This is a feature of Windows, it tries to keep USB ports unique, so when you plug in a particular device, it holds the same COM port number it was first assigned. There is a way around this. Our board uses an FT232R chip, and the chip manufacturer has an application note to tell Windows to use the same COM port. See Application Note, Section 7.1, Ignore Hardware Serial Number.

In a nutshell, you have to edit the registry and add a key with a value of “01”:HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\UsbFlags\IgnoreHWSerNum04036001

Please heed the application note warning about editing the Windows Registry.