Resolving MICROMASTER 4 AOP 'Requires Rebuild' Error

Resolving MICROMASTER 4 AOP 'Requires Rebuild' Error

The Siemens Advanced Operator Panel (AOP) used with the MICROMASTER 4 inverter family (MM420, MM430, MM440) is a battery-backed, plug-on keypad that holds the multilingual firmware assets required to render menus. When the AOP's internal lithium coin cell discharges below its data-retention threshold, the language-block area of the AOP EEPROM is erased on the next power-down. On the following power-up, the bootloader cannot find a valid language descriptor and locks the panel into a rebuild state. The AOP then displays one or more of the following messages and refuses to accept any keystrokes:

• NO LANGUAGE BLOCK
• MUST LOAD FROM PC
• AOP REQUIRES REBUILD

1. Affected Drives and AOP Variants

The AOP plugs into the drive's RS-232-style D-sub front port and is hot-swappable. Power for the AOP logic is drawn from the drive's internal 24 V rail. The AOP contains its own microcontroller, an EEPROM for language packs, and a non-rechargeable lithium coin cell that holds the language descriptor and any saved parameter sets when the drive is de-energized.

2. Root Cause: Internal Lithium Coin Cell Failure

Per the Siemens Knowledge Base article 21367554 - MICROMASTER 4 (AOP) lifetime and exchange of the AOP battery, the AOP's internal lithium cell is the data-retention source for the language blocks.

Once the cell voltage drops below the retention threshold of the EEPROM, the next power-down event clears the language descriptor and any associated parameter sets held inside the AOP. On the next power-up the bootloader performs a checksum on the language area; when the descriptor is missing or invalid, it forces the rebuild state and refuses to mount any language pack.

3. Secondary Triggers

Although battery failure is the dominant root cause, two other conditions can produce the same boot message:

1. Firmware mismatch after inverter flash. If the inverter was flashed to a firmware version that the existing AOP bootloader does not recognize, the AOP may refuse to mount its language descriptor and report rebuild. Confirm compatibility in the MICROMASTER 4 Operating Instructions release notes.
2. EEPROM wear or write-cycle exhaustion. Repeated RAM → AOP parameter uploads combined with hot-plugging can in rare cases corrupt the language descriptor. Battery replacement does not repair this; the AOP must be re-flashed or replaced.

4. Diagnostic Procedure

Before replacing the AOP, confirm that the drive itself is healthy and that the failure is isolated to the keypad.

4.1 Power-Cycle and Observe

1. Switch off the drive's line supply. Wait for the DC bus to discharge (typical: 5 minutes, or until the AOP backlight is fully off).
2. Remove the AOP from the drive.
3. Re-apply line power without the AOP. The inverter should start in its last commanded state (or in fault, depending on P1210 auto-restart setting).
4. Re-attach the AOP while the drive is powered. Observe the boot text sequence. Any of the messages listed above confirms the AOP failure mode.

4.2 Verify the Drive via PC Tool

1. Connect DriveMonitor (legacy) or STARTER / SINAMICS Startdrive (newer) to the drive via the drive's COM port (RS-485 USS) or via PROFIBUS.
2. Read r0000 (inverter state). A value of 0 indicates Ready. 1 indicates Run. Either confirms the drive logic is alive.
3. Read r0021 (actual motor speed) and r0026 (actual DC-link voltage) to confirm the inverter is processing feedback.
4. Read P0010 (commissioning parameter). If P0010 = 0, the inverter is in normal operation. A non-zero value indicates the drive was left in commissioning mode and is unrelated to the AOP fault.
5. Try a parameter write to P0010 = 1 then back to 0. Successful write confirms the PC-to-drive path works. The drive is fully usable without the AOP.

4.3 AOP Boot Sequence Map

5. Communication Paths That Still Work Without the AOP

Use DriveMonitor or STARTER to read the full parameter set, save a .dnx / .par project file to disk, and confirm that no drive-side fault accompanies the AOP failure.

6. Recovery Options

6.1 Option A: Siemens Industry Online Support Service Request

1. Navigate to Siemens Industry Online Support.
2. Open a service request under "Automation & Drives / Drives / MICROMASTER 4 / AOP".
3. Include the AOP serial number, the drive order number (e.g., 6SE6440-2UD31-1DA1), the inverter firmware version (read via PC tool from r0018 or P0018), and a clear text of the boot message.
4. Siemens may supply the language-block file for re-flashing via DriveMonitor's Load to AOP function. Field success with this path is limited; the support response will indicate whether re-flash is feasible.

