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Check the HMI or operator panel for any active file transfer dialogs or communication status indicators showing ongoing RS232C activity.

Wait for the current RS232C transfer to complete normally, monitoring the progress indicator until it reaches 100% and closes.

If the transfer appears frozen, cancel the operation using the appropriate abort function in the communication menu, then acknowledge the alarm.

Canceling an active transfer may result in incomplete or corrupted file transfer. Note the file name for re-transfer.

After cancellation, perform an NC START reset and verify the RS232C interface status shows idle before attempting a new transfer operation.

Acknowledge the alarm using the NC START key or alarm reset button on the operator panel.

Navigate to the communication or service menu and check the RS232C interface status to ensure it has returned to idle state.

Review the alarm log buffer to identify what action preceded this alarm (emergency stop, mode selector change, reset button).

Restart the interrupted RS232C operation from the beginning, ensuring no mode changes or resets occur during the transfer process.

Instruct operators to avoid pressing NC RESET or changing modes during file transfers to prevent data corruption.

Power cycle the NCU (control unit) to reset the RS232C interface hardware and clear any stuck communication buffers.

Coordinate machine shutdown with production schedule. Ensure all axes are in safe position before powering down.

Verify RS232C cable connections at both the NCU and external device ends, checking for bent pins, loose connections, or damaged shielding.

Check RS232C communication parameters (baud rate, parity, stop bits, handshaking) in Setting Data under the serial interface configuration match the external device settings exactly.

Test with a known-good RS232C cable and verified external device. If alarm persists, check machine data MD11210 (serial interface configuration) and contact Siemens service for potential NCU communication board diagnostics.