The Host and Hubs

The USB connects USB devices with the USB host. The USB physical interconnect is a tiered tree topology. A hub is at the root of each tree. Each wire segment is a point-to-point connection between a hub and a device. Hubs are themselves USB devices.

There is only one host in any USB system. The USB interface to the host computer system is referred to as the Host Controller. A root hub is integrated within the Host Controller to provide one or more attachment points.

Hubs have status bits that are used to report the attachment or removal of a USB device on one of its ports. In the case of an attachment, RTUSB-32 enables the port, opens a control pipe to the device's endpoint 0 (the Default Control Pipe), and assigns a unique address to the device. Then, client software drivers are called to give them a chance to handle the device (either standard class drivers or application supplied drivers). If the new device is a hub, RTUSB-32's internal hub driver will handle the device.

On USB 1.1 buses, a hub's sole local intelligence is detecting and reporting to the host port attach and detach events. USB 2.0, on the other hand, must also be able to translate High-Speed transactions to Low- or Full-Speed when a USB 1.1 device is attached to one of its downstream ports. Such translated transactions are called split transactions. A USB 2.0 hub can operate several independent USB 1.1 buses (each supporting the Full-Speed bandwidth of 12 Mbs) on each of its downstream ports.

USB 3.0/3.2 buses actually consist of two separate buses: a USB 2.0 bus for Low-, Full-, and High-Speed devices, and a SuperSpeed/SuperSpeedPlus bus. These two busses use separate signals in the USB 3.0 cable. A USB 3.0 Hub consists of two logical hubs: one USB 2.0 hub for the USB 2.0 bus and one SuperSpeed/SuperSpeedPlus hub which handles only SuperSpeed/SuperSpeedPlus devices.

Left The Universal Serial Bus

Down Transfer Speeds