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USB3.0 Loopback plug identified as "Unknown Device"

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  • USB3.0 Loopback plug identified as "Unknown Device"

    I am using the loopback plugs for an automated test on a system with 4 usb 3.0 ports.

    Sometimes, 1 or more plugs aren't recognized and appear as "Unknown Device" in Windows's Device Manager
    From what i can see, unknown device have a VID=0x0000 and a DID=0x0000. But do have the other 3 plugs as siblings.

    If I Unplug and reconnect the faulty plug physically, everything goes back to normal but this is not a valid solution.

    Here are the steps I do to create the problem :

    1. Plug the loopbacks in the system
    2. Power up the system from G3 to S0
    3. Boot Windows 7
    4. Run the test
    5. Shutdown the system to G3
    6. Start over

    And the problems occurs more than 1 out of 4 times.

    I run windows 7 with Intel(R) USB 3.0 eXtensible Host Controller version
    (I also tried

    If I disable XHCI in the BIOS or use USB2.0 ports, there is no problem but again this is not a solution.

    Do you have any idea of what I might be doing wrong?

    I have screenshot and can reproduce it easily so if you want them by email just ask


  • #2
    We haven't been able to replicate any behavior like this in our testing, the only similar thing we have seen on an Intel host controller is that with multiple USB3 plugs connected sometimes when connecting another plug (eg from 3 to 4) on the front panel ports it will refuse to enumerate properly. However this could also just be some bad wiring on the front panel ports.

    Are the plugs all using firmware version 1.4 (displayed on the LCD when they first power on)?

    Is it only specific ports displaying the issue (eg the front panel ports) or does it occur across all of them? Does it still happen if you only have 3 or 2 plugs connected?


    • #3
      All plugs are using firmware 1.4
      I can reproduce the problem with only 1 or 2 plugs
      Every USB3.0 ports I have cause the problem and USB2.0 ports won't.

      I also reproduced the problem with a DELL XPS I7-3770 on the front panel USB3.0 (This computer don't have rear pannel USB3.0). The DELL also have Intel(R) USB 3.0 eXtensible Host Controller version
      The other boards I had problem with are custom system with rear panel USB3.0

      In my recent test, I have found that :
      I can reboot or shutdown the computer without any problem (s0-->s5 and s5-->s0). No matter if the USB still have power or not while in s5. But as soon as i go to G3 (Mechanical off (unplug the power cord)) the next boot in windows will show me at least 1 "unknown device" with the VId = 0 and DID = 0.
      When I see "Unknown Device" under Universal Serial Bus Controllers in Device Manager I can also see the "Passmark USB3.0 Loopback plug".
      Sometimes, the reenumeration caused by the function :
      SendVendorCommand(CurUSBDevice, SET_CONFIG, 2, (UCHAR*) &usb3Config, sizeof(usb3Config));
      will remove the unknown device but sometimes it will simply make the passmark plug disappear. To see the error everytimes i have to look in device manager for the unknown device



      • #4
        Windows seems to detect 2 devices instead of only 1.
        I think the usb plug might link with the usb 2 phy while the system is in the BIOS and that it don't react correctly when the usb3 driver come into play when Windows is loading. I think the plug might be linked by usb2 and usb3 at the same time...

        Passmark plug programatically tells me that are are linked at superspeed and test result also show that they can reach SS bandwidth but the text on the plug still say that it is linked at 480 MB/s.

        This could also explain why I can see the problem when they are on usb2.0 ports or when i deactivate usb2.0 compatibility on usb3.0 ports (in the BIOS set XHCI to Enabled instend of AUTO)



        • #5
          We've been able to semi-reproduce this behavior on a Gigabyte board, here's what we've seen;

          XHCI disabled: Plugs display 480Mb/s, Test ok, shutdown (power off) and repeat: no issues.
          XHCI enabled: Plugs display 5Gb/s, Test ok, shutdown (power off) and repeat: no issues.
          XHCI "Auto" : Plugs switch between 480Mb/s and 5Gb/s duirng boot process. Test ok, shutdown (power off) and repeat: saw device not recognised error however plugs are able to be tested ok and ran at 5Gb/s.
          XHCI "Smart Auto" : Plugs display 480Mb/s. Tested at 5Gb/s speeds, shutdown (power off) and repeat: saw device not recognised error however plugs are able to be tested ok and ran at 5Gb/s. Unrecognised devices disappear once test starts.

          We also saw device not recognised issues when changing the XHCI mode and not disconnecting the plugs, so we'd recommended reconnecting the plugs on any change like this.

          So it looks like the auto settings are likely not working well with the USB3 plugs as they try to emulate USB2 ports up until the point the USB3 driver loads, while the USB3 plugs will connect at a speed appropriate to the port when it is plugged in (until a re-enumeration occurs). So the switch between the USB2/USB3 is not working very well.

          In all these instance we were still able to run the USB test in BurnInTest correctly but we would recommend switching the XHCI mode to enabled for testing purposes.
          Last edited by Tim (PassMark); Mar-03-2014, 04:58 AM.


          • #6
            Ok we will do this
            Thanks for the support

            Running the test can also "fix" the issue but not always on our side. Sometimes, about 1 out of 10 times, the plugs are unable to be tested.