From my maintenance experience, I can't identify it. First, the computer can't recognize USB devices. \x0d\ Secondly, USB devices cannot be recognized by computers. First of all, it is necessary to judge whether it is a computer problem or a USB device problem. We can use other USB devices to test whether there is something wrong with the computer. \x0d\ If it's a computer problem. Has your computer been damaged by a virus? Is the USB2.0 driver installed or is the version incompatible? Is there any software conflict (solve the software problem first)? Generally, you will be prompted to install the driver. \x0d\ If it is a computer hardware problem. First, we use a multimeter to measure four USB wires (red +5V white data-data-green data-data-black ground wire). The multimeter uses the red probe and the black probe (white line and green line) to measure the grounding. The resistance difference of is less than 50 Ω, and both lines are from South Bridge. If the resistance changes too much, it can be judged that it is a south bridge problem. It shows that the USB module of the South Bridge is burnt out. \x0d\ Let's look at the external USB device. Many people need drivers now. Even if there is no driver, there will be no unrecognizable situation. There are several unrecognizable situations. If a USB data cable is blocked or has poor contact, it will generally touch that place, because it may rust and swell after a long time, and something may block it. Second, USB cable doesn't work. There will be no response when the red line is blocked, and the white line is blocked and unrecognizable. The green line is impassable and unrecognizable. The white line and the green line cannot be reversed. It is also unrecognizable. Black line blocked, no response. This measurement method has been mentioned above. \x0d\ attention. The low voltage of 5V on the red line will also lead to illegibility. \x0d\ Three USB boards have internal problems. 1 The crystal vibrates badly, so it cannot vibrate. 2 Bad crystal oscillator capacitor (20P) There are two \x0d\3 reset capacitors with poor resistance. The reset voltage is on the high side of the bottom. 4. Bad three-terminal IC provides insufficient voltage for IC. 5.\x0d\ The main IC is broken \x0d\ Good luck!