Fanuc A20B-2902-0341 Module A20B29020341 A2OB-29O2-O341

FANUC A20B-2902-0341 | Flash ROM 4 MB / SRAM 256 KB Memory Module — Series 16-C / 18-C, Japan Origin

Part Number: A20B-2902-0341

Manufacturer: FANUC Corporation (Japan)

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

FROM Capacity: 4 MB (Flash ROM)

SRAM Capacity: 256 KB

Compatible Systems: FANUC Series 16-C, 18-C and compatible

Status: Discontinued by Manufacturer

Overview

The A20B-2902-0341 is a combined Flash ROM and SRAM memory module for FANUC Series 16-C and 18-C CNC systems. It is an SMD-type daughter board that installs on the main CPU board, providing both non-volatile software storage and battery-backed volatile data storage in one compact module. The board interfaces between the NC (Numerical Control) and MC (Machine Control / PMC) sections of the controller, carrying the system firmware, ladder program, and active operational data that the CNC requires to function.

The description "interfaces between NC and MC" describes what the stored content does, not a physical signal path. The FROM content is what the NC and PMC need to execute their respective functions — it is the common software foundation for both. The 256 KB SRAM holds the data the controller writes and reads during machine operation, serving both the NC and PMC sides of the control architecture.

This module is discontinued. FANUC no longer produces new units. Series 16-C and 18-C machines continue operating in production plants worldwide, and the A20B-2902-0341 remains an active maintenance item in those facilities.

Key Specifications

FROM 4 MB — System Software Storage

The 4 megabytes of Flash ROM on this module hold the entire software stack for the Series 16-C or 18-C controller. CNC operating software, servo control software, spindle control software, PMC ladder runtime environment, and any loaded option software all reside here. When the controller powers on, it reads the FROM contents and loads them into DRAM for execution. The FROM is the source — all the controller's processing behaviour originates from the code stored here.

Four megabytes was a generous allocation for the software generation of the Series 16-C/18-C era. It accommodated the full control software suite plus installed options without capacity constraints for most configurations. Series 16-C and 18-C machines with extensive option software packages may approach this limit, but for standard machining centre and turning centre configurations, the capacity was more than sufficient.

Flash ROM is inherently non-volatile. It retains its content without any power source, without a battery, and through extended storage. A FROM module removed from a machine can sit in a drawer for years and still contain its software intact when reinstalled.

SRAM 256 KB — Active Data in Battery-Backed Memory

The 256 kilobytes of SRAM hold the machine-specific and job-specific data that distinguishes one machine's configuration from another. Part programs, tool offset tables, parameter settings, workpiece coordinate data, and macro variables all live here. The backup battery on the module maintains the SRAM contents through power-off events.

256 KB is a moderate SRAM allocation. It suits the standard program and data requirements of Series 16-C and 18-C machines in typical machining applications. Machines with very large program libraries or extensive macro variable use may find the capacity limiting. In those cases, additional SRAM expansion modules may have been installed in the system's expansion slots.

The backup battery is a consumable. It has a service life measured in years — typically three to five years in normal industrial service, longer if the machine operates continuously (which reduces battery drain cycles). When the battery voltage drops, the FANUC controller issues a low-battery alarm. Replace the battery before the next planned machine shutdown.

NC to MC Interface — What the Module Stores

The description "interfaces between NC and MC" reflects the module's content rather than its physical architecture. The FROM on this board stores the software for both the NC (the motion control and interpolation functions) and the MC (the PMC ladder — the programmable machine controller that handles I/O sequencing and machine logic). They are stored together on this module and loaded together at power-up.

This combined storage means that a failure or corruption of this module simultaneously affects both NC and PMC functions. The controller cannot execute either motion control or machine logic without its correct software. Both sides of the control system depend on the content of this single module.

FAQ

Q1: The Series 16-C machine shows a ROM parity alarm on startup. Is the A20B-2902-0341 faulty?

A ROM parity alarm indicates FROM content integrity failure. Reseat the module first — a partially seated module produces the same alarm as corrupt content. If the alarm persists with the module fully seated, the FROM content may be corrupted or the module hardware may have failed. Source a replacement module and load the correct software version before recommitting the machine to production.

Q2: After replacing the A20B-2902-0341, what must be done before the machine can run?

A replacement module installs blank — no FROM content, no SRAM data. First, load the correct CNC software version from a backup or original source using the boot screen restore procedure. Then restore all SRAM data: parameters, part programs, tool offsets, and workpiece coordinates. Confirm all parameters are correct by comparing against a parameter printout. The machine cannot produce correct parts until parameters are verified.

Q3: The PMC ladder program is missing after a module replacement. The NC appears to function. Why?

If the FROM was loaded with CNC system software but the PMC ladder was not included in the load, the controller boots and the NC functions — but the PMC has no ladder to execute. Without the ladder, machine I/O does not respond. Load the PMC ladder program separately from the PMC programming screen or from a memory card backup. The ladder is a separate file from the main CNC system software and must be loaded independently.

Q4: Can the A20B-2902-0341 be used to replace other 4 MB FROM modules in the A20B-2902 series?

Interchangeability between A20B-2902 series FROM module variants depends on whether the SRAM capacity and system compatibility match. The -0341 specifically carries 256 KB SRAM and is matched to Series 16-C/18-C. Another variant in the same series may have a different SRAM allocation or be matched to a different controller series. Use the exact part number match for confirmed compatibility.

Q5: The machine has been running without issue for years. Should the SRAM battery on this module be replaced proactively?

Yes. Battery replacement on a scheduled basis is better practice than waiting for the low-battery alarm. A battery that reaches the alarm threshold still has some margin before data loss — but that margin is finite and unpredictable. If the machine powers off between the alarm and battery replacement, SRAM data is at risk. Replace the battery during planned maintenance every three to four years, with the machine powered on during the swap to keep SRAM live.