Used FANUC A20B-2901-0762 A20B29010762 A2OB-29O1-O762

FANUC A20B-2901-0762 | 256K SRAM / 128K FROM Module with Analog Spindle — Series 20 / 21 CNC

Part Number: A20B-2901-0762

Manufacturer: FANUC Ltd. (Japan)

Product Type: Combined Memory and Spindle Module — SRAM + FROM + Analog Spindle Interface

SRAM Capacity: 256K

FROM Capacity: 128K 

Memory and Spindle in One Module

The A20B-2901-0762 is a combined memory and spindle module for the FANUC Series 20/21 CNC. It integrates three functions on a single PCB:

256K SRAM — Battery-backed static RAM that retains the CNC's machine parameters, PMC ladder, part programs, and tool offsets through power-down cycles. The SRAM battery is the only thing keeping this data alive when mains power is off. A failed battery or dead SRAM causes parameter and program loss at the next power cycle.

128K FROM — Flash ROM that holds system-level firmware required for CNC operation. This is non-volatile memory — it retains content without battery backup. When FROM files become corrupted, the CNC fails to boot correctly.

Analog spindle interface — Provides the ±10V analog velocity command output to the spindle amplifier, translating the CNC's digital spindle speed command into the analogue signal the amplifier uses as its speed setpoint.

This combined architecture reflects FANUC's Series 20/21 module design philosophy — consolidating memory and spindle interface functions on a single plug-in board to minimise the total board count in the CNC cabinet.

Related Variants

Within the A20B-2901-0762 sub-family, the spindle interface type determines the variant:

Confirm the correct variant — SRAM size, FROM size, and spindle type — against the CNC's hardware documentation before ordering.

Key Specifications

FAQ

Q1: The CNC powers up with an SRAM alarm or shows all parameters reset to defaults. Is the A20B-2901-0762 the cause?

An SRAM alarm or parameter loss at power-up is caused either by a dead or missing SRAM battery, or a failed SRAM memory chip on the module. Check the battery voltage first — a fresh battery sometimes recovers a marginal SRAM situation. If the battery is good and the alarm persists, the SRAM chips have failed and the module must be replaced.

Q2: The CNC fails to boot with a FROM error. Is this the A20B-2901-0762?

FROM errors at boot indicate corrupted or missing firmware on the FROM portion of this module. The FROM file must be reloaded using FANUC's boot utility. If reload fails consistently (write errors during the FROM write cycle), the Flash memory chips on the module have degraded and the module must be replaced.

Q3: Can an A20B-2901-0760 (256K FROM variant) replace the 0762 (128K FROM)?

Physically the boards are similar, but the FROM capacity differs. Whether the larger FROM variant can substitute depends on the specific CNC's firmware revision and slot assignments. Consult FANUC's hardware compatibility documentation for the specific Series 20/21 CNC before attempting this substitution.

Q4: After replacing A20B-2901-0762, what data must be reloaded?

All data stored in both the SRAM and FROM portions must be reloaded: machine parameters, PMC ladder programs, part programs, tool offsets, and the FROM system firmware. A complete backup of all CNC data before module removal is essential. Without a backup, full recommissioning from documentation is required.

Q5: How long does the SRAM battery last, and what is the replacement procedure?

FANUC SRAM backup batteries typically last 2–5 years depending on usage patterns. The battery should be replaced while the CNC is powered on — this allows the SRAM content to remain powered from the main supply during the battery swap, preventing parameter loss. Replacing the battery with the machine powered off risks SRAM content loss if the battery is completely dead.