FANUC A20B-2902-0348 | Combined FROM / SRAM Memory Module — A20B-2902 Series, CNC Controller, Japan Origin

Part Number: A20B-2902-0348

Manufacturer: FANUC Corporation (Japan)

Product Type: Combined FROM / SRAM Memory Module (SMD Daughter Board)

Board Series: A20B-2902

Memory Types: Flash ROM (FROM — non-volatile) + SRAM (battery-backed)

Compatible Systems: FANUC Series 16, 18, 21 CNC and compatible

Design: SMD plug-in daughter board

Overview

The A20B-2902-0348 is a combined FROM/SRAM memory module from FANUC's A20B-2902 series — a family of compact SMD plug-in memory boards that serve as the primary persistent storage devices in FANUC Series 16, 18, and 21 CNC controllers. It integrates two functionally independent memory types on a single daughter board: Flash ROM for non-volatile software and program storage, and battery-backed SRAM for parameter and data retention.

The A20B-2902 series is among the most important module families in FANUC's Series 16/18/21 controller ecosystem.

These controllers — deployed in enormous numbers across turning centres, machining centres, and multi-axis manufacturing cells worldwide — depend entirely on the correct FROM/SRAM module to start up and operate.

The FROM holds the operating system and PMC ladder. The SRAM holds the parameters and offsets the machine runs by.

Both memory types are present on the A20B-2902-0348, and both must be intact for the machine to function.

The board uses surface-mount construction throughout. It plugs into a dedicated socket on the main CPU board, fitting cleanly within the physical space allocated for the memory module in the controller chassis. Installation is straightforward: remove the old board, seat the new one, restore data.

But the data restore step — reloading the FROM with the correct software and restoring the SRAM data from backup — requires preparation before the board is removed.

Key Specifications

FROM — Non-Volatile System Software Storage

Flash ROM on the A20B-2902-0348 stores the software that makes the CNC controller work. This includes the CNC operating system itself, the PMC ladder program that controls the machine tool's automation logic, and in many configurations, the machine builder's custom application programs and macro libraries.

Flash memory achieves non-volatile storage through floating-gate transistors whose charge state persists without power.

The data survives power interruptions, dead batteries, and extended storage. When the CNC powers up, the operating system loads from the FROM module and begins execution. 

If the FROM content is corrupted — through a failed write during a software update, physical damage to the chips, or cell wear — the controller cannot start.

FROM content is machine-specific.

The operating system version must be matched to the hardware generation. The PMC ladder is the machine builder's ladder, unique to that machine configuration.

A blank replacement FROM module must have the correct FROM files written into it before the machine can run. 

These files come from the machine builder's documentation, from a FANUC service representative, or from a backup copy made from the original module.

SRAM — Battery-Backed Parameter Storage

Static RAM on the A20B-2902-0348 retains its content during power-off through a backup battery.

The battery supplies the small continuous current the SRAM cells need to hold their charge states, keeping the data intact. 

When the battery voltage eventually drops below the SRAM's retention threshold, data is lost.

The SRAM stores the data that changes during normal machine use but must persist between shifts and power cycles. CNC parameters define how the controller behaves — feed rate limits, axis stroke limits, servo gains, communication settings.

Tool offset tables hold the length and radius compensation values for every registered tool. Work coordinate offsets define the datum positions for the parts being machined. All of this lives in the SRAM.

Losing SRAM data is a production event. It doesn't damage the machine mechanically, but the machine cannot run correctly until all data is restored.

A complete backup of the SRAM data — captured to a PC, USB, or FANUC memory card — before any maintenance work on the controller is the standard practice that limits the recovery effort to a data reload rather than manual re-entry.

A20B-2902 Series — Many Variants, Specific Applications

The A20B-2902 series contains a large number of FROM/SRAM module variants. The FROM capacity and SRAM capacity differ across variants. Some variants serve the Series 16-C and 18-C platforms; others serve Series 21; some bridge both.

A few include additional functions — analog spindle interfaces, PMC ladder execution circuits — alongside the memory.

The A20B-2902-0348 is one specific variant in this family. It serves its defined controller platforms with its specific memory configuration. Replacing it with a different variant — even one that appears physically similar — carries the risk of a wrong capacity mismatch or hardware incompatibility.

Always confirm the exact part number from the installed board's label before sourcing a replacement.

FAQ

Q1: The controller shows a FROM checksum alarm at every power-on. Nothing else changed. Is this board the cause?

A persistent FROM checksum alarm at power-on indicates the FROM content has been corrupted or a FROM cell has failed.

First check if the alarm occurred after a software update — an interrupted FROM write can corrupt the affected sector. 

If no update was in progress, the FROM hardware has failed.

The module requires replacement followed by a complete FROM rewrite. Have the correct FROM file set ready before starting the work.

Q2: The CNC generates a battery low alarm. How much time is available before SRAM data is lost?

The low battery alarm appears when the battery voltage drops below the warning threshold, not the critical retention threshold.

The gap between these thresholds typically provides several weeks under normal conditions, though this varies with battery age and cabinet temperature. 

Treat the alarm as requiring action within days. Replace the battery with the controller powered on to maintain SRAM content during the change.

Q3: The replacement A20B-2902-0348 module arrived. Is there anything to check before installation?

Confirm the replacement module's full part number matches the installed module exactly, including any revision suffix.

Inspect the connector contacts for any damage from shipping or storage. Confirm the backup of all CNC data is complete and verified before removing the original module. 

Have the FROM file set ready for reloading. With these steps in order, installation can proceed with confidence.

Q4: After replacing the module and reloading data, one axis's servo parameters seem slightly different from the backup values. What should be checked?

After a data reload, compare all parameters for the axis in question against a second backup copy or against the machine's documentation.

A single-digit parameter discrepancy can sometimes result from a backup that captured data during a parameter edit, rather than the stable final state. 

If the current parameter value is producing correct machine behaviour, and the backup shows only a trivial numerical difference, the restored values are likely correct.

Q5: The machine uses a later revision of the A20B-2902-0348 (e.g., /02A instead of /01A). Are these interchangeable?

Minor hardware revision suffixes within the same base part number represent PCB layout changes or component substitutions that maintain the same functional specification.

In most cases these are interchangeable. 

FANUC typically designs later revisions to be backward-compatible with earlier revision installations.

If there is uncertainty for a specific application, confirm with the machine's maintenance documentation or a FANUC service resource.