What is the difference between a 2-of-4 decoder and a 1-of-4 decoder?
Also, how would I design a 1 of 4 decoder with an active high Enable
input, selective inputs and active low inputs?

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Clarification of Question by infogamer-ga on 30 Mar 2005 12:56 PST

Where could I find a site about truth tables(explaning how to write
truth tables for a 1-to-4 decoder, in particular)and how to build an
equation and a circuit from the truth table.(What does the Enable and
select(s)on the diagram of the decoder have to do with the outputs?)

There is no such thing as a "2 of 4" decoder, but there is a "2 TO 4"
decoder.  The terms "1 of 4" and "2 to 4" refer to the same device. 
"1 of 4" refers to the fact that only one of the four outputs are
active at any one time (assuming the enable is active), and "2 to 4"
refers to the fact that the device takes two input lines and provides
four output lines.  Either way, they are the same thing.  If you see
"2 of 4" written anywhere, it is a typo, and is supposed to say "2 to
4".

As for the rest of the question, what do you mean by "design a 1 of 4
decoder"?  Are you looking for a logic gate diagram or what?  Also, if
you want to invert the logic level of an input or output, you can just
put a NOT gate before the input or after the output.  Maybe you could
clarify?

You can also get truth tables for loads of ICs at the GIICM:

http://www.kingswood-consulting.co.uk/giicm/

Check out the 74139.  It's a "1 of 4" decoder.