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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A20B-3900-0170 |
Part Number: A20B-3900-0170
Manufacturer: FANUC Corporation (Japan)
Product Type: FROM/SRAM Analog Spindle Module (PCB)
Reference Manual: B-64115EN
The A20B-3900-0170 is a dual-function PCB from FANUC Corporation. It combines FROM/SRAM memory and an analog spindle interface on a single plug-in module.
FANUC designed it for CNC control systems configured to drive the spindle through an analog signal path — a distinct and specific hardware configuration that this board exists to support.
The analog spindle interface is the board's defining feature.
FANUC CNC systems support two spindle control methods: serial and analog. Serial spindle drive uses a digital communication link to the spindle amplifier. Analog spindle drive uses a voltage signal — typically 0 to ±10 V — to command spindle speed.
Machines built around older or analog-type spindle amplifiers require the analog interface board. The A20B-3900-0170 provides that interface. Remove it, and the CNC has no path to command the spindle.
The FROM and SRAM on this board serve the same roles they serve on any FANUC memory module. FROM holds system software non-volatilely.
SRAM retains runtime data under battery backup. Both memory types are integrated onto the same physical board as the spindle interface circuitry — the A20B-3900-0170 is not a pure memory module. Its spindle function changes what it is and how it must be handled.
| Parameter | Value |
|---|---|
| Part Number | A20B-3900-0170 |
| Manufacturer | FANUC Corporation |
| Product Type | FROM/SRAM Analog Spindle Module |
| Board Series | A20B-3900 |
| Spindle Interface | Analog (voltage command type) |
| Memory Type | FROM (Flash ROM) + SRAM (battery-backed) |
| Reference Manual | B-64115EN |
| Installation | Plug-in module, FANUC CNC controller board |
| Origin | Japan |
| Data Retention on Swap | Data lost — full system data reload required |
| Operating Temperature | 0 – 55°C |
| Storage Temperature | −20 – 60°C |
| Humidity | 75% RH max (non-condensing) |
| Condition Available | New / Refurbished / Repaired |
In a FANUC CNC system, the spindle control path depends on which type of spindle amplifier is installed.
A serial spindle amplifier communicates digitally with the CNC. An analog spindle amplifier takes its speed command from an analog voltage input. For machines using analog spindle amplifiers, the CNC needs hardware that produces this voltage signal.
That hardware is the analog spindle module — the A20B-3900-0170.
When the CNC commands a spindle speed, the analog spindle module converts that command into a proportional voltage.
The spindle amplifier reads this voltage and drives the spindle motor accordingly.
Feedback from the spindle position encoder returns to the CNC through a separate path, allowing the CNC to monitor and regulate spindle speed and position.
This interface is fixed in hardware. Unlike some CNC functions that can be remapped through parameters, the spindle interface type is determined by which board is installed.
A machine configured for analog spindle drive will not run correctly with a pure memory module in this slot.
The spindle control path depends entirely on the A20B-3900-0170 being present.
Replacing the A20B-3900-0170 means losing all data stored on the board.
System software, parameters, spindle tuning data, and any stored programs that reside on the FROM/SRAM portion of this module are gone when the board is removed. A new board installs blank.
This is not a risk that can be managed after the fact.
A complete backup of all CNC data — programs, parameters, pitch error compensation, tool data, spindle parameters — must exist before the board comes out. Without a valid backup, the machine cannot be restored to its previous state after a board swap. The backup must be current.
An outdated backup restores an outdated configuration.
Backing up FANUC CNC data is done through the BOOT screen's DATA BACKUP function or through external memory devices via the RS-232 or memory card interface. Confirm the backup is complete and readable before proceeding with any board work on this module.
The A20B-3900 series contains multiple board variants. Some are pure memory modules — they carry only FROM and SRAM with no additional interface functions.
The A20B-3900-0170 is not one of those. Its analog spindle interface circuitry is present on the board regardless of the memory contents.
Substituting a different A20B-3900 memory module for the -0170 removes the analog spindle function from the system.
The machine will boot, but spindle control will fail.
The correct replacement is another A20B-3900-0170, or a board confirmed by FANUC documentation as a direct equivalent for the specific CNC model in use.
Q1: What happens if the A20B-3900-0170 is replaced with a standard A20B-3900 memory module?
The analog spindle interface is no longer present in the system. The CNC will power up, but any spindle motion command will fail.
The machine will alarm on spindle-related faults. Only a board with the analog spindle interface function — the correct part number — restores spindle control.
Q2: The spindle does not respond to speed commands. How do I determine if the A20B-3900-0170 is at fault?
First confirm the analog command voltage is present at the spindle amplifier's command input — use a voltmeter on the analog output connector from the CNC. If no voltage changes when the CNC commands spindle speed, the fault is in the analog output path, which points to this board.
If voltage is present but the spindle doesn't respond, the fault is downstream — in the spindle amplifier or motor.
Q3: Is a system data backup mandatory before replacing this board?
Yes. All data on the FROM and SRAM portions of this module is lost when the board is removed. Programs, parameters, and system data must be restored from backup after installation.
There is no way to recover this data without a prior backup.
The backup must be performed before the board is removed.
Q4: The backup battery on the board is dead. Is the board still usable?
The FROM portion retains its data without the battery — FROM is non-volatile. The SRAM portion will have lost its data if the battery has been dead for an extended period.
The board is still usable physically, but the SRAM-held data must be restored.
Replace the backup battery and reload all system data before returning the machine to operation.
Q5: The CNC is showing a memory alarm rather than a spindle alarm. Could this board still be the cause?
Yes. If the FROM contents are corrupted — due to a power surge, failed write operation, or board fault — the CNC may alarm on memory rather than on a spindle-specific fault.
A memory alarm that appears alongside an inability to boot the control completely, or that appears immediately after a power event, is a reasonable indication that the FROM/SRAM module has been affected.
Check the alarm code against the B-64115EN manual for the specific fault description.
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