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Help needed for using a decoder

Discussion in 'Electronic Basics' started by [email protected], Dec 29, 2005.

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  1. Guest

    Hi all, I was wondering if anyone could help with the following
    question.

    For the function, F(A, B, C, D) = (0, 2, 4, 5, 8, 10, 12, 13) give the
    minimised sum-of-products solution.
    Devise a realisation that uses a decoder with active-low outputs and
    one other gate.

    I have a solution of (NOT B AND NOT D) OR (B AND NOT C) but I dont know
    how to use a decoder in this situation. Can I use a 3-line-to-8-line
    one for this? Any suggestions?

    Also very quickly, why are general purpose encoders not very useful?

    Thank you

    strictly_mk
     
  2. PeteS

    PeteS Guest

    As you end up with three terms in your solution, (and I am not sure of
    the terminology above to help with your minimisation - is D the least
    significant bit?) then a three to eight decoder (such as the '138)
    would work, but then you'd have to OR the various outputs together.

    I answered a post yesterday that used a '156 decoder, which would be
    perfect in this particular case, as you have two two product terms and
    the '156 has two 2-4 decoders. You'll still have to OR some outputs
    together (but don't be foled by the term OR - a NAND is just as
    usefully an OR and a NOR an AND if you know how to do it - see
    DeMorgan's theorem).

    As decoders decode minterms by definition, the tough stuff is done in
    hardware. You'll have to figure out the rest (I could, but someone else
    beat you to my 'help with the homework on the season of goodwill'
    quota).

    Datasheet at:
    http://www.onsemi.com/pub/Collateral/SN74LS155-D.PDF
    Who says? They have their uses, although they are not generally as
    useful as decoders, in terms of the issues we come up against. Great
    for encoding banks of switches (although there are other methods)
    amongst many other things.
    You're welcome

    Cheers

    PeteS
     
  3. Rich Grise

    Rich Grise Guest

    I don't even know what this means.

    Can you be more specific?

    Thanks,
    Rich
     
  4. PeteS

    PeteS Guest

    A,B,C,D, the function evaluates true when the values of A.B,C,D == 0,
    2,4,5,8,10,12,13, or alternatively (for D = msb)

    D B C A Y
    0 0 0 0 1
    0 0 0 1 0
    0 0 1 0 1
    0 0 1 1 0
    0 1 0 0 1
    0 1 0 1 1
    0 1 1 0 0
    0 1 1 1 0
    1 0 0 0 1
    1 0 0 1 0
    1 0 1 0 1
    1 0 1 1 0
    1 1 0 0 1
    1 1 0 1 1
    1 1 1 0 0
    1 1 1 1 0

    Cheers

    PeteS
     
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