3 bit adder/subtraction

Discussion in 'Homework Help' started by aka1adrian, Apr 13, 2011.

  1. aka1adrian

    Thread Starter New Member

    Apr 13, 2011
    2
    0
    Can anyone help me design a circuit that has inputs of two 3 bit numbers, A and B, and two selct bits S1 and S0. There is a 4 bit output, X which depends on A and B according to this if S1 S0 X=
    0 0 A
    0 1 A+B
    1 0 A-B
    1 1 B-A

    Using 7483, 74151 and primitive gates. I would appreciate if some one could help me design this in Multisim. Thank You
     
  2. guitarguy12387

    Active Member

    Apr 10, 2008
    359
    12
    Can you post your attempt at the problem?

    Adders/subtractors are pretty standard... they should have been covered in a basic digital logic textbook.

    Hint: use a mux controlled by the select lines
     
  3. aka1adrian

    Thread Starter New Member

    Apr 13, 2011
    2
    0
    This is the attempt I did but it is way to complex. Can anyone try to simplify it? This needs to be built in class and I fear I will not be able to do it with all of the gates that there are. Any help anyone can give would be greatly appreciated.
     
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  4. guitarguy12387

    Active Member

    Apr 10, 2008
    359
    12
    Wowzers, that's too many gates hah! I have no idea what's happening. You should be able to do this with more or less one mux, one adder, and some supporting logic.

    Okay, you already have full adders and muxes available to you.

    Google "ripple carry adder"

    Better yet,

    http://en.wikipedia.org/wiki/Adder-subtractor
     
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