Multiplexer Circuit Drawing a 18:1

Discussion in 'Homework Help' started by xellax, Oct 28, 2009.

  1. xellax

    Thread Starter New Member

    Oct 27, 2009
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    0
    Hello!
    This is my first post on these forums and I'm not entirely sure if it will be my last. I'm in an Computer architecture course [since I'm in IT it's mandatory] anyways, our teacher isn't a very good one, and we don't have a textbook. We were given an assignemnt last week which is due today, and I've been crazily searching the internet for things to help me!

    TBH she hasn't taught us about multiplexers or truth tables, and so far all I know is what I'm learned off of google. Our whole class is having problems with these questions and I was hoping someone would have figured it out by now - no luck!

    I'm just looking for someone to help explain how I am supposed to draw multiplexers. I saw on wiki that it kind of gives an explanation, but I didn't quite understand it. So far I have 18 inputs draw to 18 AND symbols and then the output of those AND to an OR, I basicall need help understand how to draw and connect the selectors

    This is the question in case I misinterpretted it.

    (3.a) If there are 18 input bits, and 1 output bit, how many bits are
    needed for the selection signal?
    (3.b) Design a circuit that multiplexes 18 input bits (x0, x1, …, x17) to 1
    output (y0) using the number of selection bits (SEL) from the answer
    of (3.a). It is required that the output:
    y0 = xi if SEL = i where i <= 18, and
    y0 = 0 if SEL = x0 where i > 17.
    Namely, if the SEL selects an input larger than 17, then the first input
    x0 is used. You are allowed to use the 4-bit multiplexers and logic
    gates if needed.

    From what I've learned so far, Iwould need five selectors as 2^4 is 16, which would be the max input for 4 selectors.

    Any help or websites or anything to drawing this would be great. It is due today and I'm obviously not expecting a miracle.

    Thanks so much!
     
    Last edited: Oct 28, 2009
  2. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    Construct a true and complemented version of each selector bit. A complemented version of a selector bit is created by passing it through an inverter.

    Now setup 18 six-input AND gates. For the 0-input use input-0 and the complement of each seletor bit.

    Y0 = I0 & S0* & S1* & S2* & S3* & S4*

    Now for each of the inputs in turn use the input bit and the combination of selector bits and their complements which are all equal to 1 to select that input

    At the end you have an 18 input OR gate that combines the 18 Y values from Y0 to Y17

    Is that enough of a hint?
     
  3. xellax

    Thread Starter New Member

    Oct 27, 2009
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    0
    Construct a true and complemented version of each selector bit.

    Do you mean a truth chart? If so could you point me in the direction of somewhere that explains how those work? If you didn't mean that, then I'm completely lost.

    As I said our prof is terrible and we on't even have a text, we're all flying blind, it kind of sucks, and if I can't understand my assignment, I'm kind of paranoid for my exam.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Get the data sheet for a 74LS151. It's 16 to 1, but completely illustrates Papabravo's directions.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    NO! I already answered that question. To make a complemented version of an input signal you have to use a device called an inverter, who's output is the complement (opposite) of the input. If the input is a '1', the output is a '0', and if the input is a '0' then the output is a '1'. On a drawing it looks like a little triangle with a bubble on the output.
     
  6. xellax

    Thread Starter New Member

    Oct 27, 2009
    4
    0
    Oh! Aha I already had that :D
    I was callign it a NOT symbol, because for question one we had to draw Logic gates and that's what it was called. Ok So I understand that part!

    I also have the 18 AND gates, yeah I'm understanding :)

    OMG I think I get it :D I'm going to draw and then upload what I drew, if I dod that will someone tell me if its right?

    Thanks SO much!!
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    Way to go man! We will certainly review whatever you care to produce and upload.
     
  8. xellax

    Thread Starter New Member

    Oct 27, 2009
    4
    0
    Woah that took a while!
    I'm hoping this is right because if not, I'm back to square one.

    I uploaded it as an attachment I do believe

    Thanks in advance for any input!
     
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