What causes Siemens SINUMERIK alarm 120008?

Normal control operation generated this status message. User action or program command triggered this notification. System configuration or commissioning activity produced this message. Machine data or parameter change created this notification.

How do I fix Siemens SINUMERIK alarm 120008?

1. Read the complete message text on the SINUMERIK screen — note any parameter values shown. 2. If action is required, follow the instructions in the message. 3. Acknowledge the message on the operator panel to continue operation. 4. Consult the Siemens SINUMERIK alarm documentation for message 120008 if clarification is needed. 5. If this message appears unexpectedly, review recent parameter or configuration changes.

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Siemens SINUMERIK Controller

PLC · Intermediate · 30-120 minutes

Alarm 120008

Siemens 840D 120008 - Control unit switchover, PLC timeout

This alarm occurs due to a timeout during control unit switchover between MMC and NCU, often caused by PLC communication issues or program errors. Resolve by addressing PLC logic or program syntax errors.

Siemens SINUMERIKPLCIntermediate30-120 minutes

120008

Siemens 840D 120008 - Control unit switchover, PLC timeout

Control unit switchover, PLC timeout

Root Cause Summary

This alarm occurs due to a timeout during control unit switchover between MMC and NCU, often caused by PLC communication issues or program errors. Resolve by addressing PLC logic or program syntax errors.

Safety

Refer to Siemens documentation
Ensure qualified technician performs repairs

Causes and Fixes

1
SAFETY: Power down and lockout
Follow LOTO procedures before accessing PLC or control systems. Ensure machine is in safe state.
2
Check PLC logic
Review the PLC program for errors or delays in handling MMC requests.
3
Verify NCU status
Ensure the NCU is responsive and not in a fault state.
4
Reset and retry
Clear the alarm and attempt the switchover again after corrections.

Service Recommendedlow confidence

Call a technician if:

·Always - control unit switchover issues require qualified technician
·If PLC logic corrections do not resolve the timeout
·If the alarm persists despite program fixes

Prevention

Validate PLC logic before switchover operations
Use program simulation to catch syntax errors early

Common Mistakes

Overlooking PLC logic delays
Ignoring specific alarm subcode (001, 002, 003) details

Seeing Multiple Alarms?

Seeing another alarm with this one? Enter the code.

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Always verify with your machine manual or a certified technician before performing service procedures.