Code 201688: <location>SI Motion CU: Actual value synchronization not permissible
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical <location>SI Motion CU: Actual value synchronization not permissible 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: Drive object: SERVO, VECTOR Reaction: OFF2 Acknowledge: IMMEDIATELY (POWER ON) SINAMICS-Alarms Cause: It is not permissible to simultaneously enable the ac tual value synchronization and a monitoring function with absolute re ference (SCA/SLP). Note: This fault results in a STOP A that cannot be acknowledged. Remedy: Either de-select the function "actual value synchr onization" or the monitori ng functions with absolute reference (SCA/SLP) and carry out a POWER ON. Note: SCA: Safe Cam / SN: Safe software cam SI: Safety Integrated SLP: Safely-Limited Position / SE: Safe software limit switches See also: p9501 (SI Motion enable safety functions (Control Unit))
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 201688 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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