USB is the abbreviation of Universal Serial Bus (Universal Serial Bus) in English, and its Chinese abbreviation is "serial line". It is an external bus standard, which is used to standardize the connection and communication between computers and external devices. It is an interface technology applied in PC field. USB interface supports plug-and-play and hot-plug functions of devices. USB was proposed by Intel, Compaq, IBM and Microsoft at the end of 1994.
USB uses four-wire cable, two of which are serial channels for data transmission, and the other two provide power for downstream devices. For high-speed and high-bandwidth peripherals, USB transmits data at full speed of 12 Mbps; for low-speed peripherals, USB transmits data at 1.5 Mbps transmission rate. The USB bus will automatically and dynamically convert between the two transmission modes according to the peripheral conditions. USB is a token-based bus. Similar to token-ring networks or FDDI token-based buses. USB master controller broadcasts tokens. 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 the USB bus power supply by supporting suspension/recovery operations. USB system adopts cascade star topology, which consists of three basic parts: Host, Hub and functional devices.
Host, also known as root, root or root Hub, is installed on the main board or as an adapter card on the computer. The host contains the main controller and root Hub, which 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.
Hub is a specific component of the USB structure. It provides a point called Port to connect devices to the USB bus. It also detects the devices connected to the bus and provides power management for these devices. It is responsible for bus fault detection and recovery. Hubs can provide energy for buses, and also for themselves (power from outside). The equipment that provides energy for itself can be inserted into the hubs that provide energy for buses, but the equipment that provides energy for buses can not be inserted into the hubs that provide energy for themselves or support more than four downstream ports, such as buses providing energy. When the equipment needs more than 100 mA power supply, it can not be connected with the hub which provides power from the bus.