FANUCA20B-3900-00744 MB FROM Flash Memory Plug-In Module for Series 16i / 18i / 21i CNC Main Board

Product Description

What this part is

A Fanuc A20B-3900-0074 — a 4 MB FROM (flash ROM) memory module that plugs into a connector on the main CPU board of a Fanuc Series 16i / 18i / 21i CNC. It is the non-volatile store for the CNC system software, the PMC ladder program, macro executors and option software; the contents are copied into system DRAM at power-up for execution.

Correcting the source data

This is an internal plug-in memory module, not a removable PCMCIA-style memory card. It seats on the main board inside the control and requires the control to be opened; it is not the card you push into the slot on the front of the operator panel to transfer programs. Buyers looking for a front-slot data card need a different item.

Decoding the order code

A20B — Fanuc printed-circuit-board / module product class3900 — the i-series plug-in memory module family (FROM and SRAM modules share this block)0074 — the 4 MB FROM specification within that familyBoards are commonly stamped with a revision suffix such as /01A; the revision does not change the memory capacity or fit

Published specifications

Type: FROM (flash ROM) plug-in memory moduleCapacity: 4 MB (4 M)Function: CNC system software, PMC ladder and macro storage — non-volatileHost boards: Fanuc Series 16i, 18i, 21i main CPU boardsMounting: internal connector on the main boardWeight: approx. 2.0 ozOrigin: Japan (Fanuc nameplate)

What these specs mean in practice

FROM is flash, so it retains its contents with no battery. That is the practical difference from the SRAM modules in the same A20B-3900 block, which hold part programs and parameters and do depend on the CNC backup battery. If you are chasing lost parameters or an SRAM battery alarm, this module is not the fix.

Installation and setup notes

A replacement FROM module arrives without your machine’s software. The CNC system software, PMC ladder and any option software must be loaded to it — normally from a boot-monitor restore of a known-good backup. Fit the module capacity the control was configured for; a smaller FROM than the installed software set will not boot.

What is not included

Module only. No main CPU board, no CNC rack, no SRAM module, no battery, no memory card or card adapter, no CNC or PMC software, no ladder program, no cables and no documentation.

Quantity and configuration

Sold as one module.

Interchange guidance

A20B-3900-0074 and A20B39000074 are the same part. Other numbers in the A20B-3900 block are different capacities or different memory technology — a 0075 or 0304 is not a substitute for a 4 MB FROM, and the SRAM modules in the block are a different function entirely. Note that A03B and A20B are Fanuc’s own numbering schemes, and the A86L prefix seen on other Fanuc-branded parts marks bought-in components; none of those are GE Fanuc IC-series numbers.