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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.