8 MB FROM (Flash ROM) Memory Module, DIMM — Program Memory C for i-Series CNC Master Boards and R-J3 Robot Controllers

What this part is:
A Fanuc A20B-3900-0072, an 8 MB FROM module in DIMM form. It plugs horizontally onto the main system board (the motherboard) of a Fanuc i-series CNC control, and it is also catalogued as an installable option on Fanuc R-J3 robot controllers, where it fits the A16B-3200-0330 or A16B-3200-0331 main PCB.

Dealers list it as the “8 MB PROGRAM MEMORY C” or FROM card for the larger 16i / 18i Model A controls, and describe it as containing the NC BASIC files and other system files.

Correcting the source data — FROM, not general-purpose memory:
Our previous listing called it simply an “8MB Memory Module” and left it there. That is technically true and practically misleading, because in a Fanuc i-series control there are two quite different memory modules in the same physical DIMM form factor:

FROM — flash ROM. Holds the CNC system software: the NC basic package, the servo and spindle control software, and the purchased option software. Non-volatile, no battery.SRAM — static RAM. Holds your data: part programs, parameters, tool offsets, PMC ladder and macro variables. Battery-backed.
The A20B-3900-0072 is the FROM module. One distributor page in circulation describes it as an SRAM module; the weight of evidence — including two independent listings that call it a FROM board carrying the NC basic files, and one that gives the alternative Fanuc number A20B-3900-0012 — is that it is FROM. We have stated FROM and flagged the disagreement rather than quietly picking a side.

Why the distinction is worth money: swapping a FROM module changes what software your control runs. Swapping an SRAM module changes what data it holds. Confusing the two is how a machine ends up with a blank control or a lost parameter set.

Firmware: read this before you buy:
This is the single most important thing about a FROM module and it does not apply to ordinary memory.

A FROM module is only useful if the software written into it matches your machine — the correct CNC series (16i, 18i, 21i), the correct model generation, the correct machining or turning package, and the correct set of purchased options. Two identical A20B-3900-0072 modules from two different machines are not interchangeable in any meaningful sense: they are the same hardware carrying different software.

What that means in practice:

If you are replacing a failed FROM module, you will need to reload the system software from your machine’s backup, or have it loaded to match your control’s series and edition. Fanuc’s boot system handles the load, but you must have the data.If you buy this module expecting it to arrive with your machine’s software already on it, it will not. We make no claim about what software, if any, is currently written to this module. Treat it as bare hardware.Option software is licence-tied to the control. Options are enabled against the CNC’s serial number; a FROM module from another machine does not transfer another machine’s paid options to yours, and attempting it is not a supported path.
If you have your machine’s FROM backup and a boot procedure, this is a straightforward hardware replacement. If you do not, source the software before you source the module.

Published data:
Type: FROM (flash ROM) memory module, DIMM form factorCapacity: 8 MBFanuc designation: Program Memory CContents when programmed: NC basic files and other system filesCNC platforms: Fanuc Series 16i, 18i and 21i (i-series master board / motherboard), listed specifically for the larger 16i / 18i Model A controlsRobot platform: option module fitted horizontally to the main PCB in Fanuc R-J3 controllers (A16B-3200-0330 / A16B-3200-0331)Alternative Fanuc part number: A20B-3900-0012
Module dimensions, pin count, flash device type, operating temperature and current draw are not published and are left blank.

Handling and installation notes:
A FROM DIMM is a bare, static-sensitive assembly. Handle it by the edges on an earthed mat, and store it in antistatic packaging — the flash devices carry the machine’s software and an ESD event that only partially damages a flash device produces the worst kind of fault, one that appears intermittently under load rather than failing cleanly.

Fit it with the control powered down and the mains isolated, and seat it fully in the socket; a partially seated DIMM on a Fanuc master board typically presents as a boot alarm rather than an obvious mechanical problem. And before you pull the old module, note its orientation and position on the board — i-series master boards carry several DIMM sockets and they are not all the same function.

What is not included:
Module only. Not included: the CNC master board or motherboard, the SRAM module (a separate part), the memory backup battery, the CNC system software or any option licence, the boot media, and any mounting hardware.

Interchange guidance:
Search forms include A20B-3900-0072, A20B39000072, A20B 3900 0072, A20B-3900-0072/xxx and A20B-3900-0012. Related Fanuc numbers: A16B-3200-0330 and A16B-3200-0331 (R-J3 main PCBs this fits as an option). One unit in stock.

The i-series controls this serves are legacy but far from dead — a very large installed base of 16i / 18i / 21i machines is still in daily production, and Fanuc Corporation remains in business with Fanuc America supporting the range. That combination is exactly why the surplus market for i-series FROM and SRAM modules is active: the machines are worth keeping, the modules are no longer freely available new, and a control retrofit costs many multiples of a memory module.