Used Fanuc A20B-3900-0041 Memory Card Circuit Board A20B39000041 A2OB-39OO-OO41

A20B-3900-0041 | FANUC i-Series 8MB DRAM Memory Module — 16i / 18i / 20i / 21i / DIMM Plug-In / A20B-3900 Series

A20B-3900 — Plug-In Memory Modules for i-Series CNC

The A20B-3900 series is FANUC's family of plug-in memory modules for the i-series CNC platform. These compact DIMM-format boards add specific memory resources to the CNC main board's CPU and memory architecture:

FROM modules: Flash ROM modules storing system software and option programmes. The A20B-3900-0014 (4MB FROM) and A20B-3900-0060 (FROM) are examples from this family.

DRAM modules: Dynamic RAM providing additional working memory for the CNC processor. The A20B-3900-0041 (8MB DRAM) is the DRAM module confirmed for the i-series platform.

These modules plug directly into the CNC main board — small, self-contained boards that slide into defined connector positions alongside the CPU card and other functional modules.

Key Specifications

A20B-8100-0135 Configuration — Where This Module Lives

Fixtech.com confirms the A20B-3900-0041 pairs with A20B-3300-0050 CPU card in the A20B-8100-0135 mainboard. The complete A20B-8100-0135 (FANUC 18i-A LCD Mounted Main PCB) configuration includes:

The parenthetical notation "A20B-3300-0050(A20B-3900-0041)" indicates the DRAM module plugs into or pairs directly with the CPU card — the two work together as the CNC's processor-and-memory unit.

Application Scenarios

i-Series DRAM failure: A FANUC 18i-A CNC develops memory error alarms or fails to boot — a POST (Power-On Self Test) failure attributable to faulty DRAM. The A20B-3900-0041 DRAM module is replaced. CNC boot completes normally.

CNC memory expansion or refresh: An older FANUC 16i CNC undergoes a maintenance refresh. The A20B-3900-0041 is replaced alongside FROM modules to restore known-good memory to the CNC controller.

FAQ

Q1: Is A20B-3900-0041 a DRAM module or a FROM module?

"DRAM Dimm memory module / 8MB Memory for the Fanuc 16i, 18i, 20i and 21i control." This is a DRAM (volatile working memory) module — not a FROM (non-volatile Flash ROM) module. The A20B-3900-0014 is the separate FROM module for a similar i-series platform.

Q2: Does replacing A20B-3900-0041 affect stored CNC parameters or programmes?

No. As DRAM, the A20B-3900-0041 is volatile — it loses its contents whenever the CNC power is off. CNC parameters and programmes are stored in battery-backed SRAM (separate module) or FROM. Replacing the DRAM module does not affect any persistent CNC data.

Q3: Can A20B-3900-0041 be bench tested before installation?

Like any DRAM device, the A20B-3900-0041 requires a host system to test — it needs the address, data, and control signals from the CPU card to be exercised. 

Q4: What failure symptoms point to A20B-3900-0041 as the fault?

CNC failure to complete POST (Power-On Self Test), memory parity error alarms, or random CNC resets that correlate with the CNC operating temperature (warm DRAM cells failing under thermal stress) all indicate DRAM module suspect. The CNC's POST memory test specifically exercises DRAM on startup — a POST memory error alarm points directly to the DRAM module.