What is USB (Universal Serial Bus)?

The USB standard enables communication between digital devices. Almost all peripheral devices, such as a keyboard, mouse or printer, can be connected with a USB. There is however not much that’s ‘standard’ about the technology. Over the years, it has gone through several upgrades and will probably continue to be modified in the future.

You’re most likely aware of the term USB, as USB cables and ports are a part of everyday life. From the digital camera to the printer all the way to the external hard drive and the smartphone, many digital devices can communicate with each other using this type of connection. But how did it all begin?

What is USB?

The definition of USB, short for Universal Serial Bus, is a universal interface based on a serial bus system. This interface allows external devices to be connected to a computer or other digital device, allowing for data transfer or power supply. When connected, data transfer can occur in both directions. With the power supply function, a single USB cable enables certain peripheral devices, such as external hard drives in 2.5-inch format, to be used without an external power chord.

USB was created in the late 1990s as a universal computer interface that eventually replaced other interfaces such as serial and parallel ports. USB uses serial data transfer, or more precisely, bit-serial data transfer. Data is sent bit by bit in succession over one data channel instead of simultaneously over several lines. This method enables higher data transfer speed. In addition, unlike parallel data transfer, different runtimes don’t have to be considered for the respective data lines. Nevertheless, several lines can be used at the same time for serial data transmission. Standards such as ethernet, HDMI and SATA also use serial data transfer methods.

Many USB devices can be connected to a computer or another end device while the system is running. This is called hot plugging. The USB interface of the computer usually recognises the inserted device automatically and starts the interaction (plug and play or PnP). It’s also possible to unplug a USB device when the computer is running.

The current USB versions 3.1 and 3.2 are well suited for high data transfer rates. This has helped to make external hard drives a popular method for backing up data from computers. These hard drives are small and portable, and because they get their power via the USB cable, they don’t require an additional power supply. Moreover, the capacities of external hard drives have increased considerably over time and now offer multiple terabytes of storage.

What are the main features of USB?

• External interface with a universal standard for a wide range of end devices
• Capable of hot plugging (the ability to connect and disconnect while in operation)
• Plug-and-play (being automatically recognised and operated by the connected device)
• Capable of supplying power at the same time
• Expandable via a USB hub (one USB port can be turned into multiple ports)
• Stable, space-saving plug with reverse polarity protection
• Backwards compatible with older USB standards

What are the USB connector types?

Throughout its existence, the Universal Serial Bus standard has gone through many different designs and features. With each new model smaller than the previous one, the connector size has had to shrink as well. This has resulted in the emergence of various mini and micro variants. Boxes and boxes worth of obsolete computer cables can be found in many attics and garages, waiting for the right moment to be taken away. All USB connector types are protected against polarity reversal and uniquely designed to prevent them from being mixed up.

Below you can find a brief overview of the different types of USB connectors:

USB-A

This is the most common type amongst the first-generation connectors. Type A connectors were mostly used for keyboards and mice. The rectangular, flat plug contains 4 lines and a shield. Mini and micro versions are available as well as a SuperSpeed version with 9 wires.

USB-B

Type-B connectors are often found on devices used around the house or office, such as hard disks or printers. The sockets have square-like dimensions and four lines plus the shield. The B type also has a SuperSpeed version as well as mini and micro versions.

USB-C

The miniaturised USB-C connector was introduced in 2014 to accommodate slim PCs and smartphones. This version also introduced a new reversible design, allowing for the plug to be inserted regardless of which way it is facing. The smaller C-type will also serve as the basis for the new USB-4 generation.

How does USB work?

Data is transmitted in series via a bus. The individual bits are sent and received one after the other on one line. The data signal is transmitted symmetrically: one line transmits the data signal, the other line an inverted signal. This method ensures a high data transfer reliability. External electrical fields cancel each other out, which prevents interference during data transfer.

The older USB 1.0 and USB 2.0 versions transfer data in both directions over the same signal line. Additional lines were put into place for USB 3.0 and subsequent versions. In order to allow the USB cable to also act as a power supply, additional wires have been added. The connector supplies a constant voltage of at least five volts. The number of volts varies depending on the USB version and connector type. Other factors that determine the maximum current are the USB type and connected devices as well as other specifications.

What are the different USB versions?

While the letters A, B and C indicate the connector design, the different version numbers stand for a specific electrical protocol. This defines what each format needs to achieve technically. Differences between the versions are shown in the table below:

What are USB transfer rates?

The Universal Serial Bus facilitates prioritisation amongst the different types of data transfer and provides a range of transfer rates within each version’s specified transfer rate capacity. The host controller takes care of the bus logic and coordinates the exchange of data between the connected USB devices.

The table below shows how the data transfer capacity differs between versions. In reality, these rates are, however, usually significantly lower:

Which devices are USB compatible?

One of the reasons for the development of the USB standard was to create a uniform transfer system for peripheral devices. This is why today almost all external devices can be connected to PC or laptop via a USB cable. These devices include:

• External hard disks, USB sticks
• Printers and scanners
• Digital cameras and webcams
• Keyboards and computer mice
• Drives and burners for DVD or CD
• Smartphones, tablets and smartwatches