TopTechnical DictionaryUSB - universal serial bus

USB - universal serial bus

USB interface logo


USB port is mostly used as a mean of communication between computers and external devices, including keyboards, mice, multimedia devices or external storage devices. It can also be used to connect other devices to external equipment, e.g. a recorder connected to CCTV cameras. The device is often fitted with several USB ports to connect the external storage device or mouse for easy navigation through the menu. Similarly, Wi-Fi routers have an additional USB port for connecting the printer, which then becomes a network printer.


Common USB ports from the left: USB type A, mini USB type C and micro USB type B.


The main advantage of the USB port apart from its universal applications is that the device is ready for use immediately after connecting. Plug and Play function allows the device connected via the USB port to be identified immediately without any changes into hardware configuration. The external devices can be connected and disconnected without switching off or resetting the main device.


In PCs, the USB port is used as a basic communication interface with other external devices. While PCs can have several USB ports, they are all managed by a single controller known as a host controller. USB communication requires a single host controller and thus it is not possible to directly connect two PCs together. Sometimes, the number of available ports may not be sufficient. To increase the number of ports, we can use a special splitter, commonly referred to as a USB hub. USB interface architecture allows to connect up to 127 devices including the hubs. In practice, the number of devices may be lower.


Two types of hubs are available:


  • passive – USB hubs for connecting low-power devices, e.g. keyboards, mice or flash drives; the total power input cannot exceed 500 mA;
  • active – USB hubs with external power supply for connecting devices with power input up to 500 mA per port; compatible with external storage devices.

  • USB interface has been modified and improved for many years. Currently, USB standard can be classified into three categories by data rates available:


    USB 1.0 / 1.1


    The 1st generation USB interface offers throughput up to 1.5 Mb/s (approx. 0.2 megabyte per second). However, the standard had many issues, e.g. it did not allow USB extension cords and the cable length was strictly limited. Introducing an improved 1.1 standard has eliminated some of the issues with an increase in data rate up to 12 Mb/s (1.5 megabyte per second).


    USB 2.0 (Hi-Speed)


    The standard is still popular with the data rate up to 480 Mb/s (i.e. 60 megabytes per second). In practice, the writing speed is 25-30 MB/s and the reading speed is 30-42 MB/s in half-duplex mode. Compared to the previous standard, it was a truly revolutionary solution. With the ever-growing popularity of the USB, a new standard has been developed.


    USB 3.0 / 3.1 (SuperSpeed)


    Theoretical maximum data rate is 5 Gb/s for 3.0 and up to 10 Gb/s for 3.1 USB. The data are transferred in both directions simultaneously (full-duplex). The device may require up to 900 mA, and the SuperSpeed cable uses two additional shielded pairs of conductors.


    For 20 years from introducing the USB standard, its properties have changed significantly, however, the standard is still backward compatible with all the previous versions. USB interface can still be called “universal”.

    Although, several types of connectors and ports are used, the cable on one end is usually terminated with the most common plug - USB-A (Fig. 1).


    Fig. 1. The most popular connector - USB-A


    The other end of the USB can be:


    USB-B – most commonly used for connecting printers, scanners etc.


    mini USB-B – for connecting and charging smaller electronic devices, e.g. phones, Bluetooth headsets, portable multimedia players.


    micro USB-B – the most common connector for tablets, smartphones and other devices previously using mini USB.


    Each connector has its own port, and each has its counterpart in A version, however, those are not very popular and rarely used. USB cable can have a connector on one side and a port on the other side and can be used as an USB extension cable with different lengths, e.g. available in Delta offer: 1.5 m, 3 m or 5 m (Fig. 2).


    Fig. 2. USB extension cable


    Apart from data transmission, USB ports can also be used as a source of power supply for any device connected to the port. Most modern phones has a micro-USB port that can be used for both data transmission and for charging the battery. USB port voltage is 5V, however, the specification requires voltage between 4.75V and 5.25V.


    USB pin layout is shown below (Fig. 3), whereas tables show detailed description and colour of each conductor.


    Fig. 3. Pin layout - common USB connectors


    USB 1.0 / 2.0:




    Power supply +5V


    Data transmission -


    Data transmission +




    USB 3.0:




    Data transmission - (receive)


    Data transmission + (receive)




    Data transmission - (transmit)


    Data transmission + (transmit)


    Currently, many small electronic devices can be supplied with 5V provided by USB port. Delta offer includes high performance mains chargers with USB port or micro USB port with current rating of 1A or 2A i.e. significantly higher than provided by a standard USB port in your PC.


    A new type C connector has been introduced with USB 3.1. While the previous versions were backward compatible, the new one is not. Apart from different dimensions (type C connector is much smaller), a certain innovation has been introduced allowing the connector to be connected in any direction. The interface also provides power supply for devices that require up to 100 W. High bit rate and wide range of applications will make it possible to transmit high resolution signal to the monitor and supply it via the same cable, which might prove to be a serious competition to HDMI standard.