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Navigate to Diagnostics > Alarms in the SINUMERIK operator panel and note the complete fault code (%2) and timestamp (%4). Document the axis name (%1) and fault value (%3) for reference.

Record the exact error code before power cycling, as it may be cleared.

Inspect the PROFIBUS DP cable from the 840D control to the SINAMICS drive for physical damage, kinks, or loose connectors. Verify both connectors are fully seated and the 120Ω termination resistor is installed on the last slave device in the DP network.

Power down the system before disconnecting PROFIBUS cables to prevent transient faults.

Check MD27000 (PROFIBUS_DP_MODE) and verify the DP cycle time matches the SINAMICS drive configuration. Confirm MD27001 through MD27010 parameters align with the drive's DP address and expected cycle timing. Use the HMI menu: Diagnosis > PROFIBUS DP Status to view slave alive status.

If the drive appears offline in DP Status, perform a warm restart of the 840D NCK (Control > Restart) and observe whether the drive reconnects. If it does not, contact the SINAMICS drive manufacturer with the fault code to identify drive-side DP stack corruption.

A warm restart does not clear persistent hardware faults; if the alarm returns immediately, the issue is likely hardware-based.

Decode the SINAMICS fault code (%2) using the SINAMICS S120 Modular Drive documentation (typically a 5-digit code). Cross-reference with SINAMICS Alarms.pdf to determine if the fault is encoder-related (A3x.x), current regulation (A4x.x), or communication-related (A6x.x).

The fault code format depends on the SINAMICS firmware version; consult the drive's commissioning manual for your specific hardware revision.

Access the SINAMICS drive via STARTER software or the integrated SINUMERIK Service interface (Diagnostics > Extended > SINAMICS Drives). Verify that the encoder connection is secure, the motor phase sequence matches the drive's motor data (MD1024, MD1025, MD1026 for axis motor parameters), and current limits in MD1019 are appropriate for the motor.

Confirm the axis dynamic parameters (MD1043 Feed Rate Limit, MD1044 Acceleration, MD1047 Jerk) do not exceed the SINAMICS drive's rated performance. Check for a mismatch between the mechanical load inertia and the drive's tuning parameters by reviewing MD1021 (Drive Identification) and the commissioning report.

If the fault persists after parameter verification, perform a diagnostic download from the drive (STARTER > Diagnostics > Error Memory) to capture the drive's internal event log. If no encoder communication is detected, verify the encoder cable continuity with a multimeter and replace if damaged.

Do not attempt to override fault codes; always address the root cause to prevent damage to the servo loop.

Verify the axis assignment by navigating to Diagnostics > Machine Data > Axis %1. Confirm that MD1009 (AXIS_DRIVE_ADDRESS) matches the PROFIBUS DP slave address assigned to the SINAMICS drive (typically 1–15). Cross-check against the hardware configuration documented during installation.