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Demultiplexer 8 to 1

  1. Oct 21, 2009 #1
    I understand a demultiplexer has only one input and many outputs and a multiplexer only has one output and many outputs, I figured out my truth table for a 8 to 1 multiplexer which is

    S2 S1 S0 OUT
    0 0 0 D0
    0 0 1 D1
    0 1 0 D2
    0 1 1 D3
    1 0 0 D4
    1 0 1 D5
    1 1 0 D6
    1 1 1 D7

    to figure out my truth table for my demultiplexer i get quite confused on the proper way to go about it
     
  2. jcsd
  3. Oct 21, 2009 #2

    berkeman

    User Avatar

    Staff: Mentor

    Seems like you are missing something in your truth table above, and that is part of what is confusing you about the demux circuit.

    You show the 3 control inputs, but you should also show the 8 inputs and one output. When an input is selected byt the 3 control inputs, the other inputs are don' cares (X). The output value is the value of the selected input, i.e., 0-->0 and 1-->1.

    So for your demux table, you show the 3 control inputs, and one input, then show the values of the 8 outputs. It's a 16 row table, I believe. You will also need to define whether the non-selected outputs are 0 or 1 by default.
     
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