The diagram above illustrates USB data flow on three logical levels: client software to function, USB driver to device, and the USB physical level, where actual transmission occurs. On the highest logical level, USB is the data transport agent for data transfers moving between client software and function endpoints envisioned as a bundle of logical data flow "pipes." There is one logical endpoint for each client software-function pipe. USB supports isochronous, bulk, interrupt and control data transfer types. Devices are managed by USB system software. "Endpoint 0" is required of every device for control transfers which define USB function configuration. Control transfers allow the host to access different parts of a device and communicate between client software and its function using configuration/command/status control type information. Data content includes standard, class and vendor type requests. Communication occurs via bidirectional pipes, with a full-speed payload of 64 bytes and a low-speed payload of 8 bytes. Since the control transfer is a host-based token, bus access is guaranteed. Endpoint 0 must support control transfers. Other endpoints are optionally determined by implementation requirements, with a total of 16 endpoints available to each device. Under the host-based token policy of the Universal Serial Bus, the USB host handles most protocol complexity, generates the data transfer tokens, initiates data transfers at full-speed (12 Mb/sec.) or low-speed (1.5 Mb/sec.) and manages the logical connection from the host to function. Isochrony is guaranteed and managed by the host. This fundamental design asymmetry allows the design of extremely simple and low-cost USB-compliant peripherals. The USB physical layer (bus interfaces, ports and USB cable) is source terminated, provides limited power distribution and also allows hot insertion and removal. Intel fourth-generation PCIsets, the 8x930Ax peripheral controller (Product Brief, Data Sheet) and the 8x930Hx peripheral controller (Data Sheet) family represent an end-to-end solution that makes USB hardware design implementation simple, fast and cost-effective.

Legal Stuff © 1997 Intel Corporation