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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A20B-3900-0165 |
Part Number: A20B-3900-0165
Manufacturer: FANUC Corporation (Japan)
Product Type: Printed Circuit Board (PCB)
The A20B-3900-0165 is a FROM/SRAM memory module manufactured by FANUC Corporation in Japan. It carries 32 MB of Flash ROM (FROM) for non-volatile storage and 3 MB of battery-backed SRAM for volatile runtime data. These two memory types serve different roles in the controller's operation, and both are housed on this single plug-in PCB.
The board is designed for FANUC robot controllers — specifically the R-J3iC and R-30iA control units.
It installs horizontally onto the main controller board inside the robot control cabinet. At power-up, the system loads the FROM contents into the controller's working memory.
The SRAM retains its data when power is removed, provided the backup battery is in good condition.
The A20B-3900 series is a distinct family from the A20B-2902 CNC control board series, though both use FANUC's standard A20B prefix.
The -3900 series covers memory and system support modules for FANUC robot controllers.
The -0165 suffix places this board at the 32 MB FROM / 3 MB SRAM specification point within the series.
| Parameter | Value |
|---|---|
| Part Number | A20B-3900-0165 |
| Manufacturer | FANUC Corporation |
| Product Type | FROM / SRAM Memory Module PCB |
| Board Series | A20B-3900 |
| Product Category | System Memory Card — Robot Controller |
| FROM Capacity | 32 MB (Flash ROM, non-volatile) |
| SRAM Capacity | 3 MB (battery-backed, volatile) |
| Compatible Controllers | FANUC R-J3iC, FANUC R-30iA |
| Installation Method | Horizontal plug-in to main controller board |
| Origin | Japan |
| Operating Temperature | 0 – 55°C |
| Storage Temperature | −20 – 60°C |
| Humidity | 75% RH max (non-condensing) |
| Alternate Part Numbers | A20B-3900-0225, A20B-3900-0285, A20B-3900-0299 |
| Condition Available | New / Refurbished / Repaired |
The two memory types on the A20B-3900-0165 serve distinct purposes. Flash ROM (FROM) is non-volatile. It retains its contents without power.
The robot controller's system software, application programs, and configuration data are stored in FROM. When power is lost and restored, this data is intact.
SRAM is volatile. It relies on a backup battery to retain data when main power is removed.
The 3 MB SRAM on this module holds runtime data — active variables, current status information, and other data the controller generates during operation.
The backup battery is a maintenance item. When the battery voltage drops below the threshold, the SRAM data cannot be retained through power-off cycles.
The A20B-3900 series spans several FROM/SRAM capacity combinations. The -0165 is the 32 MB FROM / 3 MB SRAM variant.
Boards with the same FROM but different SRAM sizes carry different suffix numbers and are not interchangeable.
The same part number in different revision generations (e.g., A20B-3900-0165 vs.
A20B-3900-0225 or A20B-3900-0285) are alternate part numbers for the same specification — these can be substituted for one another, but always confirm compatibility for the specific controller generation before installation.
FANUC PCBs are manufactured in Japan under FANUC's own quality control process. Component selection, assembly, board-level testing, and inspection are all completed within FANUC's production chain.
That matters for boards being installed in machines where reliability is critical and unplanned downtime is costly.
Third-party replacement PCBs for FANUC controllers exist in the market.
Quality varies considerably across suppliers and products. For machines in active production — where a secondary failure shortly after a repair is a significant problem — genuine FANUC parts reduce that risk.
The board's provenance is part of the repair's reliability.
The A20B-3900-0165 is an active maintenance spare for FANUC R-J3iC and R-30iA robot systems currently in production. These are widely deployed robot controllers in manufacturing environments — automotive, assembly, material handling, and general industrial automation.
Both controller generations have large installed bases globally.
A robot controller that loses its FROM/SRAM module stops functioning.
The system software cannot load without FROM. Runtime data cannot be retained without SRAM.
The module is not a peripheral — it is a core component of the control system. When the battery fails and SRAM data is lost, or when FROM content becomes corrupted, the module must be replaced.
A correct spare on hand means the robot is back up in the time it takes to swap the module and reload the programs — not the time it takes to source a replacement from overseas.
The alternate part numbers A20B-3900-0225, A20B-3900-0285, and A20B-3900-0299 cover the same FROM 32 MB / SRAM 3 MB specification in later production versions.
When sourcing, any of these verified alternates may serve the same function — confirm against the controller documentation before installation.
Q1: The robot controller shows a SRMERR or memory fault alarm. Is the A20B-3900-0165 the likely cause?
Memory alarms on R-J3iC and R-30iA controllers often point to the FROM/SRAM module when FROM content cannot be read correctly or SRAM data integrity checks fail.
Check the backup battery voltage first — a low or dead battery causes SRAM data loss and triggers related alarms.
If the battery is within specification and the alarm persists, the module itself is a strong suspect. Note the exact alarm code before replacing — some memory alarms indicate other faults.
Q2: The SRAM backup battery has been replaced but stored programs and settings are gone. What happened?
When the battery voltage drops below threshold, SRAM content is lost. Replacing the battery restores the power source, but the lost data does not return — SRAM is volatile and cannot recover erased content.
Programs, mastering data, and system settings that were held in SRAM must be reloaded from a backup.
This is why backing up robot programs and parameters to an external device at regular intervals is essential maintenance practice.
Q3: Can the A20B-3900-0165 be replaced with A20B-3900-0225 or A20B-3900-0285?
Yes, in most cases. These alternate part numbers carry the same FROM 32 MB / SRAM 3 MB specification and represent later production revisions of the same module.
They were used in the same R-J3iC and R-30iA controller families.
Confirm the swap against the controller's documentation before installation, and reload all programs and parameters after installation regardless of which revision is fitted.
Q4: After installing a replacement A20B-3900-0165, the robot requires mastering. Why?
A new module installs without any of the previous system data. Robot mastering data — the encoder reference position values for each axis — is typically stored in SRAM.
When the module is replaced, this data is absent and the robot's position reference is undefined.
Mastering must be performed using the correct mastering method for the robot model before the robot can be used.
This is normal procedure for any FROM/SRAM module replacement.
Q5: How should the A20B-3900-0165 be stored as a spare?
Store in a dry, temperature-stable environment in anti-static packaging. The module's SRAM battery will slowly discharge during storage — check the battery condition before installation if the module has been stored for an extended period.
A module with a depleted battery can still function once installed with a fresh battery, but confirm the battery is within specification as part of the installation procedure.
Do not power the module in isolation outside the controller.
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