Fanuc PC Daughter Board A20B-2902-0250 A20B29020250 A2OB-29O2-O25O

FANUC A20B-2902-0250 | PMC Control Module, Series 15-B / 16 / 18 — PMC-RB4/RC4, Japan Origin, Discontinued

Part Number: A20B-2902-0250

Manufacturer: FANUC Corporation (Japan)

Product Type: PMC Control Module (Plug-in Daughter PCB)

PMC Engine: PMC-RB4 / RC4

Compatible Systems: FANUC Series 15-B, Series 16, Series 18

Status: Discontinued by Manufacturer

Overview

The A20B-2902-0250 is the PMC control module for FANUC Series 15-B, 16, and 18 CNC systems.

It is a plug-in daughter board that installs on the CNC's main CPU board and provides the processing engine for FANUC's built-in Programmable Machine Controller. Specifically, it enables PMC-RB4 and RC4 ladder logic execution — the PMC engine designations for this controller generation.

The PMC in a FANUC CNC is the embedded PLC. It runs the machine builder's ladder program: the control logic that manages every machine function outside of the CNC's axis motion and spindle control.

Tool changes, coolant on/off, door interlocks, clamp confirmation, lubrication cycles, hydraulic sequence valves — all of it is ladder. Without the PMC module running correctly, none of these machine functions operate.

The A20B-2902-0250 is discontinued by FANUC. It is no longer manufactured.

The demand for this board comes entirely from the installed base of Series 15-B, 16, and 18 controlled machines that remain in production. 

That installed base is still substantial — many of these machines have decades of useful mechanical life remaining.

Key Specifications

PMC-RB4 / RC4 — Engine Designation Context

FANUC assigns engine designations to its PMC generations that indicate processing capability and ladder instruction set. The RB4 and RC4 designations identify the PMC engine installed in Series 15-B, 16, and 18 controllers.

These engines define the maximum ladder step count, the available instruction set, and the I/O capacity the PMC can handle on these controllers.

Each FANUC CNC generation uses a specific PMC engine.

Earlier Series 15-B controllers and the Series 16 and 18 generation used the RB4/RC4 engines.

Later i-series controllers use different PMC engines with expanded capacity. The A20B-2902-0250 is the hardware that implements the RB4/RC4 engine on the supported platforms. 

It is not interchangeable with PMC modules from other CNC generations.

When the PMC module is absent or failed, the CNC alarms and motion is inhibited.

The controller will not allow axis movement if the PMC is not executing. This is by design — the PMC ladder manages safety interlocks, and a controller without PMC execution cannot verify that those interlocks are in a safe state.

Ladder Program and Module Relationship

The PMC module is the processor. The ladder program is the software that runs on it.

These are separate things — the module provides the hardware execution engine, and the ladder program is loaded into the CNC's memory (typically in the FROM module) and executed by the PMC processor on each PMC scan cycle.

Replacing the A20B-2902-0250 module replaces the processor hardware.

The ladder program stored in FROM is not affected by this replacement. 

After installing the new module, the ladder program that was previously running should resume execution without modification, provided the FROM contents are intact and the module is correctly seated.

This is different from a situation where the PMC module takes the ladder program with it when it fails. It does not.

The program is separate. This distinction matters practically: a failed PMC module does not cause ladder program loss.

Sourcing for a Discontinued Platform

The Series 15-B, 16, and 18 CNC generation is mature. FANUC discontinued these controls and their associated components as the i-series generation replaced them.

The A20B-2902-0250 has not been in production for years.

Every available unit comes from surplus original stock, refurbished units pulled from decommissioned machines, or repaired field returns.

The supply diminishes over time as units are consumed in repairs and not returned. Stockpiling a known-good spare when one is available — before the machine fails — is a defensible maintenance strategy for any machine running this board.

A PMC module failure that grounds a production machine while the repair team searches for an obsolete spare is a controllable risk.

FAQ

Q1: The CNC shows a PMC alarm. Axis motion is inhibited. All other alarms appear normal. Is the A20B-2902-0250 the likely fault?

A PMC alarm with motion inhibited is characteristic of a PMC module fault, but it can also indicate a PMC software alarm — a fault generated by the ladder program itself rather than the hardware.

Check the PMC alarm detail on the CNC display. PMC hardware alarms (system alarms) point to the module.

PMC program alarms (ladder alarms) point to a machine function fault. The distinction determines whether the module or the machine wiring needs attention.

Q2: The PMC module was replaced. The ladder program was running before the fault. Will it resume automatically?

Yes, if the FROM module that holds the ladder program is intact and correctly installed.

The ladder is stored in FROM, not on the PMC module itself. After the PMC module replacement, the controller will reload the ladder from FROM on power-up and begin execution. 

Confirm the FROM module is fully seated and that no FROM alarms appear at startup.

Q3: Can a PMC module from a Series 16 controller be used in a Series 18 controller?

The A20B-2902-0250 serves both Series 16 and Series 18, so within this specific part number there is no distinction.

However, confirm by checking the board label — both controllers use the same part number in this module family. 

Do not assume a PMC module from a different part number is compatible without confirming the specific part number is shared.

Q4: What is the maximum ladder step count supported by the PMC-RB4/RC4 engine?

The RB4 and RC4 engines support substantial ladder step counts appropriate for fully configured Series 15-B, 16, and 18 machine tools.

The specific maximum varies by controller model and software configuration. 

If a machine's ladder program is near the capacity limit, adding ladder steps may not be possible without upgrading the PMC engine — which is not possible on this generation without a full controller upgrade.

Q5: How should a removed A20B-2902-0250 module be stored as a spare?

Store in anti-static packaging in a dry, temperature-stable environment.

The module does not contain a backup battery on its own — battery-backed memory is elsewhere in the controller architecture — so battery management is not a concern for the module in storage. 

Inspect the edge connector contacts for oxidation before installation if the module has been stored for extended periods. 

Light oxidation can be cleaned with appropriate contact cleaner; heavy corrosion indicates the module may not make reliable contact.