Recipe 3.11. Keeping USB Devices from Disappearing

Problem

Every so often, a USB device fails to be recognized and usable even though it has been connected for as long as the PC has been running.

Solution

Using the USB cable

1. Unplug the USB device and wait for Windows to signal the disconnect, or for 30 to 60 seconds to be sure Windows knows the device is gone.

2. Reconnect the USB device and wait for Windows to recognize it again.

Using a graphical user interface

1. Click Start, then right-click My Computer and select Properties.

2. Select the Hardware tab, then click Device Manager.

3. Locate and expand the device type of interest Network adapters, or other.

4. Locate the device of interest, right-click on it, then select Properties.

5. If the device properties dialog includes a power management tab, click on it.

6. Uncheck the box next to Allow the Computer to Turn Off This Device to Save Power.

7. Click OK, then close Device Manager.

Discussion

Windows power management will turn off devices when the system goes into standby and may also do so when a screen saver kicks in. Some USB devices do not reset and come back to life after being told to power off and then come back on later, but they do OK if completely disconnected and reconnected. Changing the power management characteristics for a device can keep it from losing power and getting confused. This symptom can also affect built-in and PCI network adapters that support and comply with power management.

See Also

Power management is a significant topic of discussion among PC and periph-eral design engineers at both the hardware and software/driver levels. Microsoft and Intel are the best sources for information on how chipsets and Windows support power management. For details, visit the following web pages: http://www.microsoft.com/whdc/archive/winpowmgmt.mspx, http://www.microsoft.com/whdc/system/pnppwr/powermgmt/devicepm.mspx, http://www.microsoft.com/whdc/resources/respec/specs/pmref/default.mspx, http://www.microsoft.com/resources/documentation/Windows/XP/all/reskit/en-us/prdh_dmt_xapo.asp, and http://www.intel.com/intelpress/samples/ppm_chapter.pdf.