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| Place of Origin | JAPAN |
| Brand Name | FANUC |
| Certification | CE ROHS |
| Model Number | A20B-3900-0180 |
Part Number: A20B-3900-0180
Manufacturer: FANUC Corporation (Japan)
Product Type: FROM/SRAM Memory Module (PCB)
The A20B-3900-0180 is the FROM/SRAM memory module for FANUC Series 30i, 31i, and 32i CNC systems.
It carries 16 MB of Flash ROM and 256 KB of SRAM on a compact plug-in board that installs horizontally on the main CPU board inside the controller. At power-up, the system loads FROM contents into working memory.
From that point, the CNC operates from RAM — the FROM serves as the persistent source that survives every power cycle.
The Series 30i/31i/32i represents FANUC's high-end CNC platform.
These controllers run large machining centres, compound machine tools, and multi-axis systems with demanding real-time requirements.
The FROM module in these systems holds the software stack that makes those capabilities possible — the CNC kernel, servo control software, PMC ladder runtime, and management programs.
All of it lives on this board. When the board fails, the machine stops.
| Parameter | Value |
|---|---|
| Part Number | A20B-3900-0180 |
| Manufacturer | FANUC Corporation |
| Product Type | FROM/SRAM Memory Module (PCB) |
| Board Series | A20B-3900 |
| Compatible Controllers | FANUC Series 30i / 31i / 32i |
| FROM Capacity | 16 MB (Flash ROM, non-volatile) |
| SRAM Capacity | 256 KB (battery-backed, volatile) |
| Installation | Horizontal plug-in, main CPU board |
| Power-up Behavior | FROM contents load into DRAM at boot |
| Origin | Japan |
| Operating Temperature | 0 – 55°C |
| Storage Temperature | −20 – 60°C |
| Humidity | 75% RH max (non-condensing) |
| Condition Available | New / Refurbished / Repaired |
Flash ROM does one thing well: it keeps data through power cycles without a battery. Every power-down, every emergency stop, every voltage transient — the FROM contents survive intact. The 16 MB on this board holds the CNC system software, servo amplifier control software, PMC ladder logic, and other programs that the machine needs to function. None of this changes during normal operation.
The 256 KB SRAM is different. It's volatile. It relies on a small backup battery to hold its contents when power is removed. SRAM on this module holds runtime data — operator settings, active offsets, and similar state information generated during operation. If the battery runs low and power is cut, SRAM data is gone.
This makes battery condition a maintenance item, not a setup item. Check it on a schedule, not on a failure.
The two memory types work together. FROM provides the static foundation. SRAM provides the dynamic workspace. The controller needs both to operate correctly.
The FROM contents are software. That software has a version.
The version on the replacement A20B-3900-0180 must match what the machine was built and commissioned on. FANUC Series 30i/31i/32i systems are frequently customised by machine builders — their options, axis configurations, and PMC ladder programs are often specific to a single machine model or even a single serial number.
A replacement module with the wrong software version may power up but will not run the machine correctly.
Parameter ranges change between versions.
PMC interface definitions change. Servo tuning data may need updating. Installing the right software version — or reflashing the module to the correct version — is part of the replacement procedure, not an afterthought.
These controllers sit at the top of FANUC's CNC line. The 30i handles the most complex machines: 5-axis machining centres, large turning centres with driven tooling, and multi-path systems. The 31i and 32i are scaled versions for fewer axes but similar capability depth.
All three use the same basic hardware architecture, and all three depend on the FROM module as the foundation of their software stack.
Machines running on Series 30i/31i/32i have typically been in production for years.
The original FROM module accumulates operating hours. Flash memory has a finite write cycle endurance — in practice, FROM modules in CNC controllers rarely fail from write exhaustion, but they do fail from power transients, electrostatic events, and hardware faults.
When they do, the replacement path is clear: the correct part number, with the correct software.
Q1: The Series 30i CNC powers up but immediately shows a system software error. Could this board be the cause?
Yes. If the FROM contents are corrupted or unreadable, the CNC cannot load its operating software and will fault at startup.
A system software error at boot, particularly one that appears after a power event or hardware fault, is a strong indication that the FROM module is affected.
The alarm code will help narrow it further — consult the 30i maintenance manual for the specific fault code description.
Q2: After replacing the A20B-3900-0180, do parameters need to be reloaded?
Yes. The replacement board installs without the previous machine's parameters. All parameters — CNC parameters, servo parameters, PMC parameters, pitch error compensation data, and tool data — must be restored from backup after installation.
The software version on the new module must also be confirmed compatible before loading parameters. Do not attempt to run the machine before parameters are restored.
Q3: The SRAM backup battery alarm appeared but the machine continued running. Is this urgent?
Yes. Once the low battery alarm is active, the time before data loss is limited. SRAM data is retained as long as the battery provides sufficient voltage.
When it drops below threshold, all SRAM content is lost at the next power-down.
Back up SRAM data immediately and replace the battery before the next scheduled shutdown.
Q4: Is the A20B-3900-0180 interchangeable with other A20B-3900 memory modules?
Not freely. The A20B-3900 series covers a wide range of FROM/SRAM capacity combinations and software versions.
The -0180 is specifically the 16 MB FROM / 256 KB SRAM variant for Series 30i/31i/32i.
Modules with different FROM or SRAM sizes, or different software versions, are not direct substitutes.
Always confirm the part number and software version before installation.
Q5: Can the A20B-3900-0180 be reflashed with a different software version?
Reflashing FANUC FROM modules requires FANUC software tools and the appropriate software package for the target CNC model and option configuration.
This is not a field procedure performed with generic tools.
If the existing software version is unknown or incompatible, contact a FANUC-certified service partner with the machine's CNC type, model, and option list to obtain the correct software package.
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