# 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
74
0
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 Senior Member

Apr 15, 2008
471
1
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
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

4. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
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