Chapter 2: The Big Picture

2.0 Systems and Devices

Systems based on USB 2.0 are designed to support high-speed, full-speed, and low-speed devices. This provides backward compatibility to 1.x devices, while significantly extending USB performance, and consequently, increasing the number of devices that can be supported.

USB 2.0 is backward compatible with 1.x devices and has many of the same characteristics:

•uses the same connectors

•uses FS cables for HS devices

•employs the same communications model (token/data/handshake)

•uses the same device attachment recognition

•uses the same device configuration model

The 480Mb/s transfer rate of USB 2.0 is 40 times faster than the 12Mb/s transfers of USB 1.x. The faster transfers are intended to permit a greater number of USB devices on a single bus. Additionally, both 1.x devices in HS system lowspeed (LS) devices and full-speed (FS) devices are supported without significantly impacting the performance of high-speed (HS) devices. Figure 2-9 on page 37 illustrates a USB 2.0 system with devices attached to a variety of ports.

Figure 2-9: USB 2.0 System with Low-, Full-, and High-Speed Devices Attached

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