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Fanuc Alarm 900 - ROM PARITY
Alarm 900 ROM PARITY indicates a parity error has been detected in the CNC system ROM during boot-up or runtime integrity checking. This is a critical system fault caused by corrupted firmware data in the main CPU board's ROM chip, memory bit corruption, or physical failure of the ROM or CPU module itself.
900
Fanuc Alarm 900 - ROM PARITY
ROM Parity Error
A ROM parity error was detected. The CNC software ROM is defective. Contact Fanuc service. A CPU/ROM board replacement may be required.
Root Cause Summary
Alarm 900 ROM PARITY indicates a parity error has been detected in the CNC system ROM during boot-up or runtime integrity checking. This is a critical system fault caused by corrupted firmware data in the main CPU board's ROM chip, memory bit corruption, or physical failure of the ROM or CPU module itself.
Safety
- Always perform proper lockout/tagout procedures before accessing electrical panels
- Use anti-static precautions when handling ROM chips or CPU boards
- Ensure power is completely off before checking battery connections or replacing batteries
- ⚠️ Perform proper lockout/tagout before accessing electrical panels
- Be aware of potential pinch points and sharp edges when working inside the control cabinet.
- Consult the machine tool builder's documentation for specific safety procedures related to the CNC system.
- Consult machine tool builder's documentation for CNC-specific safety procedures
- High voltage capacitors may be present on the boards. Discharge them properly before handling the boards.
- Be aware of potential arc flash hazards when working with energized electrical equipment.
- Discharge high voltage capacitors properly before handling boards
- Be aware of arc flash hazards when working with energized equipment
- Watch for pinch points and sharp edges inside control cabinet
- Wear appropriate personal protective equipment (PPE) such as safety glasses and gloves.
- Avoid touching any components on the boards while the power is on.
- Ensure adequate ventilation when working with cleaning solvents.
- Wear appropriate PPE including safety glasses and gloves
- Avoid touching board components while power is on
- Be aware of the weight of the boards and components when removing or installing them to prevent dropping or injury.
- Use caution when handling sharp tools or instruments during troubleshooting.
- Double-check all wiring connections before powering on the system after any maintenance or repair.
- Ensure proper grounding of the CNC machine and control cabinet to prevent electrical shock hazards.
Causes and Fixes
- 1
Power down the CNC completely and observe if the alarm occurs immediately upon restart during POST (Power-On Self Test). If alarm appears before boot completion, this confirms ROM hardware failure rather than transient error.
Ensure proper shutdown procedure to avoid compounding data corruption. Record all alarm history from ALARM > HISTORY screen if accessible.
- 2
Open the control cabinet and locate the main CPU board (varies by series: 0i-MODEL uses compact CPU card, 16i/18i uses slot-mounted modules, 30i/31i/32i uses main board assembly). Check manufacturing date labels - boards over 15 years old have higher ROM failure rates.
Observe ESD precautions. Disconnect main power and wait 5 minutes for capacitor discharge before handling boards.
- 3
Inspect CPU board for physical damage: bulging capacitors near ROM chips, burn marks, corrosion on PCB traces, or loose socketed ROM chips (older 16i/18i series may use socketed EPROMs). Reseat ROM chips if socketed, ensuring proper orientation.
Socketed ROM chips have pin 1 orientation markers. Incorrect insertion will destroy the chip when powered.
- 4
If no physical damage is evident and reseating does not resolve the issue, the CPU board assembly requires replacement. Contact Fanuc or authorized distributor with control serial number and software series (found on system boot screen or SYSTEM > CNC CONFIG menu). Order replacement main CPU board with matching software version.
CPU board replacement requires system parameter and program backup/restore. Ensure all data is backed up to memory card or via RS-232 before replacement.
Call a technician if:
- ·Alarm persists after power cycling
- ·Multiple ROM-related alarms occur
- ·System fails to boot completely
- ·LED indicators show CPU board errors
Prevention
- Install quality surge protection
- Maintain proper grounding
- Keep control cabinet temperature below 40°C
- Regular power supply voltage monitoring
- Backup system parameters and PMC ladder regularly
Common Mistakes
- Ignoring intermittent ROM alarms
- Not checking power supply voltages first
- Attempting to repair ROM chips
- Not documenting alarm patterns
- Skipping proper ESD precautions when handling boards
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Always verify with your machine manual or a certified technician before performing service procedures.