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Access PLC program via STEP 7 or HMI diagnostics menu. Navigate to the M-function handler subprogram (check OB271 for M06 tool change, or equivalent OB for the specified command number %2). Verify the subprogram contains logic to set the acknowledgement bit/signal.

Do not modify PLC code without proper version control and machine lockout (EMERGENCY STOP engaged).

In SINUMERIK Operate, open Diagnosis > PLC > Breakpoints or use online debugging to trace PLC execution. Confirm the tool change OB is being called when command %2 is issued. Monitor the acknowledgement signal output (typically a boolean or word variable in the NCK/PLC interface DB).

Check machine parameter MD20070 (MFUNC_PARAM: M-function parameter definition) and verify that command %2 is correctly mapped to the intended PLC subroutine. Confirm timeout parameter MD21400 (MFUNC_PARAM_TIMEOUT) is set appropriately (typically 5000-10000 ms for tool change).

Test tool change manually via MDI (Manual Data Input) with G-code M06 or equivalent. Monitor Diagnosis > Signals to observe NCK waiting state and PLC acknowledgement signal status in real-time. If acknowledgement signal never toggles, the PLC is not executing the handler.

In SINUMERIK Operate, navigate to Diagnosis > System Data (SD) > PLC/NCK Interface. Verify the variable definitions for tool change acknowledgement signals. Check SD parameters for command %2 acknowledgement pointer (typically SD21400-SD21410 range for M-function definitions). Ensure the acknowledgement variable name is spelled correctly and mapped to a valid DB address.

Open the PLC symbol table (in STEP 7 or via HMI) and cross-reference the acknowledgement signal variable against NCK parameter definitions. Confirm the variable is declared as a non-retained boolean or word type and is not protected by read-only attributes. Use SIMATIC STL or LAD to verify the signal is not being overwritten by competing logic.

Check the hardware coupling (if using distributed I/O or PROFIBUS modules): verify module addresses in MD26100 (TOOL_CHANGER_TYPE) and related parameters match physical hardware configuration. Monitor Diagnosis > Modules to confirm all I/O modules are online and responding.

Reload the system data and PLC program completely. In SINUMERIK Operate, select Utilities > Backup/Restore > Load System Data, then reload the current configuration. Perform a controlled power cycle (via Utilities > Shutdown) to reinitialize NCK/PLC handshake protocol. Re-test tool change command.

This action will reset the control; ensure no program is actively running on the machine.

In SINUMERIK Operate, navigate to Machine Data > Spindle/Tool > Tool Change (or Diagnosis > Machine Data MD21400). Review the acknowledgement timeout parameter for command %2. Default is typically 5000 ms; if set below 2000 ms, increase to 8000-10000 ms to allow longer tool change cycles. Save changes and perform a controlled restart.