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Digital Logic; 4 inputs if and only if

  1. Nov 4, 2011 #1
    1. The problem statement, all variables and given/known data

    Using a truth table and a karnaugh map, build a minimized circuit (using only AND and OR gates) to have an output if and only if any two inputs A,B,C,D are true (let true = 1, false = 0).


    2. Relevant equations

    karnaugh maps.


    3. The attempt at a solution

    My only question about this is: will there be an output with more than two inputs being true (1) or will there only be an output when exclusively two inputs are true? Normally "if and only if" means if P then Q, and if Q then P. But, I'm tripping myself up with the language. Obviously, the two situations would have very different circuits.
     
  2. jcsd
  3. Nov 4, 2011 #2

    phinds

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    Yeah, sounds to me like there's a 1 output where there are exactly 2 inputs that are 1

    EDIT: OUCH ! ... that is one ugly kmap. I don't think that's going to simplify much at all.
     
  4. Nov 4, 2011 #3
    I know! but I'm not too worried about the kmap, its the logic! For example, if all inputs are true does this mean there is an output or is it exclusive to strictly two inputs?
     
  5. Nov 4, 2011 #4
    O sorry.. I misread your post, my k map seems to have 6 completely isolated outputs, no groups of two whatsoever. I think it comes out to like 14 gates or something :(
     
  6. Nov 4, 2011 #5

    phinds

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    Yep. Like I said, it's ugly
     
  7. Nov 5, 2011 #6
    Phinds, thank you for your input, I do have an unrelated question; I took a look at your profile and saw that you studied EE, did you work as an EE in the past? And if so, were karnaugh maps something that you did a lot? I'm interested in learning more about EE careers (I'm a physics major).
     
  8. Nov 5, 2011 #7

    phinds

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    I designed digital computers for NASA for 15 years in the 60's and 70's and actually did occasionally use kmaps, although not all that much. When you are working with circuits that are simple enough to be analysed by a kmap, you pretty much get used to doing them in your head. OTHER than designing simple digital electronics, kmaps are of little use. OH, I guess they are useful for simple symbolic logic problems in the same way they are useful for simple digital circuits.
     
  9. Nov 5, 2011 #8
    That sounds really awesome, I actually really want to work at NASA (I really want a space-related career), and I'm applying for an internship there for the summer. Any tips?
     
  10. Nov 5, 2011 #9

    phinds

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    I haven't been there since 1978 so don't have a clue what might be helpful these days but I'm sure good grades help. I was basically "drafted" by NASA in 1962 as a "co-op" because Ga. Tech where I went to school had a co-op program w/ Goddard Space Flight Center and I got the highest score any Ga. Techie had ever gotten on the Civil Service exam at the time. Since this kept me out of the military after I finished colledge ANd gave me the most exciting job I could imagine, I was in 7th heaven.

    By the mid-70's NASA had gone from being the most "can-do" organzation in the history of the world to just another govt agency, with managers playing CYA instead of getting good stuff done, so I left.

    EDIT: by the way, I'm SURE there are lots of good folks still there, but it IS just another govt agency now in terms of policy and management style. It was pretty free-wheeling in the 60's.
     
  11. Nov 5, 2011 #10
    That's quite an accomplishment! Well done, but it is a shame that such an amazing organization can go to the dogs. Could I have your opinion on these smaller firms such as SpaceEx? I personally believe that space is a very important part our future and I don't want to watch it crash and burn.
     
  12. Nov 5, 2011 #11

    phinds

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    Just because NASA is just another old line gvt agency doesn't mean there isn't good work getting done there. I got used to it being more like a "startup" so got discouraged as it matured, but that doesn't make it necessarily wrong for you, ESPECIALLY as you start out and really don't have to interact with senior management.

    I love it that we have private companies getting into space, but I'm very leery of their economic outlook. As I have posted elsewhere on this forum recently, I don't see ANY space activity other than tourism that has an ROI in anything like the near term. I want very much to be wrong about that because it is ONLY a decent ROI that will ultimately make space activity sustainable.

    Look at all the OTHER stuff that governments have to worry about, especially these days.
     
  13. Nov 5, 2011 #12
    very good points, and I can definitely agree with you, I'm just longing for the future where we are (hopefully) out colonizing planets, manufacturing and mining out in space, but yes, that is very far off...

    It would be interesting to see the predictions of an ROI on just space tourism, you are thinking much more realistically than I am.
     
  14. Nov 5, 2011 #13

    phinds

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    Richard Branson thinks it will work and although I'm dubious, he's a very bright guy. I'm just not sure the extent to which he's in it just for the excitement and might be kidding himself about profits.

    But, he's not the only one getting into it and one hopes they can't all be fooling themselves since space tourism is the ONLY way we're going to have a serious manned space presence in the near term (I'm thinking of the pilots, not the tourists).
     
  15. Dec 30, 2011 #14
    This task looks like that one - http://purefractalsolutions.com/show.php?a=xgktut/gktut6
     
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