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Access Diagnostics > NCK > Channel Status in SINUMERIK HMI. Identify which channel has active spindle/axis motion by reviewing the 'Active Spindle' and 'Active Axis' fields for both channels.

Do not interrupt active motion; allow current block to complete or issue controlled stop via NC_STOP.

In the NC program attempting the coupling, verify the M function or COUPLE command syntax. Confirm the target spindle/axis number in the COUPLE statement matches MD32100 (spindle assignment table) and that the source channel has released the spindle.

Check machine logic (PLC) for competing spindle M-function commands. Ensure M3/M4/M5 and spindle synchronization blocks in Channel 1 complete before Channel 2 issues COUPLE commands. Review source code for race conditions in multi-channel scheduling.

If channels are permanently assigned to different spindles, verify MD32110 (spindle assignment per channel) is configured correctly to prevent cross-channel coupling attempts. Consult machine builder documentation for intended spindle architecture.

In SINUMERIK HMI, navigate to Diagnostics > PLC > Status and locate the spindle enable command register (typically spindle number mapped in MD20110). Check the current state—if the PLC enable bit is SET (1), PLC retains control.

Review PLC source code (OB1, spindle control function blocks) for missing RESET or DISABLE commands. Search for spindle M-function handlers that set spindle enable without corresponding reset logic when coupling is required. Confirm PLC handshake logic completes before NC re-requests the spindle.

Do not force-reset PLC spindle state if spindle is physically rotating; coordinate with machine operator.

Add explicit spindle release logic in PLC: after M-function execution, ensure the spindle enable bit is cleared and wait for spindle ready feedback (MD20111 bit status) before signaling to NC that coupling is permitted. Implement inter-channel semaphore or handshake via %M memory bytes if needed.

Verify MD20111 (spindle ready signal) is properly wired from drive/spindle controller back to PLC. If feedback is missing or stuck, PLC may never recognize spindle is available for release. Check hardware diagnostics in Drives > Spindle for encoder/feedback faults.

Verify spindle number in the NC COUPLE command (e.g., 'COUPLE(1)' for spindle 1). Cross-reference against MD32100 [spindle number index] to confirm the spindle is defined and enabled (bit 0 of corresponding MD32101 should be 1).

In HMI, open Diagnosis > Machine Data > Spindles and review MD32110 (spindle assignment per channel). Confirm that the spindle referenced in the COUPLE command is NOT exclusively assigned to the other channel via this parameter. If it is, reconfigure MD32110 to allow multi-channel access or use a dedicated spindle.