# A 3-bit and 4-bit binary adder.

Discussion in 'General Electronics Chat' started by Yash_Arya, Sep 21, 2008.

1. ### Yash_Arya Thread Starter New Member

Sep 21, 2008
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Hello, I was trying to make a 4-bit binary Adder and I reffered to some sites here and there but I could'nt get my concepts clear.

Could anyone please tell me (show me?) how a circuit of a 4-bit binary adder would be? Using standard Gates and ICs only.

Also, how would it be different from a 3-bit adder?

Thank you, Greetings.
Yash Arya.

2. ### scubasteve_911 Senior Member

Dec 27, 2007
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The 3 and 4-bit adders originate from a single bit adder, just scaled up.

The following link shows a single bit adder, then, how it is hooked up to make 4-bits.

I'm pretty sure that the logic isn't optimized for low-gate count, but it will work fine. It is possible to write out karnaugh maps or do boolean algebra to reduce the number of gates needed, but that's another level of complexity.

Steve

Or, an allaboutcircuits reference is here too

3. ### beenthere Retired Moderator

Apr 20, 2004
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Get the data sheet on a 74XX83. It is a 4 bit full adder. The data sheet should include the internal logic diagram.

4. ### harsimranjit singh New Member

Sep 22, 2008
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whats 3 and 4 bit adder////i mean binary

Apr 20, 2004
15,815
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6. ### Yash_Arya Thread Starter New Member

Sep 21, 2008
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Thank you for your help people, now I think I do not need any further guidance unless I myself read the resources that you kindly provided.

However, (obviously) if someone else needs to clear something, go right ahead.

Thank you,
Yash Arya.

7. ### Yash_Arya Thread Starter New Member

Sep 21, 2008
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**I already have querries to be cleared**

I went through the page [http://www.play-hookey.com/digital/adder.html] you provided me and I have the following querries.

(1) In the Full-Adder part, if we see the truth table, We have A,B (i/p) and Cin, Cout. Here, Is A itself a multi-bit number (and same for B)? Or do A, B, Cin and Cout together make a "multi-bit number"?

(2)If, A is itself a multi bit number, then how come in the truth table we give it only "0" or "1" in its coloumn? I mean, the number will be something like "A = 1011" for example.

Thank you,
Yash Arya.

8. ### veritas Active Member

Feb 7, 2008
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For each individual full adder, A and B are only 1 bit, and Cin/Cout are for those bits. For a multiple-bit adder, Cin goes to the least significant bit, and Cout comes from the most significant bit. The other carries are internal.

9. ### Yash_Arya Thread Starter New Member

Sep 21, 2008
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Okay, so let us speak in terms of "order" of 'A' and 'B'. Suppose that 'A' and 'B' are bits of order 'n'. (i.e A[n] and B[n]) and the carry from previous bit is C[n-1].

Now to add A[n], B[n] and C[n-1], we need one full adder. Am I correct? So, for 4-bit adder, will I need four such full-adders? (Heck that will be complex)

Also, let us say that after the Addition, I recieve Sum (Sn) and Carry out(Cn).

Now what all should be taken as i/p for the next full-adder?

From what I understood, It will be A[n+1], B[n+1], C[n].

Am I correct?

Also, please tell me some suitable IC for the same. (The ICs I have in my syllabus are 7432, 7408, 7404, 7402, 7400, 7486, 74135) The ICs must be from the list only.

Thank you all for your co-operation.

Yash Arya.

10. ### veritas Active Member

Feb 7, 2008
167
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You are correct on all counts, I believe. If you have only logic gates, you will need to create logic for Sum[n] and Cout[n] from the inputs A[n], B[n], and Cin[n], and then duplicate you logic 4 times.

*edit* I would suggest making a truth table.

11. ### Yash_Arya Thread Starter New Member

Sep 21, 2008
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Very well then, I will try this out as soon as I get my next practicals!

Thank you all once again! I am really glad you helped me out!

Thank you,
Yash Arya

12. ### STEVEN DUPRE New Member

Sep 18, 2008
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SAYING HI, I'M 1/2 THROUGH THE ADDER SECTION OF THE F.A.C.E.T. HANDS ON LEARNING EQUIPMENT.IT'S VERY INTERESTING AND GETTING HARD BUT NOT ENOUGH TO THROW AN INTELLIGENT QUESTION YOUR WAY.IT'S GOOD TO KNOW WHEN IT GETS THAT TIME I CAN POST SOME SERIOUS @&*!. LATER.