Edgeport Industrial Installation Guide (90000409 Rev. J) – Page 14
Understanding Hubs
Hubs, 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). See figure 1.
Edgeport Edgeport
Hubport
PC
Host
Hubport
Edgeport
bu s-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, 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.
Only low powered 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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