FANUC A20B-3900-0061

1MB SRAM Module for FANUC i-Series CNC Control Memory

Main Description

The FANUC A20B-3900-0061 is a 1MB SRAM module used in the memory structure of FANUC i-Series controls.

Listings commonly associate it with 16i, 18i, 20i, and 21i systems, and describe it as a DIMM-style memory module that installs on the main master PCB rather than in the amplifier or operator-panel section.

In practical maintenance work, this model is best understood as a board for retained control memory support.

It is usually selected when the original CNC platform is still worth keeping in service and the correct SRAM module is needed to preserve the intended board-level memory layout. Because modules in the same family can vary in capacity and fit, the part number is more important than general appearance.

Parameter Table

Product Role in the Control

A module like A20B-3900-0061 is not a general plug-in accessory. It belongs to the controller memory section, where stable retained memory is part of normal CNC operation.

That makes it especially relevant in board-level repair, exchange stock, and service support for older i-Series controls that still run reliably but need exact memory hardware to stay in operation.

FAQ

Q1: What is A20B-3900-0061 used for in a FANUC control?
It is used as a 1MB SRAM memory module in the internal memory structure of FANUC i-Series controls. In practical terms, it belongs to the storage side of the controller rather than to the drive, display, or operator interface.

That means its role is tied to internal control memory handling, and it is usually replaced when the installed memory module itself is identified as the fault point.

Q2: Which FANUC controls is this module commonly linked with?
It is commonly linked with 16i, 18i, and 21i controls, and some sellers also include 20i and 15i references in their model mapping.

In service work, that means the board is best treated as an i-Series memory module whose fit should be confirmed against the installed control rather than assumed from family name alone.

Q3: Why is exact part-number matching important for A20B-3900-0061?
Because memory modules in the A20B-3900 family may share a similar outline while still using different capacities and control associations.

For this model, the 1MB SRAM configuration is a defining feature. In industrial repair, that matters because a memory board is chosen for its exact function in the control, not just for connector similarity.

Matching the exact module number is the safer way to preserve compatibility and avoid unnecessary troubleshooting after installation.

Q4: Where is this module installed inside the control?
Seller descriptions place it on the main master PCB as a DIMM-style plug-in module.

That installation detail is useful because it tells the buyer this board belongs to the internal control-memory stack, not to an external interface or amplifier assembly. 

For a technician, that also means inspection should include the receiving socket, board seating, and any signs of contact wear or contamination on the connector edge.

Q5: What should be checked before replacing this module?
A technician should confirm the exact installed part number, verify the CNC family, and make sure the symptom really points to the memory path rather than to the master board or software side.

It is also good practice to inspect the DIMM socket and the surrounding control area before fitting the replacement. 

After installation, the control should be checked for stable startup and normal operation to confirm that the memory board is recognized correctly. 

The final verification steps are practical engineering guidance based on the module’s listed role and installation context.