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Navigate to Diagnostics → Axis → Encoder in the 840D operator panel. Document the exact encoder type from MD20070 (ABSOLUTE_ENCODER_TYPE) and verify the configured baud rate matches hardware (typically DRIVE-CLiQ 3 Mbps for safe encoders).

Perform a visual inspection of the encoder cable from axis motor to control cabinet. Check for kinks, crushing, or exposure near high-frequency noise sources (servo drives, spindle cables). Inspect DRIVE-CLiQ connectors for corrosion, bent pins, or loose seating.

Power down the machine and verify servo drives are de-energized before handling connectors.

Swap the encoder cable with a known-good cable of identical specification. If alarm clears, the original cable has intermittent faults. If alarm persists, proceed to encoder module inspection.

If available, use SIEMENS cable tester or multimeter to verify continuity on all DRIVE-CLiQ pins (pin 1-8 per IEC61076-2-109). Document resistance readings; open circuits or resistance >5Ω indicate cable failure.

Access the Diagnostics menu and navigate to Actual Values → Encoder Diagnostics. Record the raw encoder counts, signal strength indicators, and any safety error flags. Compare against the encoder datasheet baseline (e.g., SICK DFS60, Heidenhain ERN1380, etc.) specifications for your axis.

Power down the machine completely. Physically remove the encoder from the axis shaft and visually inspect the optical window for contamination (oil film, dust accumulation, condensation). Clean gently with lens paper and isopropyl alcohol. Reinstall and test.

Lock out the axis via emergency stop and verify no motion is possible before physically removing the encoder.

If contamination is present or cleaning does not resolve the alarm, the encoder optical path is compromised. Consult the encoder OEM documentation (MD20070 references the encoder model). Request RMA and replace with an identical encoder assembly calibrated to the same gear ratio.

After encoder replacement, perform a full axis commissioning cycle: Home axis, run circular test pattern at 50% speed, and verify Diagnostics → Actual Values shows stable encoder counts with no CRC errors over 5 minutes of operation.

Access the Parameter Editor (Siemens HMI → Service → Parameters) and navigate to MD20070 and MD20075 for the affected axis. Record the configured encoder type code (numeric identifier). Cross-reference against the physical encoder nameplate and your machine documentation to confirm match.

If encoder type is incorrect, verify the physical encoder specifications: multiturn vs. singleturn, DRIVE-CLiQ vs. analog signal, and safety certification level (Cat 3/4, SIL2/3). Update MD20070 to match the installed hardware. Do NOT guess the code; consult your project documentation or contact SIEMENS support with the encoder model number.

Incorrect encoder type parameters can cause loss of position feedback. Verify changes offline before moving the machine.