A group of seven companies began the development of USB in 1994: Compaq, DEC, IBM, Intel, Microsoft, NEC, and Nortel. 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. Ajay Bhatt and his team worked on the standard at Intel; the first integrated circuits supporting USB were produced by Intel in 1995.
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. Draft designs had called for a single-speed 5 Mbit/s bus, but the low speed was added to support low-cost peripherals with unshielded cables, resulting in a split design with a 12 Mbit/s data rate intended for higher-speed devices such as printers and floppy disk drives, and the lower 1.5 Mbit/s rate for low data rate devices such as keyboards, mice and joysticks. Microsoft Windows 95, OSR 2.1 provided OEM support for the devices in August 1997. The first widely used version of USB was 1.1, which was released in September 1998. Apple Inc.‘s iMac was the first mainstream product with USB and the iMac’s success popularized USB itself. Following Apple’s design decision to remove all legacy ports from the iMac, many PC manufacturers began building legacy-free PCs, which led to the broader PC market using USB as a standard.
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 Nokia), NEC, and Philips jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480 Mbit/s, 40 times as fast as 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 backward compatible with USB 2.0.(3–1) USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus.(1–3) For this reason, the new version is also called SuperSpeed. The first USB 3.0 equipped devices were presented in January 2010.
As of 2008, approximately 6 billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.
The USB 3.1 specification was published in July 2013.
In December 2014, USB-IF submitted USB 3.1, USB Power Delivery 2.0 and USB-C specifications to the IEC (TC 100 – Audio, video and multimedia systems and equipment) for inclusion in the international standard IEC 62680 (Universal Serial Bus interfaces for data and power), which is currently based on USB 2.0.
The USB 3.2 specification was published in September 2017.
Released in January 1996, USB 1.0 specified data rates of 1.5 Mbit/s (Low Bandwidth or Low Speed) and 12 Mbit/s (Full Speed). 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. USB 1.1 was the earliest revision that was widely adopted and led to what Microsoft designated the “Legacy-free PC“.
Neither USB 1.0 nor 1.1 specified a design for any connector smaller than the standard type A or type B. Though many designs for a miniaturised type B connector appeared on many peripherals, conformity to the USB 1.x standard was hampered by treating peripherals that had miniature connectors as though they had a tethered connection (that is: no plug or receptacle at the peripheral end). There was no known miniature type A connector until USB 2.0 (revision 1.01) introduced one.
USB 2.0 was released in April 2000, adding a higher maximum signaling rate of 480 Mbit/s (60 MB/s) named High Speed or High Bandwidth, in addition to the USB 1.x Full Speed signaling rate of 12 Mbit/s.
Modifications to the USB specification have been made via Engineering Change Notices (ECN). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org:
- Mini-A and Mini-B Connector
- Micro-USB Cables and Connectors Specification 1.01
- InterChip USB Supplement
- On-The-Go Supplement 1.3 USB On-The-Go makes it possible for two USB devices to communicate with each other without requiring a separate USB host
- Battery Charging Specification 1.1 Added support for dedicated chargers, host chargers behavior for devices with dead batteries
- Battery Charging Specification 1.2: with increased current of 1.5 A on charging ports for unconfigured devices, allowing High Speed communication while having a current up to 1.5 A and allowing a maximum current of 5 A
- Link Power Management Addendum ECN, which adds a sleep power state
The USB 3.0 specification was released on 12 November 2008, with its management transferring from USB 3.0 Promoter Group to the USB Implementers Forum (USB-IF), and announced on 17 November 2008 at the SuperSpeed USB Developers Conference.
USB 3.0 adds a SuperSpeed transfer mode, with associated backward compatible plugs, receptacles, and cables. SuperSpeed plugs and receptacles are identified with a distinct logo and blue inserts in standard format receptacles.
The SuperSpeed bus provides for a transfer mode at a nominal rate of 5.0 Gbit/s, in addition to the three existing transfer modes. Its efficiency is dependent on a number of factors including physical symbol encoding and link level overhead. At a 5 Gbit/s signaling rate with 8b/10b encoding, each byte needs 10 bits to transmit, so the raw throughput is 500 MB/s. When flow control, packet framing and protocol overhead are considered, it is realistic for 400 MB/s (3.2 Gbit/s) or more to transmit to an application. Communication is full-duplex in SuperSpeed transfer mode; earlier modes are half-duplex, arbitrated by the host.
Low-power and high-power devices remain operational with this standard, but devices using SuperSpeed can take advantage of increased available current of between 150 mA and 900 mA, respectively.
USB 3.1, released in July 2013 has two variants. The first one preserves USB 3.0’s SuperSpeed transfer mode and is labeled USB 3.1 Gen 1, and the second version introduces a new SuperSpeed+ transfer mode under the label of USB 3.1 Gen 2. SuperSpeed+ doubles the maximum data signaling rate to 10 Gbit/s, while reducing line encoding overhead to just 3% by changing the encoding scheme to 128b/132b.
USB 3.2, released in September 2017, preserves existing USB 3.1 SuperSpeed and SuperSpeed+ data modes but introduces two new SuperSpeed+ transfer modes over the USB-C connector with data rates of 10 and 20 Gbit/s (1.25 and 2.5 GB/s). The increase in bandwidth is a result of multi-lane operation over existing wires that were intended for flip-flop capabilities of the USB-C connector.
USB 3.0 also introduced the UASP protocol, which provides generally faster transfer speeds than the BOT (Bulk-Only-Transfer) protocol.
Starting with the USB 3.2 standard, USB-IF introduced a new naming scheme. To help companies with branding of the different transfer modes, USB-IF recommended branding the 5, 10, and 20 Gbit/s transfer modes as SuperSpeed USB 5Gbps, SuperSpeed USB 10Gbps, and SuperSpeed USB 20Gbps, respectively:
|Specification||Name||Previous name||USB-IF branding||Data rate||Transfer speed|
|USB 3.0||USB 3.2 Gen 1||USB 3.1 Gen 1||SuperSpeed USB 5Gbps||5 Gbit/s||500 MB/s|
|USB 3.1||USB 3.2 Gen 2||USB 3.1 Gen 2||SuperSpeed USB 10Gbps||10 Gbit/s||1.21 GB/s|
|USB 3.2||USB 3.2 Gen 2 × 2||N/A||SuperSpeed USB 20Gbps||20 Gbit/s||2.42 GB/s|
The USB4 specification was released on 29 August 2019 by USB Implementers Forum.
USB4 is based on the Thunderbolt 3 protocol specification It supports 40 Gbit/s throughput, is compatible with Thunderbolt 3, and backwards compatible with USB 3.2 and USB 2.0. The architecture defines a method to share a single high-speed link with multiple end device types dynamically that best serves the transfer of data by type and application.
The USB4 specification states that the following technologies shall be supported by USB4:
During CES 2020, USB-IF and Intel stated their intention to allow USB4 products that support all the optional functionality as Thunderbolt 4 products. The first products compatible with USB4 are expected to be Intel’s Tiger Lake series and AMD‘s Zen 3 series of CPUs, due for release in late 2020.