This so-called serial port 3 is actually a modem built into the notebook. When you open the "Modem" in Device Manager, you can see that the port it uses is COM3. The line on the switch is not a serial port, but a line connecting the console port of the switch and the network port. It cannot be used as a serial port. If the notebook itself does not have a serial port, you need to go to the computer city to buy a serial port to USB converter. Remember to drive, about 30~50 yuan.