Fanuc Memory Module A20B-3900-0073 A2OB-39OO-OO73 A20B39000073

A20B-3900-0073 | FANUC 6MB Flash ROM (FROM) Module — DIMM / 16i / 18i / 21i / Power Mate i / CNC System Software Storage

FROM — What It Is and Why It Matters

In the FANUC i-series CNC, the control unit contains a stack of memory modules on the main master PCB. Each module has a specific type and role:

FROM (Flash Read-Only Memory): Stores the CNC's system software — the complete operating programme that makes the CNC function. This includes the G-code interpreter, interpolation software, servo control algorithms, PMC (ladder) executor, display driver, and all option software. Unlike regular RAM, Flash ROM retains its content after power-off without a battery. It can be erased and reprogrammed, but during normal CNC operation it is read-only.

SRAM: Stores the machine-specific data — parameters, tool offsets, work coordinates, part programmes, PMC data. Battery-backed to retain content through power cycles.

DRAM: Working memory for the CPU — filled at startup from the FROM modules, cleared at power-off.

The A20B-3900-0073 is the FROM module — the 6MB flash storage that holds all the CNC software the system needs to operate.

Key Specifications

6MB — Where It Fits in the FROM Module Range

The A20B-3900 series covers FROM modules of different capacities for the i-series CNC platform:

• A20B-3900-0071: 2MB FROM — base capacity
• A20B-3900-0073: 6MB FROM — expanded capacity
• A20B-3900-0074: 4MB FROM — intermediate capacity
• A20B-3300-0022: Higher capacity FROM modules for i-series platforms

The 6MB capacity accommodates larger CNC software images — the full option software complement for systems with advanced capabilities (multiple option packages, extended PMC functionality, graphics options). Machines with only standard functions may run on a 2MB or 4MB module; machines with many software options require the 6MB to hold the complete software image.

Application Scenarios

CNC software reload after data loss: A 16i CNC develops FROM corruption — either from a failed update procedure or flash cell degradation after many years. The A20B-3900-0073 is replaced with a new module, and the correct CNC software version is loaded from the original installation media. The CNC resumes operation with restored software.

Capacity upgrade: A 21i CNC system running a 2MB FROM module requires a software option that exceeds the 2MB module's capacity. The A20B-3900-0073 6MB replacement provides the storage needed for the expanded software image.

FAQ

Q1: Does the A20B-3900-0073 come pre-loaded with CNC software, or must software be loaded after fitting?

CNC software must be loaded from the original installation media or a FANUC backup file after fitting. If the correct software version file is not available, contact the machine tool builder or FANUC for the applicable software.

Q2: What CNC software version should be loaded onto the replacement A20B-3900-0073?

The correct software version depends on the specific CNC system and its configuration. The installed software version is visible on the CNC's system screen before the FROM is replaced. Note this version before removal. FANUC service or the machine tool builder can provide the correct software file for the specific system type.

Q3: Can a 6MB A20B-3900-0073 replace a 4MB A20B-3900-0074 in a system that normally uses 4MB?

Generally yes — a higher-capacity FROM module can replace a lower-capacity one in the same socket if the module interface is compatible. The CNC only uses as much FROM as the software image occupies. However, confirm compatibility from the FANUC maintenance documentation for the specific CNC type before substituting capacities.

Q4: What causes FROM module failure in FANUC CNC systems?

Flash ROM cells have a finite write endurance — each cell can be written a limited number of times. For FROM modules that store static CNC software (rarely updated), the write count is low and long life is typical. Power surges or incorrect update procedures can corrupt FROM data without reaching the cell's write endurance limit. Physical damage (connector corrosion, mechanical stress from repeated insertion) can also cause failure.

Q5: Is the A20B-3900-0073 specific to the i-generation CNC, or does it work with older Series 0 or Series 16 hardware?

The A20B-3900-0073 is confirmed for the i-generation CNC (16i, 18i, 21i, Power Mate i). Earlier FANUC generations (Series 0-C, Series 16/18 Model B, etc.) use different memory module formats — the A20B-2902-0xxx series FROM modules and other older module types. The A20B-3900-0073 DIMM format is specific to the i-generation platform.