USB hardware structure

USB uses four wire cables, two of which are serial channels for transmitting data, and the other two provide power for downstream devices. For any peripherals that have been successfully connected and recognized each other, data will be transmitted at the highest rate that can be supported by both devices. The USB bus will automatically dynamically switch from high speed to low speed in the compatible transmission mode according to the situation of the peripheral device and lock at a suitable rate. USB is a token based bus. Similar to 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 matches itself, and responds by receiving or sending data to the host. USB manages USB bus power by supporting suspend / resume operations. The USB system adopts a cascade star topology, which is composed of three basic parts: host, hub and functional equipment.

The host, also known as root, root node or root hub, is made on the motherboard or installed on the computer as an adapter card. The host includes a main controller and a 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. A computer may have multiple root hubs.

The hub is a specific component of the USB structure. It provides a point called port to connect devices to the USB bus, detect devices connected to the bus, provide power management for these devices, and be responsible for fault detection and recovery of the bus. The concentrator can provide energy for the bus or itself (get power from the outside).

Functional devices are connected to the bus through ports. USB can also be used as hub.