The serial number can't be found, but the program can open com3 and send and receive by itself. But the notebook has no serial number and the device manager has no serial port.
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.