1) Check the "Telephone/Modem Options" in the Control Panel to see if there is a "Modem" option to connect the occupied serial 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.

5) These configurations can also be cleared by deleting a numerical item in the registry: enter "regedit" in the run dialog box to enter the registry; Then enter HKEY _ local _ MachinerSystemCurrentControlSetControlCom name arbiter. At this time, we can find the numerical item: ComDB, whose value represents the serial number currently used. For example, if 1d is binary: 000 1101,then the number from right to left means1,3, 4, 5 ports are occupied. In this way, we only need to delete the data item ComDB and close the registry (without restarting the computer). If it's all cleared, be careful of conflicts.