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Access Diagnostics > Parameter > MD 30305 (servo parameter set selector) and MD 30310 (RESU configuration) for the affected channel. Verify the values match the physical servo drive hardware installed.

Do not modify these parameters while the axis is powered; use offline parameter editor if possible.

Check NCK > Servo > RESU Status in HMI. Note the specific error code %2 returned. Cross-reference with SINUMERIK 840D Servo documentation error code table (typically codes 1-50 indicate firmware issues, 51-100 indicate communication faults).

Power-cycle the servo drive and NCK. If alarm persists, reload the RESU firmware via SINUMERIK service mode using the appropriate .hex or .elf file for your servo model (e.g., SIMODRIVE 611U, SIMOPOS, or 1FT6 encoder variants).

Firmware upload requires service-level access; backup current parameter set before proceeding.

If firmware reload fails or alarm returns, replace the RESU hardware module. Document the serial number and firmware version of the defective unit for field replacement.

Inspect the encoder connector (typically X31/X32 on servo drive) for corrosion, loose pins, or bent contacts. Verify the shielded twisted pair cable is properly grounded at one end only, per SINUMERIK EMC guidelines. Reconnect and retorque M4 connectors to 2.5 Nm.

Ensure machine is in safe state with servo disabled (parameter MD 32000 = 0) before disconnecting encoder.

Access Diagnostics > Axes > Encoder tab for the affected channel. Monitor encoder counts and parity/CRC error rate during manual jog. If error rate exceeds 0.1% or counts freeze, encoder hardware is likely faulty.

Verify MD 30350 (encoder error threshold) is set appropriately for your encoder type (typically 3-5 for EnDat, 10+ for incremental). If threshold is too aggressive, increase by 2-3 counts and retest. If errors persist, proceed to encoder replacement.

Changing encoder tolerances may mask underlying cable or EMI issues; ensure machine environment is within SINUMERIK specifications.

If alarm clears but returns intermittently, the encoder is reaching end-of-life. Schedule replacement with a calibrated unit matching the original specification (e.g., 1FT6-0004-0 with 2048 ppr).

Access Siemens SINUMERIK Parameter Editor (offline or via HMI > Diagnostics > Parameter > Axis Data). Export the current parameter set for the affected channel to a .asc file. Compare MD 30100 (axis type), MD 30200 (load inertia ratio), and MD 30320 (encoder resolution) against the servo motor nameplate and drive hardware documentation.

Parameter export may require service-level PIN; contact your system administrator if restricted.

If discrepancies found, correct the parameters in the offline editor and validate the parameter set using the SINUMERIK parameter checker tool. Pay particular attention to MD 30105 (servo loop gain) and MD 30110 (velocity gain), which must be tuned to the motor inertia.