Windows 95/98

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USB Architecture

The USB standard breaks the traditional limitation of the number of serial and parallel expansion connectors or PC card slots available for port expansion. With USB, peripherals can behave as hub devices so only one peripheral device has to be plugged directly into the host controller. Selected peripherals, or stand-alone USB boxes, can be used as an expansion hub with other USB peripherals acting as nodes. This creates a tiered star topology.

A USB controller will act as the USB Host for the bus architecture. There is only one host on any USB system. The host controller may be implemented in a combination of hardware, firmware, or software. A root hub is integrated within the host system to provide one or more attachments points.

Hubs are "expansion bays" for the USB bus. A hub can be plugged into the Host controller or another hub, which is called cascading, to increase the number of available ports. Hubs can be built into other devices so that, for example, a keyboard may also provide additional USB ports. Hubs can be self-powered or Bus powered. A self-powered hub uses an external power source while a bus-powered hub relies on the bus for power. Listed below are some limitations of how the hubs can be setup.

  • Bus-powered hubs cannot be connected into a bus-powered hub, while a self-powered hub or device can be function on a bus-powered hub.
  • Bus-powered hubs cannot have more then four ports.
  • Bus-powered hubs cannot support bus-powered devices that require more the 100mA.
  • The Hub cascade depth cannot be greater then five. This number includes the host controller in the computer. Therefore there cannot be more the four physical hubs between a device and the host controller.

There is no mechanism in Win98 to determine if a hub is self-powered or bus-powered. If a self-powered hub that has no power is connected into a bus, it will fail to be initialized and may appear in Device Manager with a yellow failure bang. Some ac-powered devices that include hubs may not necessarily be self-powered hubs. The ac may be designed to be used to power the device but not the hub.