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Navigate to SYSTEM > PARAM and check parameters 6700-6709 (external alarm message number assignment). Verify which message numbers are defined and their corresponding addresses in parameters 6710-6719.

Access the PMC ladder program using FANUC LADDER-III or the built-in PMC menu (PMC > LADDER). Search for EXLM (External Alarm) function calls and verify the message number being called matches a defined parameter 6700-6709 entry.

Check PMC signal D9000-D9019 (external alarm message request addresses). Use PMC > PMC DATA > DATA TABLE to monitor which message number is being requested when alarm triggers.

Correct the PMC ladder EXLM function call to reference a valid message number defined in parameters 6700-6709, or add the missing message definition to the parameter table and power cycle the control.

Parameter modification requires FANUC parameter write enable (setting 8 bit 0 to 1) and may require a control restart.

Navigate to SETTING > MESSAGE and review the external alarm message definitions stored in the CNC. Check for any non-ASCII characters, special symbols, or format codes that may be improperly formatted.

If messages are defined via parameter input, check the message text in parameters 6700-series. Ensure only valid ASCII characters (0x20-0x7E) are used and format specifiers like %d, %s, %f match the data type being passed from PMC.

Verify message length does not exceed 80 characters total. Use PMC > PMC DATA > DATA TABLE to inspect the actual data being written to the message data registers (typically D registers) when alarm occurs.

Correct the message definition by removing illegal characters, fixing format specifiers, or shortening the message length. Re-register the corrected message and test the external alarm function.

Navigate to PMC > PMC DATA > DATA TABLE and manually inspect D registers associated with the external alarm message (reference parameters 6710-6719 for start addresses). Look for unexpected values or binary data instead of ASCII text.

Check battery voltage via MAINTENANCE > BATTERY. Low battery (below 3.0V) can cause PMC memory corruption. Replace battery if voltage is marginal and reinitialize PMC data from backup.

Battery replacement should be performed with control powered ON to prevent total memory loss. Follow proper hot-swap battery replacement procedure.

Review PMC ladder logic that writes data to the external alarm message registers. Verify data movement instructions (MOV, MOVB, XMOV) are writing correct ASCII data and not overwriting message boundaries.

Perform PMC memory all clear (PMC > PMC KEEP RELAY > ALL CLEAR) and reload a known good PMC ladder program backup. Reinitialize external alarm message parameters from a verified backup file.

PMC ALL CLEAR will erase all PMC memory including K relays and D registers. Ensure machine is in a safe state and all critical data is backed up before proceeding.