20122

Navigate to Diagnostics > Machine Data > MD20050 and MD20051 to verify all configured axis and spindle names on your SINUMERIK 840D control.

Document the exact spelling and case sensitivity of all axes (e.g., 'X', 'Y', 'Z', 'A', 'B', 'C').

Access the compensation table configuration (typically in Part Program > Compensation or via SINUMERIK HMI Compensation Editor) and verify that $AN_CEC_INPUT_AXIS and $AN_CEC_OUTPUT_AXIS reference only axes that exist in MD20050/MD20051.

Case sensitivity and spacing are critical; 'X ' (with space) is not equal to 'X'.

Confirm the compensation table is assigned to the correct NCK channel via MD28000 (Channel-specific compensation assignment) and that the channel definition in MD20000 matches the intended control structure.

Power-cycle the control or reload the machine data and compensation tables to re-initialize the NCK. Monitor the alarm log to confirm 20122 is no longer triggered.

Review the compensation table definition and confirm that $AN_CEC_INPUT_AXIS ≠ $AN_CEC_OUTPUT_AXIS. Input and output axes must be distinct for interpolatory compensation.

Check machine data MD20070 (kinematic structure) to ensure all axes referenced in the compensation table are declared in the correct hierarchical order and are active (not marked as 'not active').

If an axis is referenced in compensation before being declared in the kinematic structure, the NCK cannot resolve it and fires alarm 20122.

Verify that no compensation chain creates a loop (e.g., axis X compensates for Y, and Y compensates for X). Use the NCK/PLC diagnostics menu to trace compensation dependencies.

If changes were made to MD20070 or the compensation table, save the machine data, perform a 'Load Machine Data' (not just 'Edit'), and restart the NCK to force re-validation.

Access Diagnostics > System Variables and inspect the values of $AN_CEC_INPUT_AXIS and $AN_CEC_OUTPUT_AXIS directly. Confirm they contain valid, non-empty axis designations and are not corrupted (e.g., null, undefined, or garbage characters).

Use the 'Hex View' option if available to detect non-printable or corrupted characters in variable data.

Re-download or re-create the compensation table from a known-good backup or the original CAM/engineering source. Use the SINUMERIK transfer protocol (DNC, ISO upload, or native HMI file manager) to ensure data integrity.

Verify CRC or checksum of the compensation file before and after transfer if your HMI provides this feature.

If the alarm persists, perform a machine data 'Reset to Default' for compensation-related parameters (MD28xxx series) and reconfigure from scratch, then verify the table structure in the Part Program editor.

Back up your current configuration before reset; this action is irreversible.

Monitor the NCK boot sequence (via System > Diagnostics > NCK Startup Log) for any early warnings or initialization errors related to compensation table parsing.