Wide Selection of USB Devices

The USB specification supports lower-speed devices, such as a keyboards, mice, joysticks, and gamepads, at 1.5 Megabits per second and higher speed devices, such as removable storage devices, scanners, or digital cameras, at up to 12 Megabits per second (high-speed is referred to as full speed signalling in the USB specification).

Device Classes

USB devices are categorized by class. Table 1-1 lists a few examples of USB device classes.

**Table 1-1** Examples of USB device classes
USB device class	USB devices in class
Audio class	Speakers, microphones
Communication class	Modem, speakerphone, internet phone
Composite class	A single device that supports multiple functions, mice, keyboards, and others
HID class	Keyboards, mice, joysticks, drawing tablets, and other pointing devices
Hub class	Hubs provide additional attachment points for extending the USB
Mass storage class	Floppy drives, other removable storage devices.
Printing class	Printers
Vendor specific	A device that doesn't fit into any other predefined class, or one that doesn't use the standard protocols for an existing class

Low- and High-Speed Devices

Low-speed devices, which may include keyboards, mice, drawing tablets and others, are typically in a USB class called the Human Interface Device (HID) class. There is generally some cost reduction in low-speed devices because the cabling is less expensive than cabling for high-speed devices.

Low-speed devices support only short messaging and do not support bulk and isochronous transfers.

High-speed devices generally include communications devices, printing devices, bulk storage devices, audio devices, and others.

There is nothing to prevent USB devices from being in either a high-speed or low-speed category. However, some classes of devices, those that require bulk or isochronous transfer services, cannot be part of the low-speed category.

Note

High speed in the case of USB is not comparable to high-speed devices on a FireWire bus. USB is a complementary technology to FireWire, not a competing technology. USB enables the use of affordable higher-speed consumer grade peripherals on Macintosh computers.

USB Hub Devices

Hubs are also USB devices and provide attachment points to the USB for other devices or hubs. Hubs can be embedded into other USB devices (this is known as a compound class device). For example, a hub can reside in a keyboard, monitor, or printer to provide attachment points for other (typically) low-power devices.

Hubs are also in the form of standalone multi-port hubs that provide attachment points to the USB for other USB devices. Multiport-hubs are generally categorized as bus-powered and self-powered. Bus-powered hubs can request a total of 500ma from the USB and provide no more than 100ma of power at each port on the hub. Even though a bus-powered hub may request 500ma, it may not get the power depending on the devices connected upstream on the USB. Self-powered hubs (hubs that include a source of power external to the USB) can supply additional power to the USB, and are required to provide up to 500ma at each port on the hub.

While it is physically possible to connect two bus-powered hubs together in-line without damaging any devices on the USB, it should not be done because there isn't enough power on the USB to support such an attachment. If sufficient power isn't available for the downstream device, the USB software will not be able to properly configure the device's power requirements. The downstream hub most likely will not function. However, a self-powered hub and bus-powered hub can be connected together in-line.

See Chapter 11 of the Universal Serial Bus Specification for additional information about USB hubs.

The USB Root Hub

There is also a hub referred to as the root hub. The root hub is a software simulation of a hub with hardware controller support. It acts as part of the host hardware environment on the main logic board or on an I/O expansion card. The root hub is similar to the other hubs, in that it provides an attachment point or points to extend the USB from the host, however it is the initial connection point and parent of the bus at which all signals originate. A simple diagram of the USB topology is shown in Figure 2 .