Siemens 840D machine tool displays 120201 alarm processing process

Siemens 840D machine data is lost. Occasionally the NCU fails to complete the self-check when the NCK is cleared. Sometimes the 120202 alarm "Waiting for NC / plc connection" appears.
Check that the purple communication cable is normal, and there is no problem in connection and shielding. After judging that the NCU is faulty, it is normal after maintenance.
  120201 communication failure alarm often occurred during processing, but it returned to normal instantly.
The coordinate value of the axis on the panel becomes "#", the feed axis may appear at any position, the MCP operation panel displays normally, the NCU main board displays "6", and there is no problem in checking the MPI cable.
  Later, the connector of the purple communication line was replaced. After several days of operation and observation, the alarm no. 120201 did not occur.