USB uses a four-wire cable. Two of them are serial channels for transmitting data, and the other two provide power for downstream devices. Any peripherals that have been successfully connected and recognized by each other will be supported by both devices Data at the highest rate. The USB bus will automatically change dynamically from high speed to low speed in a compatible transmission mode according to the peripheral conditions and the matching is locked at the appropriate rate. USB is a token-based bus. Similar to a Token Ring network or FDDI token-based bus. The USB host controller broadcasts the token, and the device on the bus detects whether the address in the token is consistent with itself, and responds by receiving or sending data to the host. USB manages USB bus power by supporting suspend / resume operations. The USB system uses a cascaded star topology. The topology consists of three basic parts: Host, Hub, and functional devices.
The host, also known as the root, root knot, or root hub, is installed on the motherboard or installed on the computer as an adapter card. The host contains a main controller and a root hub (Root Hub), which controls the data on the USB bus and To control the flow of information, each USB system can only have one root hub, which is connected to the main controller. A computer may have multiple root hubs.
A hub is a specific component in the USB structure. It provides a point called a port to connect devices to the USB bus. It also detects devices connected to the bus and provides power management for these devices. It is responsible for bus fault detection and recovery. . The hub can provide power for the bus, or it can provide power for itself (from external sources).
Functional devices are connected to the bus through ports. USB can also be used as a hub.