Using a comparator to compare 2's complement numbers

Discussion in 'Homework Help' started by wind_blast942, Nov 18, 2010.

  1. wind_blast942

    Thread Starter New Member

    Sep 7, 2010
    22
    0
    Hi all

    I have this question where i am supposed to use a 74'85 comparator (with some other gates) to output a 1 whenever a 2's complement number is between -5 to 3.

    My first thought since the table

    0011 3 0010 2 0001 1 0000 0 1111 −1 1110 −2 1101 −3 1100 −4 1011 −5

    is to check whether the 2 last bits are 00 or 11, since the first 2 bits are always (00,01,10,11), but i am not sure how to implement it on a comparator.

    Any kind soul could point me in the right direction?

    thanks.
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    I would get the datasheet for the 7485. Note that the device has outputs for A>B and A<B as well as equality.

    Start with constructing a truth table and see what you can do.
     
  3. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    A comparator compares two numbers. In your case, one is variable and the other is set by you.

    When your number is positive (MSB=0) you compare with 4 and look at the "<" output.
    When your number is negative (MSB=1) you compare with 2 and look at the ">" output.

    Does that logic put you in any thoughts?
     
  4. wind_blast942

    Thread Starter New Member

    Sep 7, 2010
    22
    0
    Ah ok i get it now.

    So basically need to get a SOP expression as a function of (+ve/-ve flag) to generate 2 or 4, plug into the b0-b4 input, plug the input into the a0-a4 input, and then combine the (+ve/-ve flag) with the "<" or ">" output to give a 1 whenever (+ve AND <) or (-ve AND >) is true.
     
  5. Georacer

    Moderator

    Nov 25, 2009
    5,142
    1,266
    You got it! MUXs can help you here if you don't want to use many ICs. You can use three 2-to-1 MUX for the threshhold input and one 2-to-1 to get the correct output signal (">" or "<").
     
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