How does Universal Serial Bus USB

USB (universal serial bus)

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


The different versions of the USB interface

Modern USB interfaces meet all data rate requirements for modern peripheral devices such as external hard drives, digital cameras, video cameras, network cards, smartphones, video conference systems and many other components. In 1996 the data rate for USB 1.0 was 1.5 Mbit / s and later in the full-speed version 12 Mbit / s, 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 1.0 is referred to as low-speed (LS) with a gross data rate of 1.5 Mbit / s. This version works with NRZ-I coding and bit stuffing. 188 kB / s can be transmitted via this interface. The full-speed version (FS) of USB 1.0 has an 8 times higher data rate of 12 Mbit / s. USB 1.0 already introduced the charging of batteries, USB Battery Charging (USB-BC), which is supported by all USB versions.

The high-speed version (HS) of USB 2.0 brings it to 480 Mbit / s. It works like USB 1.0 with NRZ-I coding and bit stuffing. Version 2.0 also introduces the On-the-Go (OTG) operating mode, with which the devices can communicate directly with one another without a personal computer. Another function that was introduced with USB 2.0 is the power supply for external devices, the USB Power Delivery (USB-PD) with which powers of up to 100 W can be transmitted.

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 presented 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 that of USB 2.0. USB 3.0 achieves this data rate through the 8B / 10B coding. To compensate for the cable attenuation, USB 3.0 uses the emphasis technology, for error corrections the cyclic block check (CRC).

The USB 3.1 version passed in 2013 doubles the data rate from USB 3.0 to 10 Gbit / s. This version is called SuperSpeed ​​+ and uses the USB-C plug that can be plugged in on both sides, via which USB Power Delivery (USB-PD) also works. Parallel to the development of the USB versions, the development of the USB connector also took place, from the standard USB connector to the mini USB connector and the micro USB connector to the USB-C connector.

Data transfer from USB

The USB interface is designed as a plug-and-play interface. Peripheral devices can be connected during operation and used immediately. The USB interface recognizes the addition or removal of a device by the loading or unloading of the terminating resistors.

The data transfer within the USB system is packet-oriented, with the personal computer or a USB hub taking over the control and management of the data packets. The data packets that take over the data traffic between the USB interface and the peripheral device are limited in time to 1 ms. Each data packet has a start of frame (SOF) for the start identifier and an end of frame (EOF) for the end of the packet.

The start of frame (SOF) is always sent and is also used for synchronization when there is no data traffic. The peripheral devices are identified by their own ID number, the 8-bit proprietary identifier (PID). In addition, the data packets have a 16-bit long cyclic block check (CRC) for error detection.

The development of the USB interface

With wired USB, data is transferred using a special USB cable. It is a four-wire, 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 power the peripheral devices and can be used to charge device batteries. This application takes place 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, the Wireless USB Promoter Group has launched activities for the development of wireless USB, a wireless USB technology. This wireless USB should be able to transmit data with a data rate of 480 Mbit / s over a distance of 10 m, based on the radio standard of Ultra Wideband (UWB). 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 monitoring technology and in the PC-related connection of end devices. This WirelessUSB works 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, with the USB stick being used as an external storage component.