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Siemens SINUMERIK Controller
System · Intermediate · 30-120 minutes
Siemens 840D 4290 - Sign of life monitoring: local P-bus not alive
Alarm 4290 indicates loss of sign-of-life communication between the COM processor and NCK on the local P-bus during the IPO (Interpolator) cycle. The COM computer must update a heartbeat signal each SERVO cycle; failure to do so within one IPO cycle triggers this alarm, typically indicating a COM processor freeze, P-bus communication failure, or real-time task scheduling breakdown.
4290
Siemens 840D 4290 - Sign of life monitoring: local P-bus not alive
Sign of life monitoring: local P-bus not alive
Siemens SINUMERIK 840D system notice 4290: Sign of life monitoring: local P-bus not alive. This is a system-level notification regarding machine data or configuration. Note the message details and take the indicated action.
Root Cause Summary
Alarm 4290 indicates loss of sign-of-life communication between the COM processor and NCK on the local P-bus during the IPO (Interpolator) cycle. The COM computer must update a heartbeat signal each SERVO cycle; failure to do so within one IPO cycle triggers this alarm, typically indicating a COM processor freeze, P-bus communication failure, or real-time task scheduling breakdown.
Safety
- Refer to Siemens documentation
- Ensure qualified technician performs repairs
Causes and Fixes
- 1
Navigate to Diagnostics > System Information > Task Monitoring (or Service menu > Monitoring > Task Load) and verify SERVO_CYCLE actual execution time is less than MD10000 (typically 8-12 ms). Confirm PLC cycle time in MD31000 does not exceed 50% of SERVO_CYCLE.
If PLC cycle exceeds SERVO_CYCLE, P-bus updates are delayed and will trigger 4290.
- 2
Check OEM PLC program for infinite loops, missing waits, or synchronous function calls. Review recent changes to MD11000 (CLOCK_FEED) or timing-dependent logic. Reduce PLC cycle load by moving non-critical code to background tasks.
- 3
Verify COM firmware version matches NCK/IPO firmware (check HMI > Maintenance > Versions or Machine Data MD10001). Mismatched versions can cause handshake protocol failures.
- 4
If cycle load is normal, perform a soft reset (power cycle control without power-off) and monitor for immediate recurrence. If alarm returns within 10 minutes, suspect COM processor hardware failure or P-bus interface card malfunction.
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