Can Some One Explain To Me This Prom Circuit..

Discussion in 'Homework Help' started by transgalactic, May 3, 2008.

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  1. transgalactic

    Thread Starter Active Member

    Apr 29, 2008
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    i need to implement a circuit that has 8 bit input number
    and its output is the complement two of this number.
    i need to implement it using
    2^5 X 4 2^4X4 proms
    here is the solution:
    http://s290.photobucket.com/albums/ll279/t...nt=IMG_8819.jpg

    but i cant understand how they constructed the tables??

    why there is a data line goes into the address line of another prom???
     
  2. Caveman

    Active Member

    Apr 15, 2008
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    In PROM #1, the first 5 lines look like the output is the two's complement of the input, but drops the lowest bit. The last 3 lines don't look right though.
    However, they will apparently never be executed.

    In PROM #2, it looks the same except the last 3 lines are correct as well.

    The D3 output of PROM #1 is sort of like a carry output of an adder.
    The equivalent input for the most significant bit is always 0. So when you invert it, it should always be a 1. The only way it is a zero is when you do the second half of two's complement (increment by 1), if it has a carry. So that bit acts like a reverse carry. It is 1 if nothing carried out, and 0 is something did.

    If you look at the PROM #2, you will notice that it needs a reverse carry since if the A0 is 1 and goes to 0, it actually adds 1.

    All of this stuff is about pattern recognition and creativity. It is really hard to explain on a forum like this, but I hope it helps at least a little.
     
  3. transgalactic

    Thread Starter Active Member

    Apr 29, 2008
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    0
    why do you say that the MSB has to be zero in every number???
    why do you say that incrementing a number by 1 make the MSB to be 0
    100001+1=100010 (the MSB stays 1)

    i cant understand why we plug the data and the addresses
    i know that it some sort of carry
    although its really hard to imagine because its not a live number
    its some sort of collection of addresses and data
    i cant imagine this in my head

    can you please explain that on a simple number
    because i am having trouble to understand this concept
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    This is an established method for translation tables. You program the EPROM with the desired outputs. The value to be translated serves as the address to select the translated value.
     
  5. transgalactic

    Thread Starter Active Member

    Apr 29, 2008
    74
    0
    why do you say that the MSB has to be zero in every number???
    why do you say that incrementing a number by 1 make the MSB to be 0
    100001+1=100010 (the MSB stays 1)

    i cant understand why we plug the data and the addresses
    i know that it some sort of carry
    although its really hard to imagine because its not a live number
    its some sort of collection of addresses and data
    i cant imagine this in my head

    can you please explain that on a simple number
    because i am having trouble to understand this concept
     
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