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Home > How To > What is USB?
What is USB?

What is USB?

By  Gay Pizano 2020-01-04 1438 23

USB is a kind of interface technology used in PC, which is really plug and play. The full name of Universal Serial Bus,USB in English is abbreviated, and it is called Universal Serial bus in Chinese. It first appeared in 1995, with the development of Pentium machine. Since Microsoft added support for USB interface in Windows 98, USB interface has been popularized, and there are more and more USB devices, such as digital cameras, cameras, scanners, joysticks, printers, keyboards, mice and so on, among which cameras and USB drives are the most widely used.

USB includes the old USB 1.1 standard and the popular USB 2.0 standard. The traditional USB 1.1 has a maximum transmission rate of 12Mbps, and the products that meet the USB 1.1 standard are called "full-speed USB" by general manufacturers. However, when the high-speed USB2.0 was first launched, the highest transmission rate was only 240 Mbps, and then the USB2.0 promotion group (USB Promoter Group) increased the rate to 480 Mbps in October 1999, 40 times faster than the traditional USB 1.1. USB2.0 is backward compatible with USB1.1, and of course USB1.1 devices are also "upward compatible" with USB2.0, but cannot achieve the transmission capacity of USB2.0 and automatically transmit at a low speed. The maximum length of the USB 2.0 cable is 5 meters, but if five USB adapters are used, the maximum length can be up to 30 meters.

The traditional USB 1.1 certified product icon is black and blue and marked with "USB" and "Certified", while the USB 2.0 certified product icon is made up of blue, white and red and marked with "Certified" and "Hi-Speed". These certified product icons are part of the USB Promotion Group brand program to ensure the quality of USB products. Although

USB 1.1 has the data transfer rate of 12Mbps, it is often only about 1MB/s in practical use, so it is only suitable for low-speed devices such as printers and scanners. At present, the actual data transfer rate of USB 2.0 has been able to reach more than 20MB/s, and it can better support fast devices such as mobile hard drives, but because USB 2.0 has downward compatibility, if you do not have the following conditions, you may still be using the lower USB 1.1 transmission mode! The complete USB2.0 connection includes USB2.0 interface control card, USB2.0 driver and USB2.0 peripherals.

The first step is to verify that the motherboard has USB 2.0 ports. Open "device Manager", expand the "Universal Serial bus" section, if you see the "Enhanced" USB host controller option, this means that the computer motherboard supports USB 2.0 interface devices. The name varies from Windows to Windows. At present, the mainstream motherboards basically have USB 2.0 ports.

Second, due to compatibility issues when USB 2.0 was first introduced, some motherboards provide the ability to turn off USB 2.0 in BIOS. If it is not opened in BIOS, it is automatically backward compatible with USB 1.1 when used. There are also some motherboards that provide full-speed (Full Speed) and high-speed (Hi Speed) in BIOS, corresponding to USB 1.1 and USB 2.0 respectively. When using USB 2.0, we should pay attention to choosing high-speed state in BIOS.

Thirdly, the operating system should support USB 2.0, and USB 2.0 drivers are installed. At present, only the later versions of Windows 2000 SP3 and Windows XP SP1 can really support USB 2.0 in Microsoft products. And Windows 98 system, because most manufacturers no longer provide drivers under Windows 98, so generally do not support USB 2.0.

Finally, of course, the product should support the USB 2.0 interface. For example, cameras, some use USB 1.1, some use USB 2.0. If you want to achieve USB 2.0, you should pay attention to the selection of USB 2.0 products.

As long as one of the above four conditions is missing, the high-speed connection of USB 2.0 cannot be realized. However, colleague Lao Zhang's motherboard does not have a USB2.0 port, so it is impossible to achieve high-speed USB2.0 transmission. In addition to replacing the motherboard, you can also purchase a separate USB2.0 expansion card, which can also solve the problem.

USB uses four-wire cable, two of which are serial channels used to transmit data, and the other two provide power for downstream (Downstream) equipment. For high-speed and high-bandwidth peripherals, USB transmits data at full speed 12Mbps; for low-speed peripherals, USB transmits data at the transmission rate of 1.5Mbps. The USB bus will automatically and dynamically switch between the two transmission modes according to the conditions of the peripherals. USB is a token-based bus. Similar to token Ring Network or FDDI token-based bus. The USB master controller broadcasts the token, and the device on the bus detects whether the address in the token matches itself, and responds by receiving or sending data to the host. USB manages USB bus power by supporting suspension / restore operations. The USB system adopts cascaded star topology, which consists of three basic parts: host (Host), hub (Hub) and functional equipment.

The host, also known as root, root junction or root Hub, is done on the motherboard or installed on the computer as an adapter card. The host contains a master controller and a root hub (Root Hub), that controls the flow of data and control information on the USB bus. Each USB system can only have one root hub, which is connected to the main controller.

A hub is a specific component of the USB architecture, which provides a point called port (Port) to connect devices to the USB bus, detects devices connected to the bus, provides power management for these devices, and is responsible for bus fault detection and recovery. The bus can provide energy for the bus, and it can also provide energy for itself (getting power from the outside). The devices provided by the bus can be plugged into the hubs that provide energy from the bus, but the devices provided by the bus cannot be plugged into the hubs that provide energy by themselves or support more than four downstream ports. For example, when the needs of the energy devices provided by the bus exceed the power supply of the 100mA, they cannot be connected with the hubs that provide power with the bus.

The functional device is connected to the bus through the port. USB can also be used as Hub. 

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