USB port allocation in XP

XP has a habit of remembering which com port numbers have been used before to other devices in the past. This leads to funny situations where com port numbers that are allocated to a new device seem to be high in number (sometimes greater than 20, very often greater than 10).

Just in case others are having problems with com port allocation, it seems
XP won't tell you by default in device manager which com ports it has
previously allocated and are not currently being used, even with 'show
hidden devices' turned on.

The solution is to add a system variable (Right click My
Computer->Advanced->Environment Variables):
devmgr_show_nonpresent_devices = 1
Then use 'show hidden devices' in Device Manager. You can then delete the
obviously unused ones and free them up for re-allocation.
see http://support.microsoft.com/kb/241257

XP will allocate new com port numbers from the first available number, so if you know there is no com 4 currently used by a device and you are being given a com port number of, say, 13, all you need to do is delete com 4 and XP will allocate com 4 to the next device plugged in.

I don't know whether this works in Vista (and no idea about Windows 7), YMMV.

Great tip, Perky! :cheers:

FYI - It also Works in Vista and Windows 7.

Thanks for sharing that info!

-Andy.

I thought it might be useful. Windows seems to remember both the device you've plugged in and the physical USB port it's plugged into, so plugging the same device into another USB port later on results in yet another COM port number being allocated and remembered. Given there are many devices out there, and multiple USB port choices, you can soon end up with several 10s worth of redundant COM port numbers! Unfortunately a lot of software out there won't work if the COM port numbers are outside of a given range.

Nice to hear it works with Vista and Windows 7 too.