Edgeport Industrial Installation Guide (90000409 Rev. C) – Page 17
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.
