Karbosguide.com - Module 5c2a.
About USB
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What is USB?USB stands for Universal Serial Bus. It is a cheap and rather slow I/O bus, running at 12 Mbit/sec.It can be compared to the FireWire bus, which however is a lot speedier. USB is an open and royalty-free specification. Units can be plugged and unplugged on the fly very easily. Here you see the plugs, the two small ones, number two from the left:
There were problems with USB in the beginning, since many motherboard manufacturers produced their own versions of the port before it was fully standardized. Hence the nickname Useless Serial Bus . USB is supported by Windows 95 OSR2.1, Windows 98/Me, Windows 2000 and Windows XP.
A successUSB has become a great succes. The bus simplifies PC design - giving us a simple and unified interface for a whole lot of PC units and devices like:
All these units - and lots of others - will be connected using one single plug at the PC. USB holds up to 127 units in one long chain. The keyboard may hold a hub, so other USB units are connected here (although it more often is the monitor to include a hub, as we shall see later):
Each unit may hold two USB connectors, so they all can be daisy chained. This illustration is fiction - I never saw a setup like this, but it shows the intentions of the serial USB interface:
All units have a firmware identification code, that communicates with the OS (i.e. Windows ). The unit must have a power feed (could be minimum 100 ma) to be recognized by the USB controller and Windows 98. If one unit fails this way, Windows shows an ! on yellow background to signalize that something has to be done. This could be to unplug other USB devices to increase the available power in the chain. Many hardware manufacturers today produce their modems, cameras and scanners in versions with two-way interfaces. These devices connect either traditionally using a COM port or using the USB port.
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