History

The basic USB trident logo^[6]

USB sign is on the head of a USB Cable

A group of seven companies began the development of USB in 1994: Compaq, DEC, IBM, Intel, Microsoft, NEC, and Nortel.^[7] The goal was to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater data rates for external devices. A team including Ajay Bhatt worked on the standard at Intel;^[8][9] the first integrated circuits supporting USB were produced by Intel in 1995.^[10]
The original USB 1.0 specification, which was introduced in January 1996, defined data transfer rates of 1.5 Mbit/s "Low Speed" and 12 Mbit/s "Full Speed".^[10] The first widely used version of USB was 1.1, which was released in September 1998. The 12 Mbit/s data rate was intended for higher-speed devices such as disk drives, and the lower 1.5 Mbit/s rate for low data rate devices such as joysticks.^[11]

A USB Standard Type A plug, the most common USB plug

The USB 2.0 specification was released in April 2000 and was ratified by the USB Implementers Forum (USB-IF) at the end of 2001. Hewlett-Packard, Intel, Lucent Technologies (now Alcatel-Lucent), NEC and Philips jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480 Mbit/s, a 40-times increase over the original USB 1.1 specification.
The USB 3.0 specification was published on 12 November 2008. Its main goals were to increase the data transfer rate (up to 5 Gbit/s), decrease power consumption, increase power output, and be backwards-compatible with USB 2.0.^[12] USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus.^[13] For this reason, the new version is also called SuperSpeed.^[14] The first USB 3.0 equipped devices were presented in January 2010.^[14][15]
As of 2008, approximately six billion USB ports and interfaces were in the global marketplace, and about two billion were being sold each year.^[16]

Version history

A PCI USB 2.0 card for a computer motherboard

Prereleases

The USB standard evolved through several versions before its official release in 1996:

• USB 0.7 – released in November 1994
• USB 0.8 – released in December 1994
• USB 0.9 – released in April 1995
• USB 0.99 – released in August 1995
• USB 1.0 Release Candidate – released in November 1995

USB 1.x

Released in January 1996, USB 1.0 specified data rates of 1.5 Mbit/s (Low-Bandwidth) and 12 Mbit/s (Full-Bandwidth). It did not allow for extension cables or pass-through monitors, due to timing and power limitations. Few USB devices made it to the market until USB 1.1 was released in August 1998, fixing problems identified in 1.0, mostly related to using hubs. USB 1.1 was the earliest revision that was widely adopted.

USB 2.0

The Hi-Speed USB Logo

USB 2.0 was released in April 2000 (now called "Hi-Speed"), adding higher maximum signaling rate of 480 Mbit/s (due to bus access constraints the effective throughput is limited to 35 MB/s or 280 Mbit/s), in addition to the "USB 1.x Full Speed" signaling rate of 12 Mbit/s.^[17][18]

Overview

In general, there are four basic kinds or sizes related to the USB connectors and types of established connections:

• the older "standard" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on USB flash drives)
• the "mini" size (primarily for the B connector end, such as on many cameras)
• the "micro" size, in its USB 1.1/2.0 and USB 3.0 variants (for example, on most modern cellphones)
• the versatile "USB On-The-Go" scheme, in both mini and micro sizes.

Unlike other data cables (Ethernet, HDMI etc.), each end of a USB cable uses a different kind of connector; an A-type or a B-type. This kind of design was chosen to prevent electrical overloads and damaged equipment, as only the A-type socket provides power. There are cables with A-type connectors on both ends, but they should be used carefully.^[4] Therefore in general, each of the different "sizes" requires four different connectors; USB cables have the A-type and B-type connectors, and the corresponding sockets are on the computer or electronic device. In common practice, the A-type connector is usually the full size, and the B-type side can vary as needed.
Counter-intuitively, the "micro" size is the most durable from the point of designed insertion lifetime, as the result of latching mechanism (parts providing gripping force) being moved into plugs on the cable side.^[5]
USB connections also come in four data transfer speeds: Low Speed, Full Speed, High Speed and SuperSpeed. High Speed is only supported by specifically designed USB 2.0 High Speed interfaces (that is, USB 2.0 controllers without the High Speed designation do not support it), as well as by USB 3.0 interfaces. SuperSpeed is supported only by USB 3.0 interfaces.