output of bcd-to-decimal decoder

Discussion in 'Homework Help' started by zulfi100, Feb 22, 2013.

  1. zulfi100

    Thread Starter Member

    Jun 7, 2012
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    Hi,
    Can somebody plz guide me about the solution of this question:

    Zulfi.
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    What decimal number is represented by that input?
     
  3. zulfi100

    Thread Starter Member

    Jun 7, 2012
    320
    0
    Hi,
    Thanks for looking into my problem. Its 14.

    Zulfi.
     
  4. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    How many output pins are there on the decoder and what do they represent?

    Do you understand what BCD refers to?
     
  5. zulfi100

    Thread Starter Member

    Jun 7, 2012
    320
    0
    Hi,
    I have found 7445. It has 4 inputs and 10 outputs. From the truth table i have found, all output (Y0 to Y9) are high when ABCD=HHHL.
    BCD=binary coded decimal. We can represent decimal numbers (0..9) with 4 bit binay code. Kindly guide me, if its correct?

    Zulfi.
     
  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    That is the correct reading of the truth table in the data sheet. Now, what does that mean?

    Look at the left hand column of that table, it is marked "INVALID" for the input in question? Look at the valid columns of the table. How are the outputs shown for a valid input? A valid output is indicated as a LOW on the decimal output corresponding to the binary number on the input.

    Also note that one of the features on the front page states that all outputs are off for invalid inputs. For this device, the outputs being HIGH indicates that the output is OFF.
     
  7. WBahn

    Moderator

    Mar 31, 2012
    17,720
    4,788
    Actually, it is not 14, it is "undefined".

    The fact that it is BCD -- binary-coded decimal -- means that there are ten valid codings. But since there are sixteen possible codings, the other six are not defined and could be anything. The designer of the circuit could make them do anything they want. They could make the chip explode if one of those six inputs is applied. In general, they won't do that because it hurts chip sales. So they will try to make it do something "reasonable" and will then tell you what they decided "reasonable" was. That could be asserting none of the outputs or it could be asserting all of the outputs. If they are going for the really cheap market they could simply design it so that it does what it is supposed to for the ten valid input patterns and then let it do whatever it ends up doing for the other six. This might save them enough transistors to be able to compete at a lower price point, leaving it up to the design using the chip to deal with the issue. The designer might well be willing to do this if it lets there product compete at a lower price point despite a bit more development time spent upfront.
     
  8. zulfi100

    Thread Starter Member

    Jun 7, 2012
    320
    0
    Hi,
    Answer is 4th pin . I dont know how? Plz guide me.
    I am using the table on page =147 of Digital principles and application, 5th edition.

    However , Question is from some other book.

    Zulfi.
     
  9. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    What are the outputs you read from the table for this input value?
     
  10. WBahn

    Moderator

    Mar 31, 2012
    17,720
    4,788
    You need to post the information from the book, not just a page reference. The vast majority of us are not going to have that specific book, even if we could tell exactly which book you are talking about without the name(s) of the author(s).
     
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