1. Power-On Self-Diagnosis Power-on self-diagnosis refers to the automatic diagnostics performed by the internal diagnostic program of a CNC system when it is powered on, similar to the power-on diagnostics of a computer. Power-on self-diagnosis can automatically check critical hardware components such as the CPU, memory, I/O units, CRT/MDI units, paper tape readers, and floppy drives; determine the installation, connection status, and performance of specified devices; some systems can also diagnose certain important chips, such as PAM, ROM, and dedicated LSI. The self-diagnosis of the CNC system occurs during power-on, and it can only enter normal operation after all items have been confirmed to be error-free. The diagnostic time varies; generally, it only takes a few seconds for a CNC system, but some may require several minutes. The power-on self-diagnosis generally follows a prescribed procedure. Taking the FANUC II system as an example, during the execution of the diagnostic program, the seven-segment display on the system motherboard changes in the order of 9→8→7→6→5→4→3→2→1. The corresponding checks are as follows: 9—Reset the CPU and start executing diagnostic instructions; 8—Perform ROM test; if the ROM check fails, the display changes to 'b'; 7—Clear the RAM; the system clears the contents of the RAM to prepare for normal operation; 6—Initialize the BAC (Bus Random Control) chip. At this time, if the display changes to 'A', it indicates an error in the transmission between the motherboard and the CRT; 'C' indicates a connection error; 'F' indicates a faulty I/O board or connecting cable; 'H' indicates an incorrect identification number of the connection unit; a lowercase 'c' indicates an error in fiber optic transmission; 'J' indicates a faulty PLC or interface conversion circuit, etc. 5—Check the MDI unit; 4—Initialize the CRT unit; 3—Display the initial screen of the CRT, such as the software version number, serial number, etc. If the display changes to L, it indicates a problem with the PLC control software; if it changes to O, it indicates that the system failed initialization and the control software has a problem; 2 indicates that the system initialization work has been completed; 1 indicates that the system can operate normally. If the display changes to E, it indicates that the system's motherboard or ROM board, or the CNC control software, has a fault. Under normal circumstances, after the CRT initialization is completed, if there is a fault in other parts, the CRT can display alarm information. 2. Online Monitoring Online monitoring can be divided into two forms: CNC internal program monitoring and monitoring through external devices. CNC internal program monitoring is a method of automatically diagnosing, checking, and monitoring the status of each part through the system's internal program. The scope of online monitoring includes the CNC itself and the servo units, servo motors, spindle servo units, spindle motors, external devices, etc. connected to the CNC. Online monitoring is always effective during system operation. CNC system internal program monitoring includes three aspects: interface signal display, internal status display, and fault display. (1) Interface signal display: It can display the current status of all interface signals between the CNC and PLC, and between the CNC and the machine tool. This refers to the on/off status of digital input/output signals, aiding in fault analysis. During maintenance, it is essential to understand the meaning of each signal between the CNC and PLC, and between the CNC and the machine tool, as well as the various conditions required for signal generation and cancellation, in order to perform the appropriate checks. The "Functional Manual" and "Connection Manual" provided by the CNC system manufacturer, and the "Machine Tool Electrical Schematic Diagram" provided by the machine tool manufacturer, serve as technical guidelines for performing the above status checks. ⑵ Internal Status Display Generally, the internal status display function can display the following: 1) External causes that prevent the execution of cyclic instructions (machining programs). For example: whether the CNC system is in "position check"; whether it is in "machine locked" state; whether it is in "waiting for speed arrival" signal activation; whether it waits for the "position encoder" measurement signal during spindle feed programming; whether it is waiting for the "spindle I-turn signal" during thread cutting; whether the feed rate multiplier is set to 0%, etc. 2) Reset status display, indicating whether the system is in "emergency stop" state or "external reset" signal activation state. 3) TH alarm status display. It can display the location of the paper tape error hole when an alarm occurs. 4) Display of memory contents and abnormal status of the bubble memory. 5) Display of position following error. 6) Display of control information for servo drive components 7) Display of input pulses from position measuring elements such as encoders and gratings, etc. ⑶ Fault information is displayed in the CNC system. Fault information is generally displayed on the CRT in the form of "alarm display". The content of the alarm display varies depending on the CNC system. This information mostly appears in the form of "alarm number" plus text. Specific content and troubleshooting methods can be found in the "maintenance manual" provided by the CNC system manufacturer. Monitoring through external equipment refers to a method of automatically diagnosing, checking, and monitoring the status of various parts of the CNC machine tool using equipment such as computers and PLC programmers. For example, the PLC program can be dynamically tested in the form of ladder diagrams and function charts using computers and PLC programmers. It can display dynamic waveforms of the PLC program when the machine tool manufacturer has not provided the PLC program. It usually requires the use of necessary online monitoring equipment. With the development of computer network technology, remote diagnostic technology via network connection, as a form of online monitoring of external equipment, is becoming increasingly widespread and sophisticated. Through the network, CNC system manufacturers can directly test and monitor the on-site operation of their products, promptly resolve system problems, and provide guidance and assistance to on-site maintenance personnel. 3. Offline Testing Offline testing, also known as "offline diagnostics," involves disconnecting the CNC system from the machine tool and then testing and inspecting the system itself. Offline testing allows for further localization of system faults, aiming to minimize the scope of the problem. For example, offline testing of printed circuit boards (PCBs) can pinpoint the fault to a specific part of the PCB, or even a particular chip or device, which is essential for PCB repair. Offline testing of CNC systems requires specialized diagnostic software or testing equipment; therefore, it can only be performed at the CNC system manufacturer or a specialized maintenance department. With the development of computer technology, modern CNC offline diagnostic software is gradually being integrated with CNC control software, and some systems have even incorporated "expert systems" into fault diagnosis. Through such software, operators can diagnose CNC system or machine tool faults simply by performing some basic dialogue operations on the CRT/MDI.