USB Expansion Modules Installation Guide

Edgeport Installation Guide (90000403 Rev. D) – Page 18
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
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, 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. Since Edgeport/421, Edgeport/42, Edgeport/42+, Edgeport/416 and
Edgeport/412 operate as self-powered hubs, they are not affected by this limitation.
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.