Alarm 2120 remains active on the SINUMERIK 840D-controlled Emcoturn465 after replacement of the fan-and-battery module and two fans at the bottom of the NCU enclosure. This result does not prove that the NCU is defective; it establishes only that the replaced fan hardware did not clear the alarm. Protect the NC and PLC data before replacing the NCU or performing further hardware work.
Define the Alarm 2120 Fault Boundary
| Item | Evidence-based status | Engineering decision |
|---|---|---|
| Displayed alarm | 2120 NCK fan | Record whether it is continuously active and whether it returns after a power cycle. |
| Parts already replaced | Fan-and-battery module and two lower NCU-box fans | Do not continue replacing fans without identifying the alarm input or monitoring path. |
| Suspected component | NCU box | Treat NCU failure as a hypothesis, not a confirmed diagnosis. |
| Exact 840D hardware | Unknown | Identify the installed NCU and control version before ordering a replacement. |
| Operator storage | The display has no HDD | Plan to export archives through the available RS232 interface if no other destination is available. |
Create Separate NC and PLC Archives
The supported backup scope in the evidence is an NC archive plus a PLC archive. Do not assume that an arbitrary collection of files received over RS232 is equivalent to these two archives.
- Obtain the available service-level access. Manufacturer-level access was reported as unavailable, and the evidence does not establish whether the password was changed or whether that access level is required.
- Open Services, use the right-arrow navigation control to locate Series startup, and select the backup functions for NC and PLC.
- Save the NC and PLC backups as two separate files through the available transfer path, including RS232 when that is the only usable interface.
- Verify that both transferred files exist and can be distinguished as the NC archive and PLC archive before removing power.
Handle Access and Version Ambiguity
The available account reaches service level but not manufacturer level, and the operator reports that the older control may not present the expected Series startup option. Do not guess a password or assume that every SINUMERIK 840D software generation has the same menu path. First identify the installed control and NCU type, then document the menus and access levels actually available.
If Series startup is absent, stop before replacing the NCU unless the received files can be positively identified as complete NC and PLC archives. The evidence supplies no file names, extensions, archive validation command, or version-specific RS232 procedure, so those details must be confirmed for the installed control rather than inferred.
Replace the NCU Only After Verification
Before powering down, confirm that two separate archives have been transferred and that the replacement hardware matches the installed NCU. The evidence contains conflicting, non-authoritative statements about how long the control retains data without power; therefore, do not use an assumed retention interval as the backup strategy.
After hardware replacement and power-up, check whether alarm 2120 clears and verify that the NC configuration and PLC-controlled machine functions remain available. If the alarm persists, the NCU hypothesis is not confirmed and the fan-monitoring signal path requires further diagnosis using documentation for the identified hardware version.
FAQ
What should I back up before replacing a SINUMERIK 840D NCU?
Create two separate backups: an NC archive and a PLC archive. Verify both files before removing power from the control.
Where is the SINUMERIK 840D NC and PLC backup menu?
From Services, use the right-arrow navigation control to find Series startup, then select the NC and PLC backup functions. If the option is absent, identify the control version and available access level before proceeding.
Does alarm 2120 prove that the SINUMERIK 840D NCU is bad?
No. In this case, alarm 2120 remained after replacement of the fan-and-battery module and two lower fans, but that result identifies the NCU only as a hypothesis; it does not confirm an NCU failure.