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Siemens SINUMERIK Controller
Safety · Intermediate · 30-120 minutes
Siemens 840D 27256 - PROFIsafe: Current cycle time [ms]> parameterized cycle time
Alarm 27256 indicates that the PLC-side PROFIsafe communication cycle time is consistently exceeding the parameterized safety communication interval defined by MD $MN_PROFISAFE_IPO_TIME_RATIO. This occurs when the NCK cannot maintain synchronization with the safety protocol's real-time requirements, typically due to PLC task overload, incorrect cycle time configuration, or communication bottlenecks.
27256
Siemens 840D 27256 - PROFIsafe: Current cycle time [ms]> parameterized cycle time
PROFIsafe: Current cycle time [ms]> parameterized cycle time
Siemens SINUMERIK alarm 27256: PROFIsafe: Current cycle time [ms]> parameterized cycle time. This alarm indicates a fault or error condition on the SINUMERIK 840D control. Refer to the Siemens SINUMERIK alarm documentation for the complete description and remedy.
Root Cause Summary
Alarm 27256 indicates that the PLC-side PROFIsafe communication cycle time is consistently exceeding the parameterized safety communication interval defined by MD $MN_PROFISAFE_IPO_TIME_RATIO. This occurs when the NCK cannot maintain synchronization with the safety protocol's real-time requirements, typically due to PLC task overload, incorrect cycle time configuration, or communication bottlenecks.
Safety
- Refer to Siemens documentation
- Ensure qualified technician performs repairs
Causes and Fixes
- 1
Access Siemens SINUMERIK HMI > Diagnosis > PLC Cycles. Monitor actual PLC cycle time (OB1 scan time) and compare against MD $MN_PROFISAFE_IPO_TIME_RATIO value. Document peak cycle times over 30-second window.
Cycle time values are typically in milliseconds; verify units in your NCK parameter display.
- 2
Review PLC source code (AWL/STL or SCL) for blocking function calls, excessive array operations, or synchronous communication blocks (S7COMM) within OB1. Use Siemens STEP 7 / TIA Portal network load analysis to identify bottlenecks.
- 3
Reduce PLC load by moving non-safety-critical logic to lower-priority OBs (OB20-OB35 for event-driven tasks). Optimize safety FB calls; ensure F-blocks are only invoked when condition changes occur, not every cycle.
- 4
If PLC cycles still exceed window, increase MD $MN_PROFISAFE_IPO_TIME_RATIO value (default typically 10). Consult machine documentation for safety cycle time margins before adjustment.
Increasing ratio delays safety response; only adjust within OEM-approved limits to maintain SIL compliance.
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