and peak currents that can reach hundreds of amperes in higher-rated modules.

The control board — the A20B-2100-0182 — handles everything digital: the FSSB optical receiver and transmitter, the DSP or ASIC-based servo algorithm processor, the gate drive output signals that tell the power board's IGBT transistors when to switch, the NV-RAM that stores motor parameters, and the diagnostic LED display that shows the amplifier's state to the maintenance engineer.

The control board's circuits operate at low voltage (5V logic, ±15V analogue) and low current. 

It connects to the power board through a set of low-signal connectors that pass the gate drive commands down and the current feedback signals up.

This separation matters for fault diagnosis.

When a servo alarm occurs, the question is which layer has failed. If the amplifier shows a coherent alarm code on its LED display (even if the code indicates a fault), the control board's CPU and display circuits are functional — the fault may be in the power board, the motor, the encoder, or the cabling. 

If the LED display is dark or shows an erratic pattern, or if the FSSB communication cannot be established at all, the control board itself is the more likely failure point.

NV-RAM — Motor Parameters Inside the Drive

The A20B-2100-0182, like other Alpha series control PCBs, contains a non-volatile RAM chip that stores the motor model number and associated servo parameter settings.

These settings define the motor's rated current, encoder type and resolution, velocity loop gain baseline, overcurrent threshold, and other drive-specific values that allow the amplifier to correctly control the specific motor it is paired with.

When replacing the control board, the motor parameters must be either transferred from the original board's NV-RAM chip to the replacement board (if the original board's failure was not in the NV-RAM itself and the chip can be safely removed and reinstalled), or re-entered manually using the four push-buttons on the amplifier's face.

The manual re-entry procedure requires the motor type number from the motor nameplate and the parameter tables published in the FANUC Alpha series maintenance manual.

Failing to set correct motor parameters after control board replacement results in abnormal servo behaviour — the amplifier may command incorrect current limits, report spurious overcurrent alarms, or perform poorly in position control — even though all hardware is physically sound.

LED Diagnostic Display — Reading the Amplifier's State

The two-digit 7-segment LED display on the A20B-2100-0182's face is the primary real-time diagnostic tool for the amplifier.

The meaning of each display code is documented in the FANUC Alpha series maintenance manual.

The display passes through a defined startup sequence when power is applied: each digit corresponds to a phase in the amplifier's self-test and initialisation process, and the sequence should reach a specific final code indicating "ready" state before the CNC's servo enable signal is issued.

Common alarm codes visible on the display include overcurrent (IPM) faults (which point to the power board's transistors or to a motor winding short), velocity error alarms (which can be encoder cable, motor, or servo parameter issues), and FSSB communication errors (which point to the optical fibre connection between the CNC and the drive).

When the control board itself has failed, the display typically shows no characters (dark), shows random segments, or stops at an early initialisation code — all of which indicate the CPU or its power supply on the control board has not operated correctly.

FAQ

Q1: The amplifier's LED display shows "8.8" on both digits at power-up and never changes. Is this a control board failure?

A display showing "8.8" (all segments lit on both digits) that never changes typically indicates that the control board's CPU has not started.

This pattern is produced by the display's power-on state before the processor activates — if it persists, the processor has not executed its startup code. 

Common causes include a failed power supply on the control board, a failed processor IC, or in some cases a failed external oscillator. 

Before concluding the board has failed, verify that the 24VDC input to the amplifier is within specification — control board processors require clean 24VDC to start reliably.

Q2: Can the A20B-2100-0182 be swapped between two identical servo amplifier modules of the same model?

Physically yes, if the modules are the same A06B-xxxx order specification and use the same control board part number.

However, the NV-RAM on the board stores motor parameters specific to the motor connected to that amplifier — swapping the control board between two amplifiers connected to different motors requires re-entering the correct motor parameters for each new configuration. 

If both modules drive identical motors with identical parameters, the swap is transparent.

Confirm motor parameters after any control board swap by checking the motor type setting against the motor nameplate.

Q3: After replacing the A20B-2100-0182, the axis shows a servo alarm immediately when the CNC enables servos. What should be checked?

An immediate servo alarm after control board replacement, before any axis motion, is most often a parameter issue — either the motor type is set incorrectly, causing an internal consistency error in the control board's parameter set, or the Type A/Type B interface switch is set wrong for the CNC interface in use.

Check the interface selection switch setting first, then verify the motor type parameter. 

If those are correct, check that the FSSB connection (optical fibre) is clean and undamaged and that the encoder cable is correctly reconnected.

Q4: The replacement A20B-2100-0182 works, but the axis occasionally generates a "servo velocity error" alarm under high acceleration. What causes this?

An intermittent velocity error alarm under high acceleration, appearing only after a control board replacement, usually indicates the velocity loop gains have not been correctly restored. The motor parameter setting defines the starting point for gains, but additional tuning parameters (velocity loop gain, integration gain) may have been customised on the original board.

If these values were not recorded before the original board failed, and the replacement board comes with default values, the velocity loop may not be tight enough to follow the CNC's acceleration demand — triggering the error alarm when the commanded acceleration exceeds what the default gains can follow.

Re-tune the velocity loop through the CNC's servo parameter adjustment function.

Q5: Can the A20B-2100-0182 control board from an Alpha series amplifier be used in an Alpha i series amplifier of the same current rating?

The Alpha and Alpha i generations, while mechanically similar, use different control board hardware with updated firmware and different FSSB protocol versions.

Mixing control boards between Alpha and Alpha i amplifier generations is not supported and will not produce correct operation. 

The control board generation must match the amplifier module generation.

The part number confirms this — the A20B-2100-0182 belongs to a specific generation context, and the compatible amplifier modules are defined by FANUC's parts documentation for that specific part number.