How is COM port number associated with a USB port / device?

Posted on 2008-11-07
Last Modified: 2012-05-05
I bought a USB-to-Serial (DB9) adapter and it works fine. However, I need the serial device that plugs into the adapter's DB9 male to show up on COM3. So, I specify that in the adapter's driver properties under Device Manager (I'm in XP Pro SP3). This works fine. So my serial device is working properly on COM3 with this adapter plugged into a USB port.

Then, I unplug the USB adapter (from the USB port #1), wait a few seconds, and plug the USB adapter into a DIFFERENT USB port #2. Now, my serial device shows up on COM6. Bad news, 'cause my legacy software is looking for it on COM3.

So, I unplug the USB adapter from port #2, and plug it back into port #1. Bingo! It's shows back up as being on COM3!

When the USB adapter is plugged into USB port #2, I can "force" the COM setting back to COM3 in the adapter's driver properties (even though it says that COM3 is "In Use"). And, after forcing the setting back to COM3, my legacy software finds the serial device on COM3 and works fine again.

I'd like to understand more about how Windows XP associates a COM port number with a USB port and any tools that I can use to better understand this COM # allocation. Is there a way to tell XP to erase any associations that it has made regarding COM #'s for USB ports?

Question by:lee88
    LVL 6

    Accepted Solution


    Windows XP is not  associating between USB and serial but the driver is doing that and it save the data in registry, so you have to search the registry for it.

    Other solution may works, add a new hardware manually COM6 and use the same resources that COM3 uses. your legacy software should be happy.

    Or consistently connect to the same port, maybe you have to remove the hardware safely before unplugging.

    LVL 3

    Assisted Solution

    Actually, a USB device plugged into two different ports will be considered as two different devices  by windowns, meaning it install the driver for both 'devices' and they may get different COM ports because of this.

    Similar, if you have a USB disk and you plug it in, it will get a drive letter, if you change that drive letter, and plug it into _another_ USB port, it will get a different drive letter.
    LVL 6

    Expert Comment

    Never mind they will get deferent com ports, but the two ports will point again into the same resources that the legacy sw understands. Try it with the adapter driver.
    LVL 91

    Expert Comment

    i would suggest a pci adapter card, instead of the usb adapter :

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