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Navigate to Siemens HMI > Diagnosis > Alarms and note the specific command number (%2) and invalid acknowledge parameter (%3) values from the alarm. These numeric values identify which tool change phase failed.

Write down all four parameters from the alarm message as they directly correlate to PLC signal assignments.

Access the NCK parameters via HMI > Machine > Parameters > MD 36100 (Tool changer interface enable) and MD 36110 series (tool change signals). Verify that the PLC acknowledge bits match the tool changer hardware interface definition in your machine documentation.

Do not modify parameters while cycle is running; navigate to manual mode first.

In the PLC program, locate the OB that handles tool change commands (typically OB 240 or equivalent in your control version). Check the MOVE, PREP, and EXEC signal acknowledgements against the NCK interface specification. Verify data types match (BOOL vs INT vs DWORD).

Perform a 'NC Reset' (soft reset) via HMI > Machine > Control > Reset, then attempt a manual tool change cycle to verify the acknowledge handshake completes without alarm recurrence.

A soft reset clears pending NCK buffers; full system reboot may be required if problem persists.

Navigate to HMI > Machine > Parameters > MD 36120 to identify the tool changer type (e.g., ATC_1, ATC_GENERIC, MANUAL). Cross-reference this against your machine's equipment list to ensure it matches the actual hardware installed.

Incorrect tool changer type will cause all tool change commands to fail; verify hardware compatibility before modifying.

In the PLC project (STEP 7 or TIA Portal), expand the tool changer signal map and confirm all acknowledge signals (e.g., 'ToolChanger_Ready_Ack', 'PrepDone_Ack', 'ExecDone_Ack') are assigned to valid input addresses and are not crossed with output addresses.

Use the PLC Diagnostics > Inputs/Outputs view to monitor the acknowledge signal in real-time while issuing a tool change command. Verify the signal toggles and stabilizes at the correct logic level (typically 1 for ready/done).

If signal remains undefined or does not toggle, check PLC I/O module wiring and terminal connections on the control cabinet.

If using ASUP, re-run the automatic tool changer configuration wizard (HMI > Maintenance > Tool Changer Setup) to regenerate the signal definitions and reload the configuration to NVRAM.

Check PLC cycle time (OB 1 scan time) via HMI > Diagnosis > System Information > Performance or Siemens PLCsim trace. If PLC cycle time exceeds 50 ms or shows intermittent spikes, this can delay acknowledge signals beyond the NCK timeout threshold.

High PLC cycle time may indicate CPU overload, excessive logic, or communication bottlenecks; consult your PLC documentation for optimization strategies.