Help! got a quiz tomorrow!

Discussion in 'General Electronics Chat' started by DLDnewbie, May 22, 2013.

  1. DLDnewbie

    Thread Starter New Member

    May 22, 2013
    6
    0
    Please help me answer those two questions, I got the answers as F1: 1 and F2:ABC am i correct? if not how did you get it?
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,719
    4,788
    It's a bit hard to tell if you are correct when your diagram has text callouts covering some of the gates such that it is impossible to tell if they are AND or NAND or if they are OR or NOR.

    Assuming that you only have AND, OR, and NOT gates, you answers are wrong. And, no, I will not just proceed to work your homework for you.

    You need to show your work. You have eight internal nodes. Label them and make a truth table for them. Or express each as a boolean expression of the inputs.
     
  3. DLDnewbie

    Thread Starter New Member

    May 22, 2013
    6
    0
    Here's the question again with no callouts covering the gates.
    To clarify things, this is not homework, I downloaded this textbook for extra practice.
    my solution is:_
    T1= AB
    T2= A+B
    T3= C(T2) =C(A+B)
    T4= T2+C = A+B+C
    T5= T1(C) = ABC
    T6= T1(T3) = AB(C)(A+B) = ABC(A+B) = AABC+ABBC = ABC + ABC = ABC
    T7= (T6)'(T4) = (ABC)'+ABC = 1

    therefore:
    F1:_ 1
    F2:_ ABC

    what steps have I made wrong ?
     
  4. DLDnewbie

    Thread Starter New Member

    May 22, 2013
    6
    0
    I posted a better explanation of the question.
     
  5. rahdirs

    New Member

    May 22, 2013
    28
    0
    In the solution you posted,
    T6 is output of an OR gate, so

    T6 = T1+T3(not T1(T3) as in your solution)
    F2 = T6 = T1 + T3 = AB + BC + CA
    T7 = T6'(T4) = T1'T3'T4 = (AB)'(C(A+B))'(A+B+C).(I think u cn xpand this)
    F1 = T7(T5)

    The error you made was in calculating T6.

    I think I recognize the book from that font,is it Morris Mano ??

    Hope it helps,
    rahdirs
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,719
    4,788
    rahdirs has pointed out your key mistake. But you need to learn to check your own work. As a practicing engineer or technician, you won't have someone else to do that for you. So on a clean sheet of paper, do the truth table (which you have to do for the second question anyway) and you would discover that both answers are wrong and also gain some very useful hints as to why. Now, if you make the same mistake both ways, then you won't catch it. But you clearly know the difference between AND and OR, so I'm guessing this was just a blunder, which we all make. That's why it is important to start with a clean sheet of paper (both physically and mentally).
     
  7. DLDnewbie

    Thread Starter New Member

    May 22, 2013
    6
    0
    @ WBahn, Is it possible to do the truth table before getting the boolean expression, just by using the diagram? if yes, how?
    @ rahdirs, thank you and yes it is Morris Mano 3rd edition, do you know other textbooks or maybe if there's a newer edition to study from? and if there's a solution guide for the book, I'd be grateful.
     
  8. WBahn

    Moderator

    Mar 31, 2012
    17,719
    4,788
    Sure. In fact, few people bother with a lot of boolean algebra as there are a lot of other ways to get at the results and most people are uncomfortable with boolean manipulations to perform optimizations.

    For the truth table you simply do exactly what I recommended in my first response. Make up a table that has one column for each of the inputs. Since there are three you will then have eight rows total. Then you make a column for each of the internal nodes, so eight of those, and then one column for the final output. You then simply walk across the table filling in the columns as you get the input columns associated with the gate that drives that node.
     
    DLDnewbie likes this.
  9. rahdirs

    New Member

    May 22, 2013
    28
    0
    @DLDnewbie:I do have solutions manual for the book,but i think i can't post ebooks here,against the rules of the forum.maybe i can mail u
     
  10. DLDnewbie

    Thread Starter New Member

    May 22, 2013
    6
    0
    here's my email <snip> , thank you so much!
     
    Last edited by a moderator: May 24, 2013
  11. WBahn

    Moderator

    Mar 31, 2012
    17,719
    4,788
    You should NOT put your e-mail address in a forum post (on this or any other publicly accessible forum). Bots are always crawling pages looking for e-mail addresses to add to spam lists.

    I recommend removing your address from the post and sending it to the person you want to via a private message.

    Note that I don't know if what he is doing is legal or not, I'm not trying to knowingly assist anyone break any laws.
     
  12. DLDnewbie

    Thread Starter New Member

    May 22, 2013
    6
    0
    @ Wbahn Thank you. I'm completely new to this forum, so i didn't know if there was a private message option and where I'd find it. how can I delete it?
     
  13. WBahn

    Moderator

    Mar 31, 2012
    17,719
    4,788
    In the lower right corner of the post (any post by you) there will be a "Edit" button. If there isn't one, then it means that you have to have some minimum number of posts before you can edit -- I don't know if there is or not.

    I will go ahead and report the post to a moderator and ask them to remove the e-mail address if you can't.
     
Loading...