The computer serial port (COM port) is not connected to the device. How to use the computer prompt string? Reason:
1) Check the "Telephone/Modem Options" in the Control Panel to see if there is a "Modem" option to connect the occupied port, and delete it if there is one.
2) Check the device manager to see if the serial driver is normal. Please update the driver.
3) Check whether other applications have modified the serial port interrupt number. Please modify it.
4) Establish a serial port through USB to RS232, and modify the serial port number that the program needs to open.