Fanuc Pc Board A20B-2902-0501 A20B29020501 A2OB-29O2-O5O1

FANUC A20B-2902-0501 | 8MB FROM + 256K SRAM Combined Memory Module — Series 16 / 18 / 21 CNC, Japan Origin

Part Number: A20B-2902-0501

Manufacturer: FANUC Corporation (Japan)

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

FROM Capacity: 8 MB (Flash ROM — non-volatile)

SRAM Capacity: 256 KB (battery-backed — non-volatile with battery)

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

Overview

The A20B-2902-0501 is a combined memory module that integrates two functionally distinct memory types on a single SMD daughter board: 8 MB of Flash ROM (FROM) and 256 KB of battery-backed SRAM. Both memory types serve the same purpose in the broadest sense — storing data that the CNC controller needs — but they store entirely different categories of data and behave very differently.

This module plugs into a dedicated socket on the main CPU board of FANUC Series 16, 18, and 21 controllers.

These were some of the most widely deployed FANUC CNC platforms of their era: the Series 16 and 18 covered mid-range to high-end machining centres and turning centres, while the Series 21 targeted compact single-path applications.

All of these systems shared a common modular memory architecture in which this type of combined FROM/SRAM module provided the controller's primary persistent storage.

The A20B-2902-0501 carries the industrial-grade credentials expected from a FANUC memory product.

The PCB uses CMOS memory devices selected for low power operation and thermal stability.

The edge connectors are designed for the plug-in insertion and removal cycles that maintenance work demands.

The battery that keeps the SRAM alive during power-off is an integral part of the module's long-term reliability — when the battery weakens, SRAM content becomes vulnerable.

Key Specifications

FROM — The Non-Volatile System Storage

Flash ROM on this module stores everything the CNC controller needs to start up and run, independent of any external media or network connection. The 8 MB capacity holds the CNC's operating system software, the PMC (Programmable Machine Controller) ladder programs, and the machine builder's custom application software. In some configurations it also holds part programs and macro libraries.

Flash memory stores data by trapping electrical charge in floating-gate transistors.

This charge persists indefinitely without power — Flash is genuinely non-volatile.

Unlike battery-backed SRAM, the FROM content survives even if the controller battery fails completely. When the controller powers up, the operating system loads from the FROM module and begins execution. 

The FROM's content defines what the controller can do and how it behaves.

Replacing the FROM module — whether due to corruption, physical damage, or a software upgrade — requires rewriting the FROM with the correct software files.

These files must come from a backup or from the machine builder's documentation.

A blank replacement FROM module starts empty; the CNC operating software and PMC ladder must be loaded before the machine can operate.

SRAM — Battery-Backed Data Retention

The 256 KB battery-backed SRAM on this module stores the data that changes during normal machine operation: parameters, tool offset tables, work coordinate offsets, and the log of active tool usage.

This data changes frequently — tool offsets are updated after each qualifying cut, parameters are occasionally adjusted during maintenance — but it must survive power-off between shifts and weekends without loss.

The battery maintains a trickle current through the SRAM cells when the main power is off, preserving the stored charges that encode the data.

As long as the battery voltage stays above the retention threshold, the SRAM holds its content faithfully. 

When the battery weakens, the voltage eventually drops below this threshold and the SRAM content is lost.

FANUC controllers typically generate a low battery warning well before the battery reaches the critical threshold. Responding to this warning promptly — replacing the battery while it still has enough charge to maintain the SRAM during the replacement — prevents data loss.

Battery replacement on a live controller avoids the data restore process that follows SRAM data loss.

Two Memory Types, One Module

Combining FROM and SRAM on the same daughter board is an engineering convenience that reduces the board count in the controller and simplifies the physical layout of the main CPU board's memory expansion sockets.

The two memory types are electrically separate on the module — they share the board's PCB substrate and connector but connect to completely independent memory circuits on the main board. 

A failure of the FROM section does not affect the SRAM, and vice versa.

This independence matters for fault diagnosis. If the controller generates FROM-related alarms — software checksum errors, boot failures, program storage faults — the FROM section of the module is suspect.

If the controller generates SRAM-related alarms — parameter loss, battery low warnings that precede data corruption, program loss after power cycling — the SRAM section and its battery are the focus.

FAQ

Q1: The CNC shows a FROM checksum error at power-on. The controller cannot complete its startup sequence. Is this the A20B-2902-0501 FROM?

A FROM checksum error at boot indicates the operating software in the FROM has been corrupted or the FROM hardware has failed.

First check whether the error is persistent across multiple power cycles — a single occurrence can sometimes be a transient. 

If persistent, the FROM content must be rewritten. If the module cannot accept a new write, the FROM hardware has failed and the module requires replacement.

Have the FROM file backup ready before starting this process.

Q2: The controller shows a low battery warning. How urgently does the SRAM battery need attention?

The low battery warning appears when the battery voltage drops below the warning threshold but before reaching the critical retention threshold.

The gap between these thresholds typically allows several weeks of continued operation. 

However, this window varies depending on the battery's age and the temperature of the controller environment.

Treat the warning as requiring action within days, not weeks. Replace the battery with the controller powered on to maintain the SRAM content during the battery change.

Q3: After replacing the module with a new A20B-2902-0501, the machine shows parameter errors and all offsets are at zero. What happened?

A new replacement module arrives with blank SRAM — no parameters, no offsets, no work coordinates.

This is expected. The CNC software must be reloaded into the FROM, and all parameters, tool offsets, and work coordinates must be restored from backup. 

This is why a complete data backup before any memory module work is essential.

If no backup exists, the machine must be re-established manually to its previous state.

Q4: Can the A20B-2902-0501 be used as a direct replacement for the A20B-2902-0341 (4MB FROM + 256K SRAM) module in a Series 16-C?

Not without verification. The -0501 has 8 MB FROM while the -0341 has 4 MB.

The FROM capacity must match the software image that will be loaded — a larger capacity module can physically fit more, but the controller software must be compatible with the larger module's hardware. 

Check the Series 16-C controller's maintenance documentation for compatible FROM module options before substituting a different variant.

Q5: The module's SRAM content was lost during a power interruption. The battery tested as good. What else could cause SRAM data loss?

SRAM data loss with a confirmed good battery can be caused by: a corrupted backup battery circuit on the module (even if the battery cell voltage is correct, a broken connection defeats the backup path), an exceptionally deep power dip that pulled the SRAM supply below retention voltage faster than the battery could respond, or physical shock that momentarily disrupted the module's connector contact during the power interruption.

Inspect the module's connector contacts and verify the battery circuit's physical connections.