At the bottom layer are the endpoint descriptors that specify attributes such as transfer type and maximum packet size. Each interface supported by a configuration has its own descriptor with fields for information such as the interface class, subclass, and protocol, and the number of endpoints in that interface.
Each configuration in turn has a configuration descriptor containing fields that describe the number of interfaces it supports and the power characteristics of the device when it is in that configuration, along with other information. By examining the descriptors at each layer, you can determine exactly which endpoint you need to communicate successfully with a particular device.Īt the top layer is the device descriptor, which has fields associated with information such as the device’s class and subclass, vendor and product numbers, and number of configurations. USB Device Component DescriptorsĮach layer of a USB device provides information about its attributes and resource requirements in its descriptor, a data structure accessible through device interface functions. Endpoint and pipe are often used synonymously although an endpoint is a component of a USB device and a pipe is a logical abstraction of the communications link between endpoint and host.
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Each endpoint has characteristics that describe the communication it supports, such as transfer type (control, isochronous, interrupt, or bulk, described in USB Transfer Types), maximum packet size, and transfer direction (input or output).Ĭommunication with a USB device is accomplished through a pipe, a logical association between an endpoint and software running on the host. An endpoint is a uniquely identifiable portion of a USB device that is the source or sink of information in a communication flow between the host and the device. For example, an interface for an audio device may have different settings you can select for different bandwidths.Įach interface contains zero or more endpoints. An interface provides the definitions of the functions available within the device and may even contain alternate settings within a single interface. Such settings can include the power characteristics of the configuration (for example, the maximum power consumed by the configuration and whether it is self-powered or not) and whether the configuration supports remote wake-up.Įach configuration contains one or more interfaces that are accessible after the configuration is set. A device consists of one or more configurations, each of which describes a possible setting the device can be programmed into. The architecture of a generic USB device is multi-layered. For information on how the USB 2.0 specification affects isochronous transfers, see USB 2.0 and Isochronous Transfers. The exceptions to this are some differences in isochronous transfers. Apple provides USB 2.0 ports on all new Macintosh computers and fully supports the new specification with Enhanced Host Controller Interface (EHCI) controllers and built-in, low-level USB drivers.įor the most part, you do not have to change existing applications to support the faster data rate because the speed increase and other enhancements are implemented at such a low level. The USB 2.0 specification is fully compatible with low-speed and full-speed USB devices and even supports the use of cables and connectors made to meet earlier versions of the specification. Version 2.0 of the specification adds another bus speed to this list:
Version 1.1 of the USB specification supports two bus speeds: Hard drives, flash memory readers, CD Read/Write drives, digital cameras, and high-end media playersĪ device that doesn’t fit into any other predefined class or one that doesn’t use the standard protocols for an existing classĭigital camcorders, webcams, digital still cameras that support video streaming Hubs provide additional attachment points for USB devices
Keyboards, mice, joysticks, drawing tablets A device in which all class-specific information is embedded in its interfaces
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