Universal Serial Bus (USB)

In this video from ITFreeTraining I will look at the Universal Serial Bus or USB. USB standardized the connections to peripherals and has become very popular. Due to its popularity, it is essential for the IT technician to have a good understanding of how USB works.

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Universal Serial Bus (USB)
0:19 Universal Serial Bus or USB was released in 1996. It provides connectors and cables to connect peripherals and devices. USB was originally designed to replace the PS/2 plug which was used for the keyboard and mouse.

The PS/2 connector was limited to only being able to be used for keyboard and mouse. USB was designed to support any device that was manufactured to be compatible with USB. For example, USB could be used for USB sticks, connecting printers or even your mobile device.

USB was designed to be hot swappable. This means that you can connect and disconnect devices from your computer without rebooting the computer. USB is also auto configurable. In the old days of computing, interrupts, ports and other settings would need to be configured. However, with USB the resources the device requires are automatically allocated.

Nowadays, USB has become very popular and it would be hard to find a computer that does not have at least one USB port. USB is used to connect every device you could think of to your computer. Let’s have a look at how it works.

USB Host Controller
1:33 At the heart of USB is the USB host controller. The USB host controller provides an interface to communicate with the hardware. Each host controller has one root hub. Regardless of how many USB controllers you have, the operating system will communicate with the USB controller using a standard interface.

In this example, there is one chip in the computer that provides a USB host controller. Nowadays, the USB Host Controller will not be a separate chip but will most likely be found in the North Bridge. In some motherboards the newer faster USB 3 controller would be found in the North Bridge and the older slower USB 2 controller would be found in the South Bridge. I will look at the different versions of USB later in the video.

Regardless of how many USB controllers you have on your motherboard, their versions and if they are on one chip or more, they essentially work the same way.

Each USB Host controller can support a maximum of 127 devices. In the real world it is unlikely this maximum will be reached for a number of reasons which I will look at later in the video. To manage a large number of devices using the one USB host controller, each USB host controller has one or more root hubs.

In this example, you can see the USB host controller is connected to the root hub. The root hub is then connected to the physical USB ports. The USB host controller may support more physical ports than what are available on the motherboard. It is up to the manufacturer of the motherboard to determine how many physical ports they are going to support.

Now you may be thinking, if one controller can potentially support 127 devices, there are no motherboards on the market that have anywhere close to 127 USB connectors. There are motherboards that have 16 USB connectors, which should be enough. So, let’s have a look at why you would need to support so many.

USB Hub
3:42 USB has the ability for one port to be expanded to many ports. This is the same principal as a power strip. Shown here is a USB hub. This USB hub allows one USB connection to be split into four connections. USB hubs can be separate devices like this one or can be included in devices like computer monitors and keyboards.

The USB hub has an input port which is referred to as an upstream port. This will be connected directly to the computer or another USB hub. In the case of this USB hub, there are four downstream ports which devices or other USB hubs can be connected to. In this case, a USB thumb drive is connected to one of the ports. Another one of the ports is connected to a second hub.

In a previous video, we looked at what would happen if you connected multiple power strips together. Let’s have a look at the problem that may occur if you connect multiple USB hubs together.

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References
“The Official CompTIA A+ Core Study Guide (Exam 220-1001)” Chapter 3 Lesson 9322 - 10272
“CompTIA A+ Certification exam guide. Tenth edition” Pages 392 – 400
“USB” https://en.wikipedia.org/wiki/USB
“USB hub” https://en.wikipedia.org/wiki/USB_hub
“USB 3.0” https://en.wikipedia.org/wiki/USB_3.0
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Trainer: Austin Mason http://ITFreeTraining.com
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