FANUCA20B-3900-00728 MB FROM (Flash ROM) Memory Module, DIMM — Program Memory C for i-Series CNC Master Boards and R-J3 Robot Controllers

Product Description

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.

FROM, not general-purpose memory

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 user data — part programs, parameters, tool offsets, PMC ladder and macro variables. Battery-backed.

The A20B-3900-0072 is the FROM module. Why the distinction is worth money: swapping a FROM module changes what software the control runs, while 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

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 the 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.

Replacing a failed FROM module therefore involves reloading the system software from the machine’s backup, or having it loaded to match the control’s series and edition. Fanuc’s boot system handles the load. Option software is licence-tied to the control: options are enabled against the CNC’s serial number, so a FROM module from another machine does not transfer another machine’s paid options. This is sold as bare hardware, and a machine FROM backup and boot procedure make it a straightforward hardware replacement.

Published data

Type: FROM (flash ROM) memory module, DIMM form factor. Capacity: 8 MB. Fanuc designation: Program Memory C. Contents when programmed: NC basic files and other system files. CNC platforms: Fanuc Series 16i, 18i and 21i (i-series master board / motherboard), listed specifically for the larger 16i / 18i Model A controls. Robot 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.

Mounting data

Form factor: DIMM, bare static-sensitive assembly with an edge connector. Mounting: DIMM socket on the CNC master board or motherboard; horizontal option position on the R-J3 main PCB. Socket context: i-series master boards carry several DIMM sockets, and they are not all the same function.

Quantity and configuration

Sold individually. One FROM module per purchase. The CNC master board, the SRAM module, the memory backup battery, the CNC system software and option licences, the boot media and mounting hardware are each separate items.

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).

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.