Used FANUC A20B-2901-0716 A20B29010716 A2OB-29O1-O716

FANUC A20B-2901-0716 | RAM Module — 128KB SRAM + 3.5MB DRAM, Series 16/18/20/21 CNC

Part Number: A20B-2901-0716

Manufacturer: FANUC Corporation (Japan)

Product Series: A20B-2901

Product Type: RAM Memory Module (SMD plug-in daughter board) 

Overview

The A20B-2901-0716 is a FANUC RAM module combining 128KB of battery-backed SRAM with 3.5MB of DRAM, designed as a plug-in SMD daughter board for FANUC Series 16, 18, 20, and 21 CNC control systems.

This module installs directly onto the main CPU board of the CNC control, expanding the system's working memory for part program storage, parameter retention, PMC ladder logic, and runtime data processing.

The A20B-2901 series represents one of FANUC's most important module families — the plug-in memory cards that determine how much part program storage, PMC memory, and working data the CNC can handle.

Different combinations of SRAM and DRAM capacity serve different application requirements: larger SRAM provides more non-volatile storage for programs and parameters; larger DRAM provides more working memory for system processing and temporary data during execution.

The A20B-2901-0716 sits at the 128KB SRAM / 3.5MB DRAM specification within the A20B-2901 range — providing the maximum DRAM capacity available in the 128KB SRAM bracket of this module family.

This makes it particularly suited for applications where high-speed processing data throughput matters more than expanding battery-backed program storage.

Key Specifications

SRAM vs DRAM — Understanding the Two Memory Types

In a FANUC CNC system, SRAM and DRAM play different and complementary roles.

SRAM (Static Random Access Memory) in a FANUC CNC is battery-backed. When the machine's power is off, the lithium backup battery on the control board maintains voltage to the SRAM chips, keeping their contents intact.

This is where the CNC stores everything it must remember between power cycles: part programs, work coordinate offsets, tool offsets, parameters, and PMC (Programmable Machine Controller) data.

Losing SRAM contents — through a dead battery, an ESD event that kills the SRAM chips, or module failure — means losing all stored programs and machine setup data.

DRAM (Dynamic Random Access Memory) requires constant power refresh to retain its data.

It loses its contents the moment power is removed. The CNC uses DRAM as working memory during operation — a scratch space for executing calculations, running the interpolation engine, processing PMC ladder logic, and temporarily holding data during program execution.

More DRAM allows the system to handle more complex operations and larger programs in execution.

It is refreshed from FROM (Flash ROM) or SRAM at startup.

Module in the System Context

The A20B-2901-0716 plugs into the designated SMD socket on the FANUC CNC main CPU board.

In Series 16/18/20/21 systems, the RAM module is one of several plug-in daughter boards that customise the main board's capabilities — alongside FROM modules (for system software storage), PMC modules, system modules (spindle type selection), and servo modules.

The specific RAM module chosen for a machine was part of the machine builder's original control specification.

A machine configured for complex multi-program storage or extensive PMC ladders may have been built with the 256KB SRAM variant (A20B-2901-0711).

A machine requiring the maximum DRAM throughput with moderate program storage uses the A20B-2901-0716.

Battery Backup — The Critical Maintenance Item

The 128KB SRAM in the A20B-2901-0716 depends entirely on the lithium backup battery to retain its contents when the machine is powered off.

The battery is located on the main CPU board, not on the RAM module itself. When the battery voltage drops below threshold — typically after 3–5 years of service depending on conditions — the CNC displays a battery low alarm. 

At this point the SRAM contents may still be intact, but replacement should happen promptly.

If the battery fails completely and the SRAM contents are lost, all part programs, parameters, offsets, and PMC data must be reloaded. For machines without documented parameter backups, this can mean days of recommissioning.

A regular battery replacement programme, combined with routine parameter backup to a USB drive or over-network file transfer, prevents this failure mode entirely.

FAQ

Q1: The CNC shows a RAM module alarm and will not boot. Is the A20B-2901-0716 the fault?

Before concluding the module has failed, check whether the backup battery is in alarm state as well.

A dead battery can cause SRAM content corruption that the system detects as a RAM error. 

Replace the battery and check if the alarm clears after reload of parameters.

If the alarm persists with a confirmed-good battery and no SRAM data corruption, the A20B-2901-0716 module itself requires replacement.

Q2: Can the A20B-2901-0716 (128KB SRAM / 3.5MB DRAM) be replaced with an A20B-2901-0711 (256KB SRAM / 3.5MB DRAM) to increase program storage?

Only if the control hardware and software configuration supports the larger SRAM module. Series 16/18/20/21 controls have defined compatibility tables for RAM modules — not all module combinations are valid for all control editions and option configurations.

Verify compatibility in the hardware manual for the specific control type before installing a different module.

Installing an incompatible module can cause boot failures or memory mapping errors.

Q3: When replacing the A20B-2901-0716, do all programs and parameters need to be re-entered?

Yes — the SRAM in the replacement module is blank from the factory. All programs, parameters, tool offsets, work offsets, and PMC data must be reloaded. This is why maintaining a current backup is essential before any memory module replacement.

Most FANUC Series 16/18/20/21 controls support program and parameter output to a USB drive or over the Ethernet/RS232 interface — use these to create a backup before the module is changed.

Q4: The CNC displays a memory error on one PMC data area but not others. Could the A20B-2901-0716 have a partial failure?

Partial failures — where one specific region of the SRAM shows errors while others remain intact — can occur in aging SRAM modules. Individual SRAM chips within the module can fail while others remain operational.

If the error is consistently associated with the same data area and persists after parameter reload, SRAM chip-level failure is likely.

The module should be replaced and the full system reloaded from backup.

Q5: How do I identify which RAM module is currently installed in the CNC to confirm I need the A20B-2901-0716?

The installed module's label is readable with the control powered down and the main CPU board accessed.

The A20B-2901 part number is printed on the module's label. Alternatively, the FANUC system configuration display — typically accessible from the system maintenance screens — shows the installed memory module configuration. 

If neither is accessible, the machine's original build documentation (machine builder's maintenance manual or spare parts list) specifies the original module type.