Enumeration
The U4xx must enumerate properly. That is, the operating system must correctly detect the device before you can control it. Confirm that the device is part of the hardware that has been detected. Under windows, you would use the Control Panel ⁄ System utility to verify the detection.
Host Hardware Issues
Some USB host chipsets have difficulty with a wide range of USB devices. Specifically, the older VIA chipset may not detect the U4xx. You could remove the VIA drivers and use the Microsoft-provided drivers. This will help in some HW configurations. Another alternative is to purchase a USB plug-in card.
The VIA drivers may install an “Enhanced USB Root Hub”. Disabling this, but still allowing “USB Root Hub Companions” may also allow the USB device to be recognized.
The “SiS” chipsets are also known to have problems detecting a number of USB devices.
App Code Verification
Please feel free to use the application example code to confirm the operation of the U4xx device.