Installation guide

USB Plus Series Installation Guide (90000410 Rev. E) – Page 13

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

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 standard USB 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

standard USB 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.