What causes FANUC alarm 951?

PMC ladder program contains an error causing abnormal execution. PMC watchdog timer timeout — PMC scan cycle taking too long. PMC RAM hardware fault. PMC program corruption after a power event. Excessive PMC program complexity causing scan time overrun.

How do I fix FANUC alarm 951?

1. Power cycle the machine to reset the PMC. 2. Check the Fanuc PMC monitor for specific alarm details and fault location. 3. If the PMC program was recently modified, restore the last known good version. 4. Contact the machine tool builder if the PMC program source is unavailable. 5. Fanuc service may be required if PMC hardware failure is suspected.

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FANUC Controller

System · Advanced · 30 minutes - 4 hours

Alarm 951

Fanuc Alarm 951 - PMC System Error

Alarm 951 PMC WATCH DOG ALARM occurs when the PMC (Programmable Machine Controller) fails to complete its ladder logic scan cycle within the allocated watchdog timer period, typically 32ms or 64ms depending on system configuration. This indicates the PMC CPU is overloaded, has encountered an infinite loop in the ladder program, or the PMC hardware has failed.

FANUCSystemAdvanced30 minutes - 4 hours

951

Fanuc Alarm 951 - PMC System Error

PMC Alarm

Root Cause Summary

Alarm 951 PMC WATCH DOG ALARM occurs when the PMC (Programmable Machine Controller) fails to complete its ladder logic scan cycle within the allocated watchdog timer period, typically 32ms or 64ms depending on system configuration. This indicates the PMC CPU is overloaded, has encountered an infinite loop in the ladder program, or the PMC hardware has failed.

Safety

Machine may not operate correctly

Causes and Fixes

1
Access PMC diagnostic screen: Press SYSTEM > PMC > PMCPRM > select 'LADDER MONITOR' or 'PMC STATUS' screen. Check current scan time display. Normal scan time should be 10-30ms. If approaching or exceeding 30ms, ladder optimization is required.
2
Check PMC parameters for watchdog timer setting: Navigate to SYSTEM > PMC > PMCPRM > SETTING screen. Verify Keep Relay setting (K900.0-K900.7) and watchdog timer configuration. Standard watchdog is 32ms (some systems allow 64ms extension via K900 settings).
3
Review recent ladder program changes using FANUC LADDER-III software. Check for infinite loops, excessive SUB/CALL nesting (max 8 levels), unoptimized logic blocks, or missing END instructions. Use ladder monitor function to identify slow-executing rungs during online monitoring.
Never modify ladder logic without backup and proper documentation. Incorrect ladder changes can cause machine malfunction.
4
If scan time is excessive, optimize ladder: Remove unused rungs, consolidate redundant logic, reduce MOVN/MOVOR operations, minimize use of multiple SUB calls per scan. Consider splitting complex programs across multiple first-level execution blocks. After optimization, monitor scan time on PMC STATUS screen to verify below 25ms.

DIY Feasiblemedium confidence

Call a technician if:

·PMC hardware replacement needed
·Complex ladder program restructuring required
·Multiple PMC alarms occurring simultaneously

Prevention

Regularly monitor PMC scan times during operation
Keep ladder programs well-documented and structured
Perform periodic PMC memory card backup
Avoid overly complex nested logic in critical scan paths

Common Mistakes

Ignoring gradual increase in scan times
Not backing up ladder program before modifications
Adding complex logic without considering scan time impact
Failing to test ladder changes under full I/O load conditions

Related Alarms

950 952 953 2010 2011

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