A Brief Discussion on the Hardware Structure of USB Connectors

USB uses four wire cables, two of which are serial channels for transmitting data, and the other two provide power for downstream devices. For high-speed and high bandwidth peripherals, USB transfers data at full speed of 12Mbps; For low-speed peripherals, USB transfers data at a transfer rate of 1.5Mbps.

The USB bus will automatically and dynamically switch between two transmission modes based on the external device situation. USB is a token based bus. Similar to token ring networks or FDDI token based buses. The USB host controller broadcasts a 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/restore operations. The USB system adopts a cascaded star topology, which consists of three basic parts: host, hub, and functional devices.

A host, also known as a root, root node, or root hub, is installed on a motherboard or as an adapter card on a computer. 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 hub is a specific component in the USB architecture that provides a point called a port to connect devices to the USB bus, while detecting devices connected to the bus and providing power management for these devices, responsible for bus fault detection and recovery. A hub can provide energy for the bus or provide energy for itself (by obtaining power from the outside). Devices that provide energy for themselves can be plugged into the hub that provides energy for the bus, but devices that provide energy for the bus cannot be plugged into the hub that provides energy for themselves or into downstream ports that support more than four ports. If a bus powered device requires more than 100mA of power, it cannot be connected to the hub that provides power for the bus.