1PCS Used FANUC A20B-2001-0940 Servo Control Board A2OB-2OO1-O94O A20B20010940

FANUC A20B-2001-0940 | 6079 Alpha SVM3 Three-Axis Servo Drive Control PCB — Type A Interface, Japan Origin

Part Number: A20B-2001-0940

Manufacturer: FANUC Corporation (Japan)

Product Type: Three-Axis Servo Drive Control PCB (Top Card)

Board Series: A20B-2001

Drive Family: A06B-6079 Series (Alpha SVM3 — Three-Axis Servo Amplifier Modules) 

Overview

The A20B-2001-0940 is the control PCB — the top card — inside FANUC's A06B-6079 series SVM3 three-axis Alpha servo amplifier modules. Within the 6079 module's two-board architecture, the A20B-2001-0940 is the intelligence layer.

It mounts at the front of the module, handles all the servo control processing for three simultaneous axes, and connects to the fiber-optic and cable interfaces that link the drive to the CNC controller and the servo motors.

The SVM3 — Servo Amplifier Module, Three Axis — is the configuration that packs L, M, and N axis control into a single 90mm module. That compactness was central to FANUC's Alpha drive system design philosophy: consolidate the axis drives, share a common DC bus from the PSM, and reduce the total cabinet footprint.

The A20B-2001-0940 is what makes the three-axis arrangement possible — it runs three independent closed-loop servo control algorithms simultaneously, managing the current commands for each axis from a single board.

The A20B-2001-0940 serves the Type A interface versions of the SVM3 — the A06B-6079-H301 through H307 range.

The Type A interface uses PWM (pulse-width modulation) signals between the CNC controller and the drive. Six PWM signals encode the three-phase motor current commands. 

This is the generation of FANUC servo drive that predates the FSSB optical fiber interface of the 6096 series — the 6079 SVM3 communicates with the CNC through hardwired PWM connections rather than optical cable.

Key Specifications

Three-Axis Servo Architecture — One Card, Three Loops

The A20B-2001-0940 runs three separate servo control loops in real time. Each axis — L, M, N — has its own current command, its own encoder feedback, and its own closed-loop algorithm running simultaneously.

The card receives position commands from the CNC for all three axes, processes the encoder feedback from all three motors, computes the current error for each, and generates the current commands that go down to the power transistors in the A16B-2202-078x wiring board below it.

This is closed-loop servo control at its core.

The card's job is to ensure each motor current tracks the commanded value with minimal error. When a cutting tool encounters variable material resistance, the motor current adjusts within microseconds. The control loop's speed and accuracy determine how well the machine holds its programmed path under load.

The three loops are independent but share a common clock.

This synchronisation is important — the CNC's interpolation generates position commands for all three axes at the same time, expecting the servo system to maintain precise axis synchronisation. The A20B-2001-0940 handles this synchronised three-axis response from a single board.

Type A PWM Interface

The A06B-6079 series uses the Type A interface — PWM signals carried on a cable between the CNC and the drive module. The A20B-2001-0940 is the receiver for these PWM signals.

It decodes the duty-cycle encoded current commands from the CNC, extracts the motor current demand for each axis, and feeds this demand into the servo control loop.

The Type A interface also carries the alarm and ready signals in the reverse direction — from the drive back to the CNC. When the drive detects a fault, it communicates the alarm code back through these signals.

The A20B-2001-0940 contains the logic for both the PWM signal decoding and the alarm reporting. When the card fails, the CNC may see alarm signals on all three axes simultaneously — the common symptom of a control card fault in the three-axis module.

Drive Module Service — Control Card vs Wiring Board

The 6079 SVM3 module's two-board structure allows, in principle, fault isolation between the control card and the power section. When an alarm appears, the type of alarm indicates which section has the problem.

Control card alarms (A20B-2001-0940) typically manifest as communication errors, VRDY (velocity ready) failures, or alarm codes related to signal processing — situations where the power stage may be intact but the control card cannot initialise correctly.

Power board alarms (A16B-2202-078x) manifest as overcurrent, transistor fault, or phase loss alarms — hardware faults in the high-current switching stage.

In practice, specialist service providers supply the A20B-2001-0940 exclusively within the complete drive module context.

Testing requires running all three axes under motor load — conditions that cannot be replicated on a bench without the complete module assembly.

FAQ

Q1: All three axes (L, M, N) of the 6079 SVM3 module show servo alarm codes simultaneously. The motors and cables have been checked and are confirmed undamaged. Is the A20B-2001-0940 at fault?

Simultaneous alarms on all three axes with confirmed good motors and cables point strongly to the control card. The A20B-2001-0940 is the common element for all three axis channels.

A control card fault — particularly one affecting the power supply section of the card or the common signal processing circuitry — will cause all three axes to alarm at once.

Swap testing with a confirmed-good control card in the complete module is the definitive diagnostic step.

Q2: The drive module shows a VRDY OFF alarm on startup. No specific axis alarm code appears. What does this indicate about the A20B-2001-0940?

VRDY OFF at startup means the drive has not completed its initialisation and reported ready to the CNC.

This can be caused by the control card failing to complete its power-on self-check, by incorrect switch settings on the card, or by missing or incorrect signals from the CNC side (the Type A PWM interface signals must be present before the drive asserts VRDY). Check the CNC-side PWM cable connections first. 

If connections are good, inspect the switch settings on the A20B-2001-0940 against the drive's documentation.

Q3: A replacement A20B-2001-0940 is being installed. Are there switch or jumper settings that must match the original?

Yes. The control card carries configuration switches that set the axis count, interface type, battery connection arrangement, and other drive-level parameters.

Before removing the original card, document all switch positions with photographs or written notes. 

The replacement card must be configured identically to the original. Incorrect settings produce VRDY failures, incorrect alarm codes, or drive behaviour that does not match the CNC's expectations.

Q4: The 6079 SVM3 module has been in service for 12 years. What age-related components on the A20B-2001-0940 are likely to fail first?

The electrolytic capacitors in the control card's internal power supply section are the primary aging concern — they supply the regulated voltages for the card's logic and interface circuits.

As their capacitance decreases and ESR rises with age, the supply rails develop ripple. The early symptom is intermittent alarm codes that appear and clear without obvious cause. 

The backup battery for absolute encoder data is another time-limited component — typical service life is 2–3 years, and its depletion causes position data loss on power-off.

Q5: The 6079 SVM3 (Type A interface) needs to be replaced. Can a 6096 SVM3 (FSSB interface) be used as a substitute if it has the same axis current ratings?

No. The 6079 Type A and 6096 FSSB versions use entirely different communication interfaces between the CNC and the drive.

The CNC's servo control board must match the drive's interface type. 

Substituting an FSSB drive for a Type A drive requires changing the CNC's servo interface hardware and configuration — it is not a direct swap. Match the drive interface type exactly to the installed CNC's servo interface.