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USB 1.1, 2.0, 3.0 & On-the-Go Components |
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Since its inception, USB has moved from a 12-Mbps (mega bits per second) bus to a 5-Gbps (giga bits per second) bus. Each incremental change has been represented by a new and improved protocol, implementation, performance and compliance specification. With each performance improvement, the USB Implementers Forum, Inc. (USB-IF) has tried to maintain backward compatibility to the previous revision level in effort to minimize detrimental impact to its installed user base. To date, USB has seen four major improvements and its latest, USB 3.0, compares very favorably as one of the fastest buses available to the PC industry and its users. |
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The Universal Serial Bus (USB) Specification 1.0 was officially released in early 1996 by a group of six forward-thinking companies. This ‘Core Group’ of forward-thinking companies was comprised of Compaq, Digital Equipment, IBM, Intel, Microsoft and Northern Telecom. Prior to the 1.0 release, the Core Group had worked closely with other concerned companies to indentify the need for a standardized ‘smart bus’ to replace the personal computer (PC) industry’s primary input/output (I/O) interfaces. These concerned companies felt the PC industry’s aging I/O interfaces, RS-232C (Serial) and IEEE-1284 (Parallel) which had been in service for more than 25-years, were not capable of supporting their coming generations of high-performance peripheral products. |
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Early in the evolutionary process of USB, the Core Group recognized the need to enlist support from all quarters of the PC industry to ensure USB’s successful implementation. They formed an industry-wide non-profit special interest group, USB Implementers Forum, Inc (USB-IF). Today, while the ‘Core Group’ has changed (Hewlett-Packard, Intel, Microsoft, NEC, ST-NXP and Texas Instruments), USB-IF continues to be a highly motivated driving force in the development, implementation and advancement of USB technology. USB has seen several major changes since its inception. These evolutionary changes have been based on the PC industry’s insatiable need for increased operational bandwidths and, most importantly, for improved performance characteristics. |
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Historically, USB’s performance enhancements can be traced as follows: |
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It is very important for first time designers to understand that the maximum bandwidths shown above do not represent USB’s actual ‘usable bandwidth.’ USB’s actual bandwidth will be somewhat reduced by protocol management requirements, transfer type, packet size, control transfer times, signaling overhead, et cetera. Also, it is very important to understand that USB 3.0 5-Gbps devices require and utilize a new and unique connector interface structure requiring a fully shielded interconnect cable. As you can see from the following cable construction and interface sketches both USB 3.0 Standard ‘A’ and Standard ‘B’ receptacles are backward compatible for use with USB 1.0, USB 1.1 and USB 2.0 peripheral devices. To use an existing USB peripheral with a USB 3.0 ‘host system,’ the end-user simply plugs his existing USB device into the appropriate USB 3.0 style receptacle and the ‘host’ will recognize the device. This backward compatibility feature is very important when more than 2.5-billion USB devices were sold and installed in 2008 and an equal number is forecast for 2009. |
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