19-09-2017, 10:42 AM
USB is an abbreviation for Universal Serial Bus. It is an industry standard that defines cables, connectors, and communications protocols for connecting, communicating, and powering computers and devices.
USB was designed to standardize the connection of computer peripherals (including keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters) to personal computers, both for communication and for power supply. It has largely replaced a variety of earlier interfaces, such as serial ports and parallel ports, as well as separate power chargers for handheld devices - and has become commonplace across a wide range of devices.
Created in the mid-1990s, it is currently developed by the USB Implementers Forum (USB IF). In general, there are three basic formats of USB connectors: the default or the standard format for desktop or portable computers (eg USB flash drives), the mini for mobile devices (now disapproved except the Mini-B, which is used in many cameras), and micro-thinner size, for low-profile mobile equipment (most modern mobile phones). In addition, there are 5 USB data transfer modes, in order of increasing bandwidth: low speed (from 1.0), maximum speed (from 1.0), high speed (from 2.0), SuperSpeed (from 3.0) and SuperSpeed + (from 3.1); different modes of hardware and cabling. USB devices have some option of implemented modes, and the USB version is not a reliable declaration of the modes implemented. Modes are identified by their names and icons, and specifications suggest that plugs and receptacles should be color-coded (SuperSpeed is identified by blue).
Unlike other data buses (eg Ethernet, HDMI), USB connections are routed, with both upstream and downstream ports emanating from a single host. This applies to electric power, with only downstream facing ports providing power; this topology was chosen to easily avoid electrical surges 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 a receptacle for each end A and B. USB cables have plugs, and the corresponding receptacles are on 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 USB On-The-Go with a hermaphrodite AB receptacle, which accepts an A or B plug. On-The-Go allows USB between pairs without ruling out the directed topology by choosing the host at the time of connection ; also allows a receptacle to perform dual function in applications with limited space.
There are cables with A plugs at both ends, which may be valid if the cable includes, for example, a USB host-to-host transfer device with 2 ports, but it could also be non-standard and erroneous and should be used with care.
The micro format is the most durable from the point of view of the designed insertion life. The standard and mini connectors have a design life of 1,500 insertion-elimination cycles, the improved mini-B connectors increased this number to 5,000. The micro-connectors were designed with a frequent load of portable devices in mind, so they have a design life of 10,000 cycles and also place the flexible contacts, which wear sooner, into the easily replaced cable, while the rigid contacts more durable ones are in the containers. Likewise, the resilient member of the retention mechanism, portions that provide the required gripping force, also moved in the plugs on the cable side.
USB was designed to standardize the connection of computer peripherals (including keyboards, pointing devices, digital cameras, printers, portable media players, disk drives and network adapters) to personal computers, both for communication and for power supply. It has largely replaced a variety of earlier interfaces, such as serial ports and parallel ports, as well as separate power chargers for handheld devices - and has become commonplace across a wide range of devices.
Created in the mid-1990s, it is currently developed by the USB Implementers Forum (USB IF). In general, there are three basic formats of USB connectors: the default or the standard format for desktop or portable computers (eg USB flash drives), the mini for mobile devices (now disapproved except the Mini-B, which is used in many cameras), and micro-thinner size, for low-profile mobile equipment (most modern mobile phones). In addition, there are 5 USB data transfer modes, in order of increasing bandwidth: low speed (from 1.0), maximum speed (from 1.0), high speed (from 2.0), SuperSpeed (from 3.0) and SuperSpeed + (from 3.1); different modes of hardware and cabling. USB devices have some option of implemented modes, and the USB version is not a reliable declaration of the modes implemented. Modes are identified by their names and icons, and specifications suggest that plugs and receptacles should be color-coded (SuperSpeed is identified by blue).
Unlike other data buses (eg Ethernet, HDMI), USB connections are routed, with both upstream and downstream ports emanating from a single host. This applies to electric power, with only downstream facing ports providing power; this topology was chosen to easily avoid electrical surges 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 a receptacle for each end A and B. USB cables have plugs, and the corresponding receptacles are on 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 USB On-The-Go with a hermaphrodite AB receptacle, which accepts an A or B plug. On-The-Go allows USB between pairs without ruling out the directed topology by choosing the host at the time of connection ; also allows a receptacle to perform dual function in applications with limited space.
There are cables with A plugs at both ends, which may be valid if the cable includes, for example, a USB host-to-host transfer device with 2 ports, but it could also be non-standard and erroneous and should be used with care.
The micro format is the most durable from the point of view of the designed insertion life. The standard and mini connectors have a design life of 1,500 insertion-elimination cycles, the improved mini-B connectors increased this number to 5,000. The micro-connectors were designed with a frequent load of portable devices in mind, so they have a design life of 10,000 cycles and also place the flexible contacts, which wear sooner, into the easily replaced cable, while the rigid contacts more durable ones are in the containers. Likewise, the resilient member of the retention mechanism, portions that provide the required gripping force, also moved in the plugs on the cable side.