Understanding hubs
Hubs, which are critical components in the USB 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).
Each hub has an upstream port, connecting to the host, and multiple downstream ports, connecting to
downstream devices, possibly 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
powered devices typically need to be connected to a self-powered hub, such as the Hubport, which
obtains power from its external power supply and provides up to 500 mA for each downstream port. A
low-powered device, 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
. Since Edgeport/421, Edgeport/42,
Edgeport/42+, and Edgeport/416 operate as self-powered hubs, they are not affected by this
limitation.
According to the USB Specification, the maximum limit of hubs cascaded in a 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. You can parallel-connect up to seven
hubs can be connected parallel
at any given level
. You must tally both external and embedded hubs
when counting downstream hubs.
Edgeport USB Expansion Modules Installation Guide
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