CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [304]
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NOTE USB 2.0 defined more than just a new speed. Many low-speed and Full-Speed USB devices are also under the USB 2.0 standard.
In addition to a much faster transfer rate, Hi-Speed USB is fully backward compatible with devices that operate under the slower USB standards. Those old devices won’t run any faster than they used to, however. To take advantage of the fastest USB speed, you must connect Hi-Speed USB devices to Hi-Speed USB ports by using Hi-Speed USB cables. Hi-speed USB devices function when plugged into Full-Speed USB ports, but at only 12 Mbps. Although backward compatibility at least allows you to use the newer USB device with an older port, a quick bit of math tells you how much time you’re sacrificing when you’re transferring a 240-MB file at 12 Mbps instead of 480 Mbps!
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EXAM TIP The USB Implementers Forum (USB-IF) does not officially use “low-speed” and “full-speed” to describe 1.5 Mbps and 12 Mbps devices, calling both of them simply “USB.” On the CompTIA A+ certification exams, though, you’ll see the marketplace-standard nomenclature used here.
When USB 2.0 came out in 2001, folks scrambled to buy USB 2.0 controllers so their new hi-speed devices would work at their designed speeds. Of the variety of solutions people came up with, the most popular early on was to add a USB 2.0 adapter card like the one shown in Figure 18-5.
Motherboard makers quickly added a second USB 2.0 host controller—and they did it in a clever way. Instead of making the USB 2.0 host controller separate from the USB 1.1 host controller, they designed things so that both controllers share all of the connected USB ports (Figure 18-6). That way, no matter which USB port you choose, if you plug in a low-speed or full-speed device, the 1.1 host controller takes over, and if you plug in a hi-speed device, the USB 2.0 host controller takes over. Clever, and convenient!
Figure 18-5 USB adapter card
Figure 18-6 Shared USB ports
USB 2.0 has remained the standard for quite a while, but, as of this writing, the future of USB is nigh! USB 3.0 (also called SuperSpeed) devices are set to appear on the market sometime in 2010, with massively increased speed (up to 4.8 Gbps), increased power to peripherals, and full backward compatibility with older devices. USB 3.0 probably won’t show up on the CompTIA A+ exams until it becomes widely adopted, but you should definitely be aware of it, because if it retains the popularity USB has enjoyed up to this point, it’s going to be huge.
NOTE Using a PC running Windows 2000 or later, open the Device Manager and locate two controllers under the Universal Serial Bus icon. The one named Standard Enhanced Host Controller is the hi-speed controller. The Standard OpenHCD Host Controller is the low- and full-speed controller.
USB Hubs and Cables
Each USB host controller supports up to 127 USB devices, but as mentioned earlier, most motherboard makers provide only six to eight real USB ports. So what do you do when you need to add more USB devices than the motherboard provides ports? You can add more host controllers (in the form of internal cards), or you can use a USB hub. A USB hub is a device that extends a single USB connection to two or more USB ports, almost always directly from one of the USB ports connected to the root hub. Figure 18-7 shows a typical USB hub. USB hubs are sometimes embedded into peripherals. The keyboard in Figure 18-8 comes with a built-in USB hub—very handy!
Figure 18-7 USB hub
Figure 18-8 USB keyboard with built-in hub
USB hubs are one of those parts of a PC that tend not to work nearly as well in the real world as they do on paper. (Sorry, USB folks, but it’s true!) USB hubs have a speed just like any other USB device; for example, the hub in the keyboard in Figure 18-8 runs at full-speed. This becomes a problem when someone decides to insert a Hi-Speed USB device into one of those ports, as it