Overview[edit]
In general, there are three basic formats of USB connectors: the default or standard format intended for desktop or portable equipment (for example, on USB flash drives), the mini intended for mobile equipment (now deprecated except the Mini-B, which is used on many cameras), and the thinner micro size, for low-profile mobile equipment (most modern mobile phones). Also, there are 5 modes of USB data transfer, in order of increasing bandwidth: Low Speed (from 1.0), Full Speed (from 1.0), High Speed (from 2.0), SuperSpeed (from 3.0), and SuperSpeed+ (from 3.1); modes have differing hardware and cabling requirements. USB devices have some choice of implemented modes, and USB version is not a reliable statement of implemented modes. Modes are identified by their names and icons, and the specifications suggests that plugs and receptacles be colour-coded (SuperSpeed is identified by blue).
Unlike other data buses (e.g., Ethernet, HDMI), USB connections are directed, with both upstream and downstream ports emanating from a single host. This applies to electrical power, with only downstream facing ports providing power; this topology was chosen to easily prevent electrical overloads and damaged equipment. Thus, USB cables have different ends: A and B, with different physical connectors for each. Therefore, in general, each different format requires four different connectors: a plug and receptacle for each of the A and B ends. USB cables have the plugs, and the corresponding receptacles are on the computers or electronic devices. In common practice, the A end is usually the standard format, and the B side varies over standard, mini, and micro. The mini and micro formats also provide for USB On-The-Go with a hermaphroditic AB receptacle, which accepts either an A or a B plug. On-the-Go allows USB between peers without discarding the directed topology by choosing the host at connection time; it also allows one receptacle to perform double duty in space-constrained applications.
There are cables with A plugs on both ends, which may be valid if the cable includes, for example, a USB host-to-host transfer device with 2 ports, but they could also be non-standard and erroneous and should be used carefully.[6]
The micro format is the most durable from the point of designed insertion lifetime. The standard and mini connectors have a design lifetime of 1,500 insertion-removal cycles,[7] the improved Mini-B connectors increased this to 5,000. The micro connectors were designed with frequent charging of portable devices in mind, so have a design life of 10,000 cycles[7] and also place the flexible contacts, which wear out sooner, on the easily replaced cable, while the more durable rigid contacts are located in the receptacles. Likewise, the springy component of the retention mechanism, parts that provide required gripping force, were also moved into plugs on the cable side.[8]
History[edit]
Large circle is left end of horizontal line. The line forks into three branches ending in circle, triangle and square symbols.
The basic USB trident logo[9]
USB logo on the head of a standard A plug, the most common USB plug
A group of seven companies began the development of USB in 1994: Compaq, DEC, IBM, Intel, Microsoft, NEC, and Nortel.[10] 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;[11][12] the first integrated circuits supporting USB were produced by Intel in 1995.[13]
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.[13] Microsoft Windows 95, OSR 2.1 provided OEM support for the devices. 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.[14] Apple Inc.'s iMac was the first mainstream product with USB and the iMac's success popularized USB itself.[15] 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.[16][17][18]
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 backward compatible with USB 2.0.[19] USB 3.0 includes a new, higher speed bus called SuperSpeed in parallel with the USB 2.0 bus.[20] For this reason, the new version is also called SuperSpeed.[21] The first USB 3.0 equipped devices were presented in January 2010.[21][22]
As of 2008, approximately 6 billion USB ports and interfaces were in the global marketplace, and about 2 billion were being sold each year.[23]
In December 2014, USB-IF submitted USB 3.1, USB Power Delivery 2.0 and USB Type-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.[24]
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