USB Endpoints

The four kinds of pipes that communicate with the four kinds of USB endpoints are:

• Control. Control pipes are used primarily to send commands and retrieve status. Control pipes are intended for non-periodic, host-initiated request and response communication of small-sized structured data. Control pipes are bidirectional. The default pipe is a control pipe. See The Default Pipe.

• Bulk. Bulk pipes are used primarily for data transfer. Bulk pipes offer reliable transportation of large amounts of data. Bulk pipes do not necessarily deliver the data in a timely manner. Bulk pipes are unidirectional.

• Interrupt. Interrupt pipes offer timely, reliable communication of small amounts of unstructured data. Periodic polling often is started on interrupt-IN pipes. Interrupt-IN pipes return data to the host when the data becomes present on the device. Some devices have interrupt-OUT pipes. Interrupt-OUT pipes transfer data to the device with the same timely, reliable “interrupt pipe” characteristics of interrupt-IN pipes. Interrupt pipes are unidirectional.

• Isochronous. Isochronous pipes offer a channel for transferring constant-rate, time-relevant data, such as for audio devices. Data is not retried on error. Isochronous pipes are unidirectional.

See Chapter 5 of the USB 2.0 specification or see Requests for more information on the transfer types that correspond to these endpoints.