Universal Serial Bus (USB) Interface

Before you start

Objectives: learn what is Universal Serial Bus, how can we use it, and what are usual USB connectors.

Prerequisites: no prerequisites.

Key terms: device, connector, hub, power, type, mini, cable, self-powered, irq, micro, pin, port, lenght

Serial Bus

USB eliminates the need for separate expansion boards for every device that we want to install in the system. We can have up to 127 different devices on our Universal Serial Bus. USB uses a tiered star topology. In every computer which uses USB, we have a device installed that’s called a Root USB Hub. From that Root Hub we have several USB ports which are located somewhere on our computer case. The thing is, we can use another USB hub and connect it to the USB port. Next, we can connect various devices to that hub, or we can connect the third hub to second USB hub. That enables us to connect even more USB devices to our computer. So, hubs can be chained together to provide additional ports.


Image 318.1 – USB Hub Example

We can have a maximum of 127 different devices on Universal Serial Bus, including the hubs. Root USB Hub uses only one IRQ channel. In comparison, each serial and parallel port has it’s own IRQ channel. All devices connected together share computer resources like IRQs and I/O addresses. Some PC systems implement multiple Root USB hubs, so they might consume more than one IRQ. But, that might not be the case, since the USB Bus exists as a PCI device. Because of that multiple Root USB Hubs can actually share one IRQ. USB supports Plug-and-Play and hot swapping (adding and removing devices without rebooting, also known as hot plugging). To install a USB device we should install the software driver before attaching the device. When we plug in the device, it will typically be automatically detected and configured.

USB Versions

There are four different versions of USB standard. There are USB 1.0, USB 1.1, USB 2.0, and USB 3.0. The main difference between them is the speed. USB 1.0 transfers data at 1.5 megabits per second (Low-speed), with the maximum cable length of 3 meters, and USB 1.1 at 12 megabits per second (Full-speed), with the maximum cable length of 5 meters. USB 2.0 can transfer data at 480 megabits per second (High-speed), with the maximum cable length of 5 meters, while 3.0 can go up to 5 gigabits per second (Super-speed), with cable length not being defined, but 3 meters being recommended. Each device on the USB Bus consumes a portion of that bandwidth. That means that if we have many USB devices connected, things will slow down a bit. Version 3.0 is backwards compatible with 2.0, and 2.0 is backwards compatible with version 1.1 devices. Most motherboards allow us to enable or disable USB support in the BIOS, or configure the USB version that will be used.


There are two ways in which we can power our USB devices. A device that uses active power has its own power supply (we plug them into an AC outlet), and it is independent of the USB Bus. All devices that draw more than 500 mA of power are required to be self-powered. A printer is a an example of a device that uses active power (current from the wall outlet). Other devices will use passive power, which means that they get the power they need to operate right off of the USB Bus. USB cables have wires to carry both power and data. Bus-powered or passive power devices get their power from the USB cable. Bus-powered devices are classified as low-powered or high-powered devices depending on the amount of power they draw from the USB bus. Low powered devices use 100 mA or less, and high-powered devices use between 100 and 500 mA. Like USB devices, USB hubs can be bus-powered or self-powered. We can’t connect high-powered device to a bus-powered hub, but we can connect low-powered or self-powered device to a bus-powered hub. Therefore, self-powered hubs that provide 500 mA per port are recommended to ensure an adequate power supply to all bus-powered devices that we may wish to connect to the hub.


The first type of USB connector is the Type A connector. This the connector we probably work with the most. It plugs directly into the computer or a hub. It’s a rectangular connector that has four wires inside.

usb a m

Image 318.2 – USB Type A Connector

The second type of USB connector is Type B connector. It is D-shaped connector which we will often use to connect printers and external hard drives. Most USB cables have an A connector on one end (to connect to the computer or hub) and a B connector on the other end (to connect to the device)

 usb b

Image 318.3 – USB Type B Connector

There are also mini and micro versions of USB connectors, which are sometimes used with digital cameras, mobile phones and portable computers. For example, 4 pin mini USB connector is a small square connector designed to plug in to devices with mini plugs such as a digital camera or mobile phones. Most USB cables with a mini connector have an A connector on the other end to connect to the computer.

4 Pin Mini USB

Image 318.4 – Type A and Mini 4 Pin USB Connector

We also have a 5 pin mini and micro USB connectors.

 Mini USB

Image 318.5 – Type A and Mini 5 Pin USB Connector

Micro B USB

Image 318.6 – Micro USB Connector

All connectors that we have mentioned are male connectors, but there’s a female version of the USB connector as well. It is typically used on an USB extension cable.

Female USB Connector

Image 318.7 – Female USB Connector

This is an exception because USB cables have male connectors on both ends. USB ports on our PCs, printers, cameras and other, are always female connectors. USB 3.0 connectors are colored in blue.

usb 3.0 A Male

Image 318.8 – USB 3.0 Type A

usb 3.0 B Male

Image 318.9 – USB 3.0 Type B

usb 3.0 MicroB Male

Image 318.10 – USB 3.0 Micro B


We can have up to 127 different devices on our Universal Serial Bus. USB hubs can be chained together to provide additional USB ports. Root USB Hub uses only one IRQ channel. There are four different versions of USB standard and those are USB 1.0, USB 1.1, USB 2.0, and USB 3.0. USB devices can be self-powered or bus-powered. All devices that draw more than 500 mA of power are required to be self-powered. USB connectors that we will often use are Type A, Type B, mini 4 pin USB, mini USB, and micro USB.