A.21 U

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UART (Universal Asynchronous Receiver/Transmitter) -

UDP (User Datagram Protocol) - a connectionless method for transmitting packets to other hosts on the network. It is seen as a counterpart to TCP.

ultrasonic - sound or vibration at a frequency above that of the ear (> 16KHz typ.)

ultraviolet - light with a frequency above the visible spectrum.

UNIX - a very powerful operating system used on most high end and mid-range computers. The predecessor was Multics. This operating system was developed at AT & T, and grew up in the academic environment. As a result a wealth of public domain software has been developed, and the operating system is very well debugged.

UPS (Uninterruptable Power Supply) -

user friendly - a design scheme that simplifies interaction so that no knowledge is needed to operate a device and errors are easy to recover from. It is also a marketing term that is badly misused.

user interfaces - are the means of communicating with the computer. For CAD applications, a graphical interface is usually preferred. User friendliness is a measure of the ease of use of a program and implies a good user interface.

UUCP (Unix to Unix Copy Program) - a common communication method between UNIX systems.