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74ls138 decoder problem

  1. Feb 15, 2015 #1
    1. The problem statement, all variables and given/known data
    I'm doing some review for an exam this week. this problem was on the review sheet, but we did not cover any examples like it during lecture. The problem is:

    Show how to implement the function f(w3, w2, w1) = ∑m(0, 1, 3, 4, 6). Use a 74LS138 and NAND gate(s)



    2. Relevant equations


    3. The attempt at a solution
    I think I'm just not understanding what's being asked and this is far more simple than I'm making it. I reduced the function to !w3(NAND(!w1w2) + w3(!w1), I'm just not seeing the method to implement it using the 74ls138. Any advice as to the method to do this? I don't need the exact answer, just the path I need to take to solve this. Thanks.
     
  2. jcsd
  3. Feb 15, 2015 #2

    gneill

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    Staff: Mentor

    The 74LS138 is a decoder. The three bits of input select one of eight outputs. You can just pick off the terms that match whatever function you're interested in. Then it's up to you to effectively OR them to yield a "1" when any of the selected terms are "hit" by the inputs. Note that it is sometimes better to select for the complement f the function if the number of terms is fewer.
     
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