Simple Multiplexer Problem

Discussion in 'Homework Help' started by Dorumon, Oct 26, 2009.

  1. Dorumon

    Thread Starter New Member

    Oct 8, 2009
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    This is a theorical question so don't ask me for the device number and stuff. Question is I have a 4 in 1 Multiplexer, and I have the following equation:

    F(ABC) = {1, 2, 6, 7}

    How do you implement this? I do have the solution but don't understand how.

    The solution utilises one of the 2 of the input as a select input(A and A' or B and B' and C or C'), a don't care and a logic level 1.

    But how does this change the fact that the Multiplexer is only able to give you 0, 1, 2 and 3 as output? And won't the output be logic level 0 if you plug in lets say C' is logic level 0? :confused:
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    It may be that if you look over how a multiplexer works - like a 74LS153, it will be more clear.
     
  3. Dorumon

    Thread Starter New Member

    Oct 8, 2009
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    But whats point of doing that? It will still select 0,1,2,3 and won't connecting to a 0 give a 0 output?

    I mean yeah, I kinda understand the concept, that the 4 inputs are being partitioned into 8 inputs of groups of 2 that can be described as each having AB with the same digits. And if Z for that grroup is 00 we plug a logic 0 and if Z for that group is 11, we plug a logic 1 else we plug plug a C or C' according.

    ABC Z
    000 0
    001 1
    010 1
    011 0
    100 0
    101 0
    110 1
    111 1

    However, wads the point of the whole thing? I dun see I6 and I7 being selected... because they don't exist... and its like prententing Z and Z' to be a logic 0 or 1 and plugging in them in the inputs which are already plugged in the Logic level 1 before that(Or not, if they are not needed to be)...So whats the point? I can't see it from a pratical point of view? Its like bluffing, its not there, but we make it to confuse people its there...

    One more thing C' is 0 wouldn't the output be 0 if C is 1? But I tot we wanted 1?
     
    Last edited: Oct 27, 2009
  4. Dorumon

    Thread Starter New Member

    Oct 8, 2009
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    Sorry, but are the outputs in alternate logic 0101010101 for 0-8?

    I've been looking at the devicesheet but I can't understand... if u plug C'... wun the output be 0? I tot we wanted a 1?
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
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    Very simply, the select bits connect one of the four separate input lines to the output.

    In your table, what is the significance of the labeled inputs? What are A, B & C? Are two selection bits and one an input? If they are all selection bit, then you are working with an eight bit multiplexer.
     
  6. Dorumon

    Thread Starter New Member

    Oct 8, 2009
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    Thats the problem, it wants me to create that the table with a 4 bit multiplexer. But I really dun understand how to, becos a 4bit multiplexer only has input 0-3... how can I get 6 and 7?

    Apparently, the correct answer uses a C, C', 0 and 1 as data inputs. Which I can kinda understand the concept but not the application.

    ABC Z
    000 0
    001 1
    010 1
    011 0
    100 0
    101 0
    110 1
    111 1

    This is the truth table.

    What we want is only 001, 010, 110 and 111. And only 2 select inputs so the only combination we can get is 00 01 10 11, so for :

    Select input 00, we have C as 1
    Select input 01, we have C', 0

    And finally we want both 110 and 111, both are expected logic 1 output, so we put 1 at there

    However... if C' is 0... the output will be 0, I told we wanted a 1? to fulfill the equation ABC = F(1,2,6,7)???
     
  7. beenthere

    Retired Moderator

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    - that means you are working with a device that has 4 inputs and only one output.
     
  8. Dorumon

    Thread Starter New Member

    Oct 8, 2009
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    Yeah I know, but how does doing the above gives me a 6 or 7? Or is it purely a theorical thingy... because I get a 1 or 0 at the end anway.

    But even so... if I connect C', I get a 0 but I want a 1. why is that so then?
     
  9. beenthere

    Retired Moderator

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    You can't have this condition -
    A more standard notation denotes the address bits as separate from the input bits. The output may be complimented. Only one single bit may be selected to appear at the output.

    The two address bits select which one of the four inputs will be at the output. The state of the selected input determines the state of the multiplexer output.

    A four to one multiplexer has only the ability to produce a logic 1 or 0 at the output. The output is determined by which of the four inputs is the selected input.
     
  10. Dorumon

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    Oct 8, 2009
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  11. beenthere

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    Nope, none at all. From the data sheet of a 74LS151, you can see how the internal logic works. It's very simple. A 74LS153 is a dual 4 bit multiplexer, but a bit harder to see the inner workings.

    I do not know where the additional terms C & C' come from in the slides.
     
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