FANUC2 MB DRAM Working-Memory Module for Series 16 / 18 / 21 CNC, Model B and C — CPU Board Daughter Card

Product Description

What this part is

This is a 2 MB DRAM memory module for Fanuc Series 16, 18 and 21 CNC controls of the model B and C generations. It is a small daughter card that plugs onto the main CPU board of the control, and it supplies the working memory the main processor uses while it is running — scratch space for the data it pulls out of the FROM and SRAM modules during machining. On a control fitted for a small configuration, 2 MB was the mid-capacity option of the period.

Decoding the order code

A20B: Fanuc printed circuit board / module drawing prefix.

2902: the module block used for Series 16/18/21 CPU-board daughter cards. Its neighbours are the memory and display modules for the same platform – 0210 is a 1 MB CMOS SRAM module, 0270 and 0278 are 14-inch colour CRT control modules, 0271 a 9-inch monochrome graphic engine, 0272 an 8.4/9.5-inch LCD control module.

0196: 2 MB DRAM module.

/02A: drawing revision, marked on the board. Fanuc revision suffixes on memory modules are usually component-level changes rather than functional ones, and are worth recording when ordering a matching spare.

Also written A20B29020196, A20B 2902 0196 and A20B-2902-0196-02A.

DRAM, not SRAM — and why the difference decides the fault:
On a Fanuc control DRAM and SRAM do entirely different jobs, and the distinction decides which fault this module addresses.

DRAM is volatile working memory. It is not battery-backed, it holds nothing when the control is off, and it stores no user data. It is the processor’s scratchpad.

SRAM on these controls is battery-backed and holds machine parameters, PMC ladder, tool offsets, macro variables and part programs. FROM is flash and holds the system software.

The practical consequence: a DRAM module change leaves parameters untouched, and it is not the route back from corrupted parameters or a lost program after a dead battery. DRAM faults show up as system alarms, random resets or failures partway through boot.

Published specifications

Function: DRAM working-memory daughter module. Capacity: 2 MB. Construction: surface-mount module. Platform: Fanuc Series 16, 18 and 21, model B and model C generations. Mounting: plugs onto the main CPU board of the control unit. Board marking: A20B-2902-0196/02A.

Quantity and configuration

Memory module only, one piece. The host is the main CPU board of a Series 16, 18 or 21 control unit, which is a separate assembly; this module is a component of that assembly and functions as part of it.

Mounting data

Mounting: plug-on daughter module on the main CPU board. Construction: bare surface-mount module with an exposed edge connector. Board marking: A20B-2902-0196/02A. A DRAM change does not require re-entering parameters.

Interchange guidance

Search forms include A20B-2902-0196, A20B29020196, A20B-2902-0196/02A and A20B 2902 0196. Other modules in the A20B-2902 block — 0210, 0213, 0270, 0271, 0272, 0278 — are different modules on the same platform rather than equivalents: different capacities, different memory technologies, or display controllers rather than memory. They are not substitutes across that block. Fanuc CNC boards of this generation are strongly dependent on system generation and firmware, and a one-digit difference in the part number frequently is not interchangeable. Fanuc has long discontinued this generation; surplus, reman and specialist board repair are the supply channels.