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Summary :
This guide provides solutions to frequently asked questions in Hardware
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What kinds of USB peripherals can I connect to my
PC?
USB carries data at the rate of 12 megabits per second, which is sufficient for
"medium to low-speed peripherals". This broad category includes telephones,
digital cameras, modems, keyboards, mice, digital joysticks, some CD-ROM
drives, tape and floppy drives, digital scanners and specialty printers. USB's
data rate also accommodates a whole new generation of peripherals, including
MPEG-2 video-base products, data gloves and digitizers. Computer-telephony
integration is expected to be a big growth area for PCs, and USB can provide an
interface for Integrated Services Digital Network (ISDN) and digital PBXs. |
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How many USB peripherals can I connect at once?
Technically, you can connect up to 127 individual USB peripherals at one time.
Due to the fact that some devices reserve USB bandwidth, the practical maximum
of devices is less than the theoretical maximum. However, PCI-USB add-in cards
provide an independent USB bus to which even more peripherals can be connected.
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Does my PC have USB 2.0 ports?
Most desktop and laptop computers sold after 2002 are USB 2.0 enabled. USB 2.0
enabled means that the computer has one or more USB-enhanced host controllers.
To determine if your computer running the Windows XP or Windows Server 2003
operating system has a USB-enhanced host controller, follow these steps:
1. Start Device Manager. To do so, click Start, and then click Run.
In the Open box, type devmgmt.msc, and then click OK.
Starting
Device Manager
2. In the Device Manager window, expand the Universal Serial
Bus controllers node. Look for a host controller with the word
"Enhanced" in the title. If you find one, your system has USB 2.0 capable ports
and has the necessary host controller drivers
Enhanced
host controller with drivers loaded
3. If you do not find an enhanced host controller in step 2,
expand the Other Devices section. Look for Universal Serial Bus (USB)
Controller. If you find this item, your system has USB 2.0 capable
ports, but you need to install drivers for this host controller. If you do not
find this item, your system does not have any USB 2.0 ports.
Enhanced host controller with drivers loaded
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Why do I
see several host controllers on my system?
There are three specifications for USB host controllers. Most host controllers
found in PCs are compliant with one of these specifications; however, each
specification is meant to support only a subset of USB 2.0 device speeds
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Is it possible to have a composite and a
compound device in one piece of hardware?
Yes. The Microsoft Natural® Keyboard Pro, which has a three-port, bus-powered
hub, is an example of a compound multifunction USB device that has a composite
device attached to port 1. Two additional ports are exposed to the end user.
The device attached to port 1 is a low-speed composite device. The device has
two interfaces, both of which conform to the USB standard device class
definition for human interface devices (HID). The composite device provides two
HID interfaces instead of multiplexing all collections over a single HID
interface using top-level collections. This design was chosen for compatibility
with older BIOSs.
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Why won't my CPU enter C3 if I attach some
USB devices?
When a USB device is attached to a PC, the USB host
controller polls the frame scheduler in memory. This is a direct memory access
(DMA) bus master operation. Any bus master traffic, interrupt, or one of
several other system activities are "break events" that will move a CPU out of
C3 because, by definition, the CPU's cache cannot be snooped while in C3.
Hardware
removal. In some cases it may be possible for the hardware to be electronically
disconnected from the universal serial bus. For example, when storage media is
removed from the USB reader, the USB reader can emulate an electronic
disconnect and reconnect when the media is reinserted. In this case, the
universal serial bus will not prevent C3 transitions, because there are no USB
devices on the host controller.
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Selective Suspend.
The only alternative available in Windows XP and later operating systems is to
support USB Selective Suspend. This feature allows a driver to suspend the USB
device it controls when the device becomes idle, even while the system itself
remains in a fully operational power state (S0).
Selective Suspend is particularly powerful if all USB function drivers support
it. If even one driver does not support it, the CPU will be kept out of C3.
For additional information on Selective Suspend, see the Driver Development
Kit.
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Is USB
support stable?
Good question. USB is a relatively new specification, Windows 98 was really the
first OS to have what Microsoft considered full USB support. Unfortunately
while the majority of USB devices work fine, Windows 98 did have many USB
related problems
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What are the differences between USB 2.0 and USB 1.1?
Basically, USB 2.0 includes everything that USB 1.1 offers and adds a
high-speed mode, which runs at 480Mbps. USB 1.1 supports two speed modes: 1.5
and 12Mbps whereas USB 2.0 has three of them: 1.5, 12 and 480Mbps. USB 2.0 also
uses the same USB 1.1 compliant cables to connect high-speed devices. However,
classic USB hubs will slow down USB 2.0 devices. In addition, a USB 2.0 host
controller is required to enable the high-speed connection with a USB 2.0
device.
USB 2.0 hubs are now given a lot more work to do than USB 1.1 as they need to
handle all the traffic from three different speed mode devices. Plugging a USB
1.1 device to a USB 2.0 hub is okay, but connecting a USB 2.0 device to a USB
1.1 hub is prohibited.
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Will USB 1.1 devices run any faster on a USB 2.0 bus?
No. However, the new USB 2.0 archiclecture allows more high-speed USB 1.1
devices, such as webcams, audio devices, to share the bandwidth. Developers
need to follow USB 2.0 spec in order to design higher speed peripherals that
can take advantage of the extra bandwidth. USB 1.1 devices still operate at
12Mbps at full-speed and 1.5Mbps at low-speed on a USB 2.0 bus. Even though USB
1.1 devices won t run any faster, they can work alongside of USB 2.0 devices
on the same bus.
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What happen if a USB 2.0 device is plugged
into a USB 1.1 system?
The entire bus under the USB 1.1 root hub will slow to 12Mbps. The operating
system will probably notify the user about the sub-optimal configuration and
recommend for a better course of action.
If several USB 1.1 hubs are connected to a USB 2.0 bus, then each of the USB
1.1 hub will get a full 12Mbps bandwidth.
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What
do I need to use a USB 2.0 device?
The requirement is similar to that of USB 1.1, but all components will have to
be USB 2.0 compliant. A successful USB 2.0 connection requires a USB 2.0 host
controller card, a USB 2.0 driver and a USB 2.0 peripheral.
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Do
USB 1.1 cables work with USB 2.0 devices?
Ideally, yes. USB 2.0 architecture uses the same cables and connectors as USB
1.1 compliant products. Unforunately, only 3 out of 11 cables on the market are
certified as USB 1.1 compliant. You may run into the cables that cause problems
connecting high-speed peripherals. To avoid negative user experience, most
vendors include USB 2.0 compliant cables with their USB 2 PCI cards and
peripherals.
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