A20B-3900-0200 Fanuc PCB Board A20B39000200 A2OB-39OO-O2OO

FANUC A20B-3900-0200 | PMC Module — Series 30i/31i/32i, A5 Engine, Japan Origin

Part Number: A20B-3900-0200

Manufacturer: FANUC Corporation (Japan)

Product Type: PMC Module (Programmable Machine Controller PCB)

Overview

The A20B-3900-0200 is the PMC module for FANUC Series 30i, 31i, and 32i CNC systems. It plugs into a DIMM slot on the 30i main board and provides the processing engine for FANUC's built-in Programmable Machine Controller — the PMC A5.

The module operates on a 5 V supply rail and dissipates approximately 5 W during normal operation.

Every machine built around a FANUC CNC has two layers of control running simultaneously.

The CNC layer manages axis motion, interpolation, and spindle control. The PMC layer manages everything else: the machine's doors, clamps, tool changers, coolant systems, lubrication, hydraulics, and all the I/O signals that make a machine tool a complete machine — not just a motion controller. 

The A20B-3900-0200 handles the PMC side of this equation on Series 30i/31i/32i systems.

Without the PMC module, the machine's field I/O is uncontrolled.

The CNC may power up, but no machine functions sequence correctly. The A20B-3900-0200 is not an optional accessory. It is part of the controller's core architecture.

Key Specifications

What the PMC Does

FANUC's Programmable Machine Controller is the CNC's internal PLC. It runs the machine builder's ladder program — the sequence logic that the machine tool builder wrote to make their machine work. This ladder program controls everything the CNC's motion control software doesn't handle directly.

Consider what a machining centre does between cuts.

A tool change involves a specific sequence: spindle stops, Z-axis moves to safe position, tool changer arm actuates, clamp releases, new tool loads, clamp engages, arm retracts, the CNC resumes. Every one of those steps is a PMC ladder output or input.

The PMC reads the clamp-confirmed sensor, verifies the arm position, enables the coolant, monitors the door interlock. All of it is ladder logic executed on the PMC module.

On a Series 30i/31i/32i system, this ladder can be very large.

These controllers support complex machines with many axes and many machine functions.

The PMC A5 engine on the A20B-3900-0200 provides the processing capacity to handle this complexity at the cycle speeds the machine requires.

PMC A5 on Series 30i/31i/32i

The PMC engine designation — A5 — identifies the specific PMC processor generation and capability level. Different FANUC CNC generations use different PMC engines.

The A5 is specific to the Series 3xi (30i/31i/32i) generation and is not interchangeable with PMC modules from earlier or later controller generations.

The A5 engine determines the maximum ladder step count the controller can execute, the I/O point capacity, and the execution cycle time. Series 30i/31i/32i machines with complex ladder programs — large tool magazines, multiple work holding positions, integrated automation systems — require the A5's processing headroom to execute their full machine sequence without cycle time impact.

The A20B-3900-0200 is the hardware that houses the A5 engine. When this module fails, the PMC ceases to execute.

The ladder stops. Machine functions controlled by the PMC stop with it.

Restoring the PMC After Replacement

Installing a replacement A20B-3900-0200 installs a blank PMC processor. The machine builder's ladder program must be reloaded before the machine can function.

The ladder program is backed up and restored through the CNC's DATA I/O screen or through a FANUC Ladder III / FANUC LADDER-III software session connected to the control.

The ladder backup is a separate file from the CNC parameter backup.

Both are needed for a complete system restoration after PMC module replacement. Confirm both backups are current before beginning the replacement procedure.

A ladder backup that predates the last machine modification restores an outdated machine configuration — confirm with the machine builder's service records that the ladder version in the backup matches the current production state.

FAQ

Q1: The machine powers up but no M-codes function. Axis motion works normally. Is the PMC module the likely fault?

This is a classic PMC failure symptom. The CNC axis control is independent of the PMC module — axes can move even if the PMC is not executing. M-codes, tool changes, coolant, and all machine I/O functions run through the PMC ladder.

If axis motion is normal but machine functions are absent, the PMC module is the primary suspect. Check the PMC status screen on the CNC for execution state and alarm codes.

Q2: After installing the replacement A20B-3900-0200, what must be reloaded before the machine runs?

The machine builder's ladder program must be reloaded first. Without it, the PMC runs with no ladder and machine I/O is uncontrolled. After the ladder is loaded and confirmed running, verify all PMC parameters and I/O assignments match the machine's current configuration.

Then perform a careful step-through of machine functions before returning to production.

Q3: Is it possible to use a PMC module from a different FANUC controller generation?

No. The PMC A5 engine in the A20B-3900-0200 is specific to the Series 30i/31i/32i architecture. PMC modules from Series 16i/18i or other generations use different processor architectures and different ladder instruction sets.

Installing an incompatible PMC module will not work — the controller will not recognise it. Always replace with the correct part number for the controller generation.

Q4: The ladder program is running but some I/O points are not responding. Could this be a PMC module fault?

Partial I/O failure with the ladder running is less commonly a PMC module fault and more commonly a wiring, I/O board, or parameter fault.

The PMC module either runs the ladder or it doesn't — it rarely fails in a way that selectively disables specific I/O points. 

Check the I/O diagnostic screen on the CNC to confirm whether the expected signal transitions are occurring at the PMC's input points.

This directs the fault to the field wiring, the I/O board, or the PMC parameter configuration.

Q5: How long can a spare A20B-3900-0200 be stored before use?

Stored correctly — dry environment, stable temperature, anti-static packaging — FANUC PMC modules retain their condition for extended periods.

The module itself has no battery and no time-critical components that degrade during passive storage. 

Keep it away from moisture, temperature cycling, and electrostatic exposure.

Inspect the DIMM connector contacts for oxidation before installation if the module has been stored for several years.