เกี่ยวกับ CPU communication of document feeder
E420 Backup data cannot be read, or the data is faulty.
E421 Faults occur when writing backup data, or the written data is
faulty.
E422 A transmission error occurs due to faults in IPC of DF/ ADF/
RDF. This error code is detected by DF and it is not notified to a copier, so
the error code is not indicated.