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universal serial bus (USB)

The Universal Serial Bus (USB) is a 4-pin, standardized I/O bus that serves as a serial interface for computer ports. Introduced in 1996, the USB interface has replaced several of the previously used serial and parallel interfaces for printers, mice, modems, keyboards, scanners, digital cameras, and other peripherals as the universal interface. The rapid development of the USB interface can be seen in the data rates and functionality of the different versions.

The different versions of the USB interface

Modern USB interfaces meet all the data rate requirements of modern peripherals such as external hard drives, digital cameras, video cameras, network cards, smartphones, video conferencing systems and many other components. While the data rate was 1.5 Mbit/s for USB 1.0 in 1996 and later 12 Mbit/s in the full-speed version, it was increased to 480 Mbit/s in the high-speed version of USB 2.0. This was followed by the super-speed versions USB 3.0 with a data rate of up to 5 Gbit/s and USB 3.1 with up to 10 Gbit/s.

USB Logo's

USB Logo's

USB 10 is referred to as Low-Speed (LS) with a gross data rate of 1.5 Mbit/s. This version uses NRZ-I encoding and bit stuffing. 188 kB/s can be transferred via this interface. The full-speed (FS) version of USB 1.0 has an 8-fold higher data rate of 12 Mbit/s. USB 1.0 already introduced battery charging, USB Battery Charging (USB-BC), which is supported by all USB versions.

USB 2.0 brings it as a high-speed version (HS) to 480 Mbps. It works like USB 1.0 with NRZ-I encoding and bit stuffing. Version 2.0 also introduces On-the-Go (OTG) mode, which allows devices to communicate directly with each other, without a personal computer. Another feature introduced with USB 2.0 is powering external devices, USB Power Delivery (USB-PD) with which power up to 100 W can be transferred.

The companies NEC-Corp, Intel, Hewlett Packard, Microsoft, NXP and Texas Instruments have set the course for a faster USB version with the USB 3.0 Promoter Group. This Promoter Group unveiled the specifications for USB 3.0, SuperSpeed USB, in November 2008. USB 3.0 has a data rate that is 5 Gbit/s, which is ten times faster than USB 2.0. USB 3.0 achieves this data rate by using 8B/10B coding. USB 3.0 uses Emphasis technology to compensate for cable attenuation, and Cyclic Block Check (CRC) for error correction.

USB connector, type A

USB connector, type A

The USB 3.1 version, adopted in 2013, doubles the data rate of USB 3.0 to 10 Gbit/s. This version is called SuperSpeed+ and uses the USB-C connector, which can be plugged in at both ends, through which USB Power Delivery (USB-PD) also works. In parallel with the development of the USB versions, the development of the USB connectors also took place, from the standard USB connector to the mini-USB connector and the micro-USB connector to the USB-C connector.

The data transmission of USB

The USB interface is designed to be a plug-and-play interface. Peripherals can be plugged in while the device is running and used immediately. The addition or removal of a device is detected by the USB interface by the load or unload of the terminating resistors.

Data transfer within the USB system is packet-oriented, with the personal computer or a USB hub controlling and managing the data packets. The data packets that handle the data traffic between the USB interface and the peripheral device are time-limited to 1 ms. Each data packet has a Start of Frame (SOF) for the initial identifier and an End of Frame (EOF) for the packet end.

USB Versions

USB Versions

The Start of Frame (SOF) is always sent and is used for synchronization even when there is no data traffic. Peripherals are identified by their own ID number, the 8-bit Proprietary Identifier (PID). In addition, the data packets have a 16-bit cyclic block check (CRC) for error detection.

The development of the USB interface

With wired USB, data is transferred over a special USB cable. This is a four-core, unshielded (for 1.5 Mbit/s) or shielded cable in which two twisted wires (D-, D+) transmit the differential signals in symmetrical form. The other two wires are used to supply power to peripheral devices and can be used to charge device batteries. This application is done with USB Power Delivery (USB-PD), which specifies five profiles for powers between 10 W and 100 W.

In addition to the wired USB interface, activities have been launched by the Wireless USB Promoter Group for the development of Wireless-USB, a wireless USB technology. This Wireless USB is said to be able to transmit data at a data rate of up to 480 Mbit/s over a distance of 10 m, based on the Ultra Wideband (UWB) radio standard. In addition, also under the name WirelessUSB, there is a standardized wireless technology that can be used for Human Interface Devices (HDI), in the industrial sector, in surveillance technology and in the connection of end devices close to the PC. This WirelessUSB operates in the ISM band with a data rate of 62.5 kbit/s.

Typical USB components include the USB hub, the USB box, the USB token and the USB stick, whereby the USB stick is used as an external memory component.

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Englisch: universal serial bus - USB
Updated at: 23.01.2018
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