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CompTIA A+ Certification All-in-One Exam Guide

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certain number of USB ports, there’s no guarantee that the motherboard

maker will supply that many ports. To give a common example, the AMD

X370 chipset supports 16 USB ports, but only a few motherboard makers

supply that many USB ports.

A USB host controller is the boss, the master, of any device (the slave)

that plugs into that host controller. The host controller sends commands and

provides power to USB devices. The host controller is upstream, controlling

devices connected downstream to it (see Figure 10-3). The host controller is

shared by every device plugged into it, so speed and power are reduced with

each new device.

Figure 10-3 Host controller and USB mouse showing upstream/downstream

USB Standards and Compatibility

The USB standard has gone through several revisions:

• USB 1.1 was the first widely adopted standard and defined two speeds:

Low-Speed USB, running at a maximum of 1.5 Mbps (plenty for

keyboards and mice), and Full-Speed USB, running at up to 12 Mbps.

• The USB 2.0 standard introduced Hi-Speed USB running at 480 Mbps.

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