# LogicWorks 4 Bit Binary Adder Question

Discussion in 'Homework Help' started by ShadowX17, Oct 4, 2012.

Oct 4, 2012
1
0
Hello, I am creating a circuit in LogicWorks so that it will add three 4 bit binary numbers. I have created the circuit, the only issue I am running into, is how I can fix the carry out? I am supposed to display an overflow for the 5th and 6th bit, but I've only managed to make it work for the 5th bit. I can't seem to wrap my mind around how to do it for the 6th bit.

I came somewhat close, except when using the XNOR gate, I get an overflow of 1 when it is only getting an output of 4 bits. Some help to fix my circuit would be great. I have attached an image of what I have done so far. As you can see from the diagram, it is showing an overflow of 1 for the 6th bit even though I am only adding 0s.

Thanks!

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2. ### WBahn Moderator

Mar 31, 2012
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6,999
If the first adder produces a carry-out, then doesn't the second adder need to be made aware of that?

Make a truth table, but do it in a summary way (because with three 4-bit inputs a full truth table would have 4096 entries). At first, just choose values for the three inputs that will result all of the various combinations of bits 5 and 6. Study that and see if you can figure out the pattern. As an alternative, look at the underlying math and determine the pattern from that (the second method is the best, if you can do it).