The "Interface Signal Method" is a method of analyzing and diagnosing faults using input/output (I/O) interface signals from various control sections of the machine. The application of the "Interface Signal Method" in the maintenance of the SINUMERIK850 CNC system is described below. First, the fault phenomenon (1) during the normal operation of the table (X-axis), the oil pump suddenly shuts down, causing the running of the table to be interrupted. (2) The alarm is displayed on the screen: 1040DAC LIMIT REACHED (Table 1).