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Truth table question

  1. Oct 4, 2015 #1
    1. The problem statement, all variables and given/known data
    The question is mostly in the picture, it has tables associated with it so I couldn't put it here.

    QUESTION: Make a truth table of the controller outputs enable/direction, as a function of the inputs S1, S2, R1, and R2. Since you have not been given the explicit output for some inputs, to minimize the number of gates in the circuit, it is desirable to mark these outputs as “don’t care”, denoted by “X” and meaning that either a 0 or 1 is acceptable.

    2. Relevant equations

    3. The attempt at a solution
    I'm a bit confused at the question,

    should my truth table be something like..

    S1 | S2 | R1 | R2| Enable Direction
    0 0 0 0 X X
    0 0 0 1 ? ?
    0 0 1 1 ? ?
    0 1 1 1 1 0
    1 1 11 0 X
    I have an input for (R1 R2 ) 0 1, but no input for S1 and S2. Should I put X X again or am I doing this wrong? If either side has no input(aka 0 0) should I put X X for both enable and direction?

    For the next part, 0 1 1 1... should it be 1 0? Meaning I am on second floor, and want to go to ground floor. I want it to move down two floors, but the direction command only says go down 1 floor for the 0 input.

    For 1 1 1 1... should it be 0 X? I want the door to open but direction doesn't have to do anything.

    Attached Files:

    • S0.png
      File size:
      57.3 KB
  2. jcsd
  3. Oct 4, 2015 #2


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    Science Advisor
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    Gold Member

    First identify all the impossible inputs, and mark the outputs as don't care. Impossible is if either the source or destination bit pair indicates a non-floor.

    Re the second question, 1 0 is fine, because once the lift gets to the 1st floor it will have a new Current Floor, and hence get a new instruction, which will be to go down again.

    Re your last question, that sounds best, unless we wish to punish the lift occupant for pressing the wrong floor by trapping them in the lift. :wink:
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