THE SELECTED SERIAL INTERFACE IS EITHER NOT INSTALED

It only remembers USB connected devices via unique serial numbers, without those attached to different USB ports you'll see different COM#

Further the support for adding/removing serial devices, and applications response or failure to handle that (closing out now stale file handles, and structures), is due to Microsoft's piss poor implementation and inconsistent strategy, and the belief the serial port is two decades past obsolescence. And a absence of well considered examples, going beyond CreateFile()

It only remembers USB connected devices via unique serial numbers, without those attached to different USB ports you'll see different COM#

Further the support for adding/removing serial devices, and applications response or failure to handle that (closing out now stale file handles, and structures), is due to Microsoft's piss poor implementation and inconsistent strategy, and the belief the serial port is two decades past obsolescence. And a absence of well considered examples, going beyond CreateFile()

For USB-to-serial adapters without a serial number, Windows will instead remember USB port + ID of the USB controller chip.

So the same adapter connected to four different USB ports is suddenly remembered as four discrete serial ports, and four COMx: names consumed.

So USB adapters without a serial number are best sent directly in the waste basket (or properly turn over for recycling).