A20B-3900-0286 Fanuc PCB Board A20B39000286 A2OB-39OO-O286

FANUC A20B-3900-0286 | FROM 64 MB / SRAM 1 MB Module — R-30iA / R-30iB Controllers, Japan Origin

Part Number: A20B-3900-0286

Manufacturer: FANUC Corporation (Japan)

Product Type: FROM / SRAM Combination Memory Module

Board Series: A20B-3900

FROM Capacity: 64 MB (Flash ROM)

SRAM Capacity: 1 MB

Compatible Systems: FANUC R-30iA, R-30iB, R-30iB Mate robot controllers

Overview

The A20B-3900-0286 is a combination FROM and SRAM memory module for FANUC robot controllers. It provides 64 megabytes of Flash ROM for non-volatile system and program storage, plus 1 megabyte of battery-backed SRAM for active data retention.

The module mounts horizontally on the main board of compatible R-30iA and R-30iB series controllers.

Two memory technologies on one module, each handling a different storage role. Flash ROM is non-volatile — it requires no power source to retain data and holds its content indefinitely. SRAM is battery-backed volatile memory — it requires continuous low-level power from a backup battery to retain data through power-off events.

The combination of both on a single module is how FANUC's robot controllers store both their operating software and their active runtime data in a compact, field-replaceable form.

This module is part of the A20B-3900-0286/0287/0288 group, which covers the 64 MB FROM family at 1 MB, 2 MB, and 3 MB SRAM allocations respectively. Selecting the correct variant means knowing how much SRAM the specific robot controller configuration requires.

Key Specifications

FROM 64 MB — What It Stores in a Robot Controller

The 64 MB Flash ROM holds the robot's operating software stack. This includes the robot control kernel, motion software, servo control software, I/O processing software, teach pendant interface software, and any optional function software loaded for the specific robot application.

For installations with vision systems, force control, or other software options, the FROM must hold both the base software and the option packages.

Compared to the 32 MB FROM variants in the same family, the 64 MB allocation provides headroom for larger software installations.

Vision option software, advanced path control packages, and multiple robot programming languages consume FROM capacity. 

Systems configured with several software options benefit from the larger allocation.

The FROM does not change during normal robot operation. Its content is established during the initial controller configuration and modified only during deliberate software updates. It survives power failures, voltage transients, and extended storage without a battery.

SRAM 1 MB — Active Robot Data

The 1 MB SRAM holds the data the robot generates and uses during operation. Robot programs live here.

The robot's mastering data — the calibration values that define home position and axis references — live here. User frame and tool frame definitions, I/O mapping, alarm history, and system variables are all resident in SRAM.

Mastering data is particularly critical.

Lose the mastering data and the robot does not know where it is in space. Physical re-mastering is required before it can run. 

This is a time-consuming procedure that must be done carefully and typically requires reference positions marked on the robot at installation. 

The backup battery is what stands between normal operation and the need for re-mastering.

1 MB of SRAM accommodates standard robot configurations. Systems with extensive program libraries, many defined coordinate frames, or large amounts of stored variable data may find 1 MB constraining — in those cases, the -0287 (2 MB SRAM) or -0288 (3 MB SRAM) variants provide more headroom.

Compatible Controllers and Alternative Part Numbers

The A20B-3900-0286 is used across the R-30iA and R-30iB controller families. Specific alternative part numbers exist within FANUC's catalog for the same functional specification across different controller generations — A20B-3900-0166 and A20B-3900-0226 carry the same FROM 64 MB / SRAM 1 MB specification and serve earlier controller generations in the same robot family. The -0286 is the variant for R-30iB and R-30iB Mate main boards.

Confirm the installed module's part number from the module label or the controller documentation before sourcing a replacement.

The controller platform determines which variant is the correct fit.

FAQ

Q1: The robot alarms on startup with a memory fault. It was fine the previous shift. What should be checked first?

An overnight power-off event that coincides with a memory alarm suggests a depleted SRAM backup battery. The battery maintained data during the last power cycle but finally failed.

Check the battery voltage — if it is below specification, replace the battery and restore SRAM data from backup.

If the battery is intact, the FROM content may be corrupted. Accessing the controller's boot screen to verify FROM integrity is the next diagnostic step.

Q2: How does replacing the A20B-3900-0286 affect robot mastering?

A replacement module installs blank — all SRAM data, including mastering data, is absent. After installing the new module and restoring from backup, confirm that the mastering data was included in the backup file.

If mastering data is present in the backup, it will be restored. If not, physical re-mastering of each robot axis is required before the robot can be used for production.

Always verify mastering data availability in the backup before proceeding with a module replacement.

Q3: What is the difference between the -0286 and the -0287 variant?

Both carry FROM 64 MB. The difference is SRAM capacity: -0286 provides 1 MB and -0287 provides 2 MB. The correct variant depends on the robot controller's software configuration and data requirements.

Installing the -0287 in a position designed for the -0286 provides more SRAM than needed but is generally compatible.

Installing the -0286 where more SRAM is required will cause the controller to alarm with an insufficient memory error.

Q4: The module was replaced and the robot restored from backup. Programs run correctly but some I/O point names are missing. Why?

I/O point names and comments are stored in SRAM but may be excluded from a standard program backup depending on the backup type performed.

If the backup only captured robot programs and not the full system data including I/O configuration and comments, the names will not be present after restoration. 

Re-enter the I/O names, or perform a full system backup and restore that includes the I/O data and comment files.

Q5: Can the module be installed on a powered-on controller?

Memory modules in FANUC robot controllers are not hot-swappable under normal circumstances.

The controller must be powered down before the module is removed or installed.

Removing the module while the controller is powered can damage the module or corrupt the main board. 

Power the controller off, replace the module, then power on and restore from backup.