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Homework Help: Even and odd parity detector with active high/low inputs

  1. Oct 13, 2016 #1
    1. The problem statement, all variables and given/known data
    Using a single 74x138, build a 3-bit, active-low, odd-parity detector. (Y0-Y7 are selected by "b2 b1 b0" in binary. Also the nibble "0 0 0" is considered even parity) You may use one NOR gate and one NAND gate, with arbitrary fan-in(>1). Label input variables "I2 I1 I0" and output variables "P".

    2. Relevant equations
    I'm just very confused about the fact that it's active low and odd parity. Does that mean the output will be a 1 for all of the inputs that have an odd number of 0's? What would happen if it was active low/even parity, active high/even parity, or active high/odd parity? Please help me to understand.

    3. The attempt at a solution
    I am thinking that I need to look for all inputs that have an odd number of 0's and the output P will be a 1. But I am not entirely sure about the answer. Also, to connect all of the outputs to a NAND gate and then to a NOR gate?
  2. jcsd
  3. Oct 15, 2016 #2


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    No this just refers to the output pin. Break that down...

    It's an "odd-parity detector" so the output will be ACTIVE when an odd number of inputs are logic 1.
    The output is "active-low" which means the output will be logic 0 when ACTIVE.
    So all together..
    The output will be Logic 0 when an odd number of inputs are logic 1.

    If the output had been described as "active-high" then the output would have been logic 1 with an odd number of logic 1's present.
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