Siemens alarm 400250: NCK sign-of-life monitoring
SINUMERIK PLC system alarm · Siemens 840D/810D powerline
What it means
The cyclic sign-of-life handshake from the NCK stopped arriving at the PLC: the monitoring timer configured in FB1 (parameter NCCyclTimeout) ran down without being retriggered. The PLC therefore assumes the NCK is no longer processing the cyclic interface and raises this alarm while continuing internally. An NCK restart normally clears it; persistent recurrence points to timer misuse or a genuine NCK hang.
Likely causes
- NCK stopped cycling (hang/reset) while the PLC stayed in RUN
- User program uses S5 timers T0-T9, which the basic program reserves, destroying the watchdog retrigger
- FB1 monitoring time set too short for the actual NCK cycle load
Triage note: Official remedy: NCK restart; do not use timers T0-T9 in the user program. Program continuation: internal.
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