6.2 Option B: AOP Battery Replacement by Service Center

Battery replacement is a Siemens service-center operation, not a documented field procedure for the standard AOP. The opening sequence for the housing, the coin-cell type, and the reassembly torque values are not published for end-user service. Battery replacement alone will not restore erased language packs - the language area must be re-populated separately.

6.3 Option C: AOP Replacement (Recommended for Most Field Cases)

When Siemens support confirms that the language blocks cannot be re-flashed, replace the AOP.

1. Order the appropriate variant from the table above.
2. On receipt, record the AOP serial number in the plant asset register.
3. Plug the new AOP onto the drive while powered. Boot should complete to r0000.
4. Set P0003 = 3 (extended access) and confirm navigation.
5. Upload the saved parameter set from the PC tool to the drive (do not upload from AOP - the AOP has a blank parameter mirror).

7. Language Block Re-installation via PC (When Supported)

If Siemens support provides the language-block file (.bin or .aop), the re-flash procedure is:

1. Connect the PC to the drive's RS-485 USS port using a Siemens USB-to-RS485 converter (e.g., 6SE6400-1PC00-0AA0) or to PROFIBUS.
2. Launch DriveMonitor, go online, and confirm drive communication.
3. Navigate to AOP → Firmware / Language. Select the provided file.
4. Click Load to AOP. The AOP will display a progress bar; do not remove power during the transfer (typical duration: 2 to 5 minutes).
5. When the AOP shows Completed - power cycle, remove the AOP, wait 10 seconds, and re-attach.
6. Confirm normal boot to r0000.

8. Verification After Repair

1. Power up the drive with the repaired or replaced AOP fitted.
2. Confirm that the AOP displays the active frequency (r0000 or r0021) within 5 seconds of plug-in.
3. Press the P key and navigate to parameter P0010. Confirm it is readable and writable.
4. Set P0003 = 3 (expert access). Confirm navigation to P3900 (factory reset) is permitted.
5. From the PC tool, read the saved parameter file back into the drive. Verify with Compare in STARTER that the active parameter set matches the saved file.
6. Run a no-load start command from the terminal strip. Confirm motor rotation matches the command direction and that r0021 tracks the setpoint.
7. Stop the drive. Cycle power. Confirm the AOP retains language and parameter navigation after the power cycle.

9. Prevention and Maintenance Schedule

10. AOP vs BOP: When to Use Which

If the rebuild failure recurs and language flexibility is not required, replacing the AOP with a BOP (order number 6SE6400-0BP00-0AA0) eliminates the language-block failure mode entirely.

11. Field Commissioning Caveats

1. Firmware cross-compatibility. AOP firmware version 2.x is matched to inverter firmware 2.x. Mixing versions (e.g., AOP V3 on an inverter flashed to V4) can produce the rebuild state even with a healthy battery. Always re-flash both to matched versions per the MICROMASTER 4 Operating Instructions compatibility matrix.
2. Hot-plug during fault reset. Removing the AOP while the drive is in a fault state can corrupt the AOP parameter mirror. Power down the drive before swapping AOPs in a faulted state.
3. Static discharge. The AOP front port is exposed. In dry environments, touch a grounded metal surface before handling the AOP to avoid ESD damage to the keypad microcontroller.
4. Cable length. If the AOP is remote-mounted via the optional door-mounting kit (6SE6400-0PM00-0AA0), keep the interconnect cable under 5 m. Longer cable runs can corrupt language-block uploads.

12. Troubleshooting Matrix

13. Spare-Parts Strategy for Plant Floors

For plants with more than 10 MICROMASTER 4 drives fitted with AOPs:

• Maintain at least one spare 6SE6400-0AP00-0AB0 in the store, with its internal battery disconnected to extend shelf life.
• Keep one spare 6SE6400-0BP00-0AA0 (BOP) for cost-driven fallback.
• Maintain a library of the most recent .dnx / .par parameter backup files for every drive, indexed by drive serial number.
• Document the AOP firmware version per drive in the CMMS; replace AOPs proactively at the 5-year mark if permanently mounted.

14. Safety Notes

FAQ