USB Hubs 101
Before you can make an informed decision about getting more USB hubs, it’s important to understand the terminology. There are three types of USB hubs: root hubs, bus-powered hubs, and self-powered hubs. Understanding the differences helps you to choose wisely.
USB ports built into your computer are connected to USB root hubs; each root hub is controlled by a Standard OpenHCD USB Host Controller (which provides USB 1.1 support), and all of the root hubs in a system are also controlled by an Enhanced Host Controller (which provides USB 2.0 support).
On some systems, you can count the number of Standard OpenHCD USB Host Controllers and multiply by two to determine the number of USB ports your system can support. To do this, open the Windows Device Manager and expand the Universal Serial Bus (USB) category. As you can see from Figure 1, this system has five Standard OpenHCD USB Host Controller for a total of ten USB ports.
Figure 1 This system supports ten USB ports and also features USB 2.0 support.
This method isn’t perfect. Some systems use only a single Standard OpenHCD USB Host Controller to drive multiple USB ports. In those cases, open the USB Root Hub listing and click the Power tab to see how many ports are available (Figure 2).
Figure 2 This system has eight ports available.
The other type of USB hub you might see in your computer’s Device Manager is a generic USB hub. A generic hub is a device that splits one USB port into multiple ports (usually four, although seven-port hubs are also available). By using multiple hubs, a single USB port on a computer can support dozens of devices.
Generic hubs fall into two categories: bus-powered and self-powered. Bus-powered hubs are inexpensive (you can get them for less than $10), but they have a major weakness: they can’t work with devices that draw more than 100mA of power.
A bus-powered hub subdivides the 500mA of power available from the root hub it’s connected to into 100mA per port and 100mA for device operation. A self-powered hub typically costs $20 or more because it includes an AC power adapter, enabling it to provide up to 500mA of power.
Different types of USB devices draw different levels of power. To see how much power a particular USB device draws, open the listing for the hub the device is connected to and click the Power tab. Figure 3 lists a mixture of low-power devices that would work on any type of hub (root, bus-powered, self-powered) and a high-power device that must be plugged into a root hub or self-powered hub.
Figure 3 The USB Mass Storage Device (a USB thumb drive) cannot be safely plugged into a bus-powered hub because it requires more than 100mA of power.
What happens if you plug a high-powered USB device into a bus-powered hub? If you’re lucky, you’ll see an error message informing you that the hub doesn’t have enough power to run the device. If you’re unlucky, you could destroy your device.
So, now that you understand what makes USB ports work, it’s time to find out where to get more. In some cases, you might have “hidden” USB ports you can take advantage of for little or no money.