Whenever a new USB serial device is plugged into a PC, Windows will assign the device a new, unique COM Port number. This could even happen if the same device is plugged into a different USB port.
If you are prototyping or experimenting with different USB devices, this can cause the COM Port numbers to become quite large over time. Although there is no reduced performance associated with high COM Port numbers, you may wish to have a specific COM Port number assigned to a specific interface board.
This guide shows instructions on how to remove and change COM Port Assignments in Windows 7. Because the process requires you to enter the Device Manager, caution is advised.
Show all Com Port Assignments
Open a Command Prompt window and type the following lines:
*This should automatically open the Device Manager for you.
In Device Manager: click View, then Show Hidden Devices.
Expand the section labeled Ports (COM & LPT) to view all COM Ports that have ever been installed on this computer. COM Ports that have been assigned but are not currently connected will have a light-grey icon.
To remove an old COM Port
Right-click on the COM Port in question and chose Uninstall.
The removed COM Port is free to be assigned to new devices that are plugged into this PC.
To change COM Port assignments
Right-click on the Serial Device you want to change and chose Properties.
In the Properties window, click the Port Settings tab, then Advanced.
In the window that opens, chose an unused COM Port from the dropdown list.