USB Host Controller and Root Hub

A USB hub is responsible for:

• Monitoring the insertion or removal of a device on its ports

• Power-managing individual devices on its ports

• Controlling power to its ports

The USB host controller has an embedded hub called the root hub. The ports that are visible at the back panel are the ports of the root hub. The USB host controller is responsible for:

• Directing the USB bus. Individual devices cannot arbitrate for the bus.

• Polling the devices by using a polling interval determined by the device. The device is assumed to have sufficient buffering to account for the time between the polls.

• Sending data between the USB host controller and its attached devices. Peer-to-peer communication is not supported.

USB Hub Devices

• Do not cascade hubs beyond four levels on either SPARC or IA systems. On SPARC systems, the Open Boot PROM (OBP) cannot reliably probe beyond four levels of devices.

• Do not cascade bus-powered hubs. This means you cannot plug a bus-powered hub into another bus-powered hub. A bus-powered hub does not have its own power supply. A USB diskette device derives all its power from the bus and might not work on a bus-powered hub.