Repost from : http://blog.cognitioninfotech.com/2009/01/how-to-clear-or-reset-com-port.html
Windows always assign a COM port no (like COM2, COM3 etc) every time you connect a USB device like modem, mobile phone or specially serial to USB convertors to system, and this no goes on increasing as every time you connect device windows reserve that port and next time it assign new number.
I recently face some issue because of this, I want to test my new thing magic m5e RFID Reader with BizTalk RFID which is connected to USB of my laptop using a serial to USB convertor, here my system assign a port no COM19, this was a big problem for me as my RFID Device Provider can only access up to COM5 ( it is in pre-beta) so i need to change COM port to less than COM5, when I tried to do that it shows me list of all 19 ports and mentioned in brackets as “in use”, so i need to free all these ports which are locked by system. I searched every where in device manager , control panel no interface is available to do this in windows, then i found a nice solution for this which allow you to free all ports but for this you need to edit registry,
here are the steps which you can use to free all post.
- Click start –> Run –> type regedit and click OK button.
- This will open a Registry editor.
- Navigate to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\COM Name Arbiter
- Now right panel , you can see a key ComDB right click to that key and click modify.
- In value Data section select all and delete reset to Zero ‘0’.
- click OK
- Close Registry editor and then restart your computer. all COM ports are now Free, you may need to reinstall any USB to serial convertor.
Note: It is always good to take back up before deleting any entry from Registry, to take back up for Com Name Arbiter, in left panel right click on COM Name Arbiter and say Export, save with some name and appropriate location.