# Question about truth tables and digital logic...

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Joined: Dec 20 2003
Posts: 634

Argh, I'm so confused! Can anyone explain how a full adder truth table is constructed?

I just can't understand it:

INPUTS OUTPUTS
A B CIN COUT S
0 0 0 0 0
0 0 1 0 1
0 1 0 0 1
0 1 1 1 0
1 0 0 0 1
1 0 1 1 0
1 1 0 1 0
1 1 1 1 1

Thanks

T

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When I clicky-clicky (;)) the

When I clicky-clicky () the link it gives me two pages of Asian characters. Maybe I'll have to dust off my knowledge of logic tables later, but ATM it's midnight...

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Joined: Apr 5 2005
Posts: 232
there is already a good explanation...

...on the page you mentioned, but however, here is a short summary:

a single digit binary number is either 0 or 1. if you have a simple adder, you have one output and one overflow line. so for example, if you add two single digit binary numbers, let's say you add 1 and 1. what you would get on the output would be a 0 and a 1 on the overflow output. as binary numbers can only be 1 or 0, you can't realize a 2 on one single line, thats what the overflow is for. so, a two would be a two digit binary number, 10 represented by the overflow line and the normal output of the adder.

if you want to take the overflow bit in consideration for the adding process, you'll have to feed it back to the input. thats what the above diagram is about, where Cin would be the overflow bit of the preceding addition step.

hope this clears the situation up a little bit?

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Joined: Dec 20 2003
Posts: 634
In fact that was incredibly u

In fact that was incredibly useful For some reason I didnt realise that cin was a value you entered... Silly I know.

Anyway, I sucessful build the full adder using nand gates only and simulated it. Works great

On to the 4bit ripple through adder now. This one looks like more of a challenge...

Edit: Well it now does the addition...but I cant get it to subtract

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