Code 201651: <location>SI CU: Synchronization safety time slices unsuccessful
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical <location>SI CU: Synchronization safety time slices unsuccessful alarm. Immediate attention from a qualified operator or maintenance technician is required to prevent secondary damage to the equipment or spoiled parts during the manufacturing cycle. Always ensure standard safety and LOTO procedures are followed before accessing electrical cabinets.
Official Troubleshooting Steps
Message value: %1 Drive object: A_INF, B_INF, CU_LINK, S_INF, SERVO, TM41, TM54F_MA, TM54F_SL, VECTOR Reaction: OFF2 Acknowledge: IMMEDIATELY (POWER ON) SINAMICS-Alarms Diagnostics Manual (DAsl), 03/2009 Cause: The "Safety Integrated" function requires a synchroni zation of the safety time slices between the Control Unit (CU) and Motor Module (MM) and between the Control Unit and the higher-level control. This synchronization routine was unsuccessful. Note: This fault results in a STOP A that cannot be acknowledged. Fault value (r0949, interpret decimal): 150: Fault in the synchronization to the PROFIBUS master. All other values: Only for in ternal Siemens troubleshooting. See also: p9510 (SI Motion clock-cycle synchronous PROFIBUS master) Remedy: Re fault value = 150: - check the setting of p9510 (SI Motion clock-cycle synchronous PROFIBUS master) and if required, correct. General: - carry out a POWER ON (power off/on) for all components. - upgrade the Motor Module software. - upgrade the Control Unit software. - upgrade the software of the higher-level control. Note: CU: Control Unit MM: Motor Module SI: Safety Integrated
Source: SINUMERIK 840D sl / 840Di sl / SINAMICS S120 Diagnostics Manual (DAsl, 03/2009)
Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.
About Siemens SINUMERIK 840D CNC Alarms
Siemens controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 201651 on the SINUMERIK 840D CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Siemens's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.
Complete Alarm List: 4014 Siemens SINUMERIK 840D CNC codes.
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