BT747 evolves constantly (2.X.*), but this evolution will impact most windows users in a positive way, and existing users must learn this information or it would remain hidden from them.
In the case where you are using USB to connect your passive tracking device to the computer, BT747 is now capable of finding the corresponding serial port itself. You will still need to select the right connection speed though.
This is pretty useful on Windows 7 (at least) with the recent MTKII chipset devices. On that system the serial port number changes with the USB port and the user had to change the com port number every time. Now the setting in BT747 can be left to 'USB' and the connection should be made regardless of the port.
If this does not work for you, you can leave a message in the forums or write me a message. I prefer the forums because then others can find the resolution to the issue too.