AnywhereUSB Installation Guide (90000405 Rev. F) – Page 10
Understanding Hubs
Hubs, critical components in the plug-and-play architecture, are wiring concentrators that enable the
attachment of multiple devices, thus converting a single attachment point into multiple attachment points.
USB architecture allows a cascaded multiple hub configuration with certain power limitations (explained
later in this section). See figure 1.
Edgeport Edgeport
Hubport
PC
Host
Hubport
Edgeport
bus-powered hub
scanner
joystick mouse
Figure 1: Example of a Typical Hub Configuration
Each hub has an upstream port, connecting to the host, and multiple downstream ports, connecting to
downstream devices, including other hubs. A hub can detect attachment and detachment of downstream
devices and enable and monitor the distribution of the power to downstream devices via their integral
hardware and the operating system.
Each USB device reports its power requirements to the operating system, which then enables and disables
the device as a function of its power requirements and the amount of available power. High-speed devices
typically need to be connected to a self-powered hub, which obtains power from its external power supply
and provides up to 500 mA for each downstream port. Only simple devices, such as a mouse, can be
connected to a bus-powered hub, which obtains power from its upstream host and provides up to 100 mA
for each downstream port.
Due to the limited available power for bus-powered hubs, cascading two bus-powered hubs is an illegal
topology, and devices connected to the second hub will not function. (USB specifications limit the
connection of a bus-powered hub to a self-powered hub or host only.)
According to the USB Specification, the maximum limit of hubs cascaded in series cannot exceed five. In
other words, you may have a maximum of five hubs between any device and the host. This does NOT mean
that the maximum number of hubs in a system is five. Indeed, up to seven hubs can be connected parallel at
any given level. You must tally both external and embedded hubs when counting downstream hubs.
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