When opening a serial port, sometimes messages such as "Unable to open serial port", "Serial port occupied" and "Serial port resource does not exist" appear.

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In fact, these messages all imply that the serial port you want to use has been occupied by other processes. You can check in the device manager of the computer, COM3 is occupied;

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When you know what this process is, you can close it in the task manager;

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When you don't know what process this serial port is occupied by, you need to handle it from the registry;

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Enter regedit in the Start menu Run, and then click OK to open the Registry Editor.

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Follow the path in the figure below to find the COM name arbiter. Then click "right-click" on "ComDb" on the right and select "Delete". The project to delete the registry is to let win7 rebuild the COM port list so that those ports are not occupied.

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Open the device manager again to check that COM3 is no longer occupied;

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If you use the serial assistant or other software to open COM3, you will no longer be prompted to open COM3 and other information.