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I have no idea about electronics, this semester school gave us a strange and confusing lecture about circuits which we shouldn't take because we are no engineer or something close. Question is we should make an 8-bits fulladder and half adder logic circuit on Logisim. Everything is fine until i am stuck with half adder circuit. I have successfully drawn 8 bit full adder

8 bit full adder

I have solved the puzzle which is connecting first Cout with second Cin. But in half adder there is only Cout. I don't know how to connect them each other.

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If you combine two half adders you get the carry-in functionality. Here's a half adder: -

enter image description here

And here's a full adder: -

enter image description here

Can you see what has happened i.e. two half adders are combined to make a full adder: -

  • Two EXOR gates provide A+B then (A+B)+Cin
  • One AND gates provide the intermediary Cout from the direct inputs of A & B
  • Another AND provides the intermediary Cout from (A+B)+ Cin
  • An OR gate produces the final Cout from the intermediary results.

enter image description here

So, in effect you have made an 8 bit adder using both "building blocks".

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  • \$\begingroup\$ I went through something like this, is it going good or is there any error? i.imgur.com/JvFHn6B.jpg \$\endgroup\$ – Sercan Kucukdemirci Mar 28 '15 at 17:24
  • \$\begingroup\$ I can't tell because there is no carry in or carry out. As a partial circuit it looks right. \$\endgroup\$ – Andy aka Mar 28 '15 at 17:27
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With the addition of an OR gate to combine their carry outputs, two half adders can be combined to make a full adder.

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