Siemens alarm 400251: NCK has not started up
SINUMERIK PLC system alarm · Siemens 840D/810D powerline
What it means
During control power-up the PLC came into RUN but the NCK never established its cyclic connection, or the run-up acknowledgement exceeded the timeout entered in FB1/OB1. The PLC side is healthy - it is reporting that the NC partner did not ramp up within the expected window. Check FB1 timeout parameters and perform a general NCK reset if the condition persists.
Likely causes
- NC boot aborted or delayed (CF card/system software problem)
- FB1 parameter NCRunupTimeout too small for the machine's run-up duration
- Reserved timers T0-T9 used by the user program, breaking the handshake
- Genuine NCK hardware fault preventing run-up
Triage note: Powerline text also cites acknowledgement limits MCP1Cycl/MCP2Cycl in OB1/FB1; sl remedy focuses on increasing FB1 NCRunupTimeout.
Need the part urgently?
Send us alarm 400251 with your machine details — typical RFQ turnaround is 24–48h.