# A question in building a circuit using 2 full adders

• Engineering
• transgalactic
In summary: You're doing great! Keep up the good work!In summary, the problem is that the student doesn't know how to build a circuit using two full adders. However, they have mastered the concepts of how to add binary numbers and how to add up decimal numbers. So their solution is correct.
transgalactic
which produces a sum of 2 bit numbers x0x1 y0y1
the output is C (carry)
S0S1

the problem is i know how to build a truth table and a KARNO table
and even how to build this circuit

i know how to build a full adder circuit

the problem is i don't know how to build this task using 2 full adders

What does the truth table of a 2x 2-bit full adder look like?

there is no 2 on two full adder
there is only a 1 on 1 plus carry in adder (AKA full adder)

i showed the formula of the sum and the carry out in the drawing

http://img160.imageshack.us/my.php?image=img86211bi8.jpg

what now??
i don't know how to use this basic component in order to solve this
question

as i say again
there is no problem for me to build a truth table for all the
possibilities and to make a KARNO table out of it
and then to build a circuit

??

Last edited:
A full adder has three inputs and two outputs. Can you name them?

i showed it all in the drawing
there is the input carrier
the firstbit
the secondbit
the output sum
the output carrier

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Hi, Trans. This sounds like a homework question, so I'm following berkeman's lead in attempting to lead you to an answer, rather than giving it away.

Binary addition is just like adding up decimal numbers longhand. It might help if you tried some examples. Like adding 0110 to 1011. It's binary so the carry-out only has one of two value-- zero or one. The carry out of the one's place is carried into the two's place. The carry out of the two's place is carried into the four's place, and so on.

ye i understand that i can even show you the sketch in the book that explains that
http://img356.imageshack.us/my.php?image=img86221gg8.jpg

so like this story of herztel and gretel
where is the next bread peace?
cause i still don't have any idea of how to subtitute 2 adders in the circuit
that i am supposed to build

what is the next step in leading me for understanding this question??

You can really think of all this as the same as adding columns of normal, base 10 numbers. Except that your doing base 2. Check it out.

_999
+200
-----
1199

There's no carry into the ones place. Thats that same as having the carry into the one's place a zero. So you tie the first carry-input to ground.

The last stage in the example is the hundreds-place. Its carry-out goes straight to the answer. It's the thousands-place digit.

When they do this in an 8 bit CPU you can add up two 8 bit numbers and get a result that needs to fit in 9 bits. So they leave the last carry out in a special bit register, appropriately called the 'carry', so that you can still use it. It can become the carry-input for the next 8 bit addition you do. This allows you to add 16, 24, and 32 bit numbers 8 bits at a time.

The biggest number you can add with two stages is 11 binary to 11 binary.

_11
+11
---
110

Some thing as base 10. You have two 2-bit numbers coming in. But you get a three bit number coming out.

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so i understood that my last carry is the next bit place
and that isntead of putting the first carry in sign 0
i should have put a ground there.

except that
my solution is ok?

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So your "Cout" could be called S2. Bit zero's carry-in is a zero if your doing it on paper. If you're building it in hardware with positive logic you tie it low or tie it to ground.

I that should do it.

thanks

good luck

Sorry... I'm having a slow day at work and had to chirp up. Carry on.

transgalactic said:
which produces a sum of 2 bit numbers x0x1 y0y1
the output is C (carry)
S0S1

the problem is i know how to build a truth table and a KARNO table
and even how to build this circuit

i know how to build a full adder circuit

the problem is i don't know how to build this task using 2 full adders

think about what you do when you add two binary numbers. which bits do you start with? what bits' results depend on what happens at the other bits?

i'm surprized it isn't depicted in your textbook.

ok i think i got the consept
do you have anything to say about my solution

is it ok by you??

i think that i depicted every thing
i got the input bits
i got the output bits
i got the carry out bit
which should have been the next bit in line on the third place

what did i missed??

listen, what you have on your jpg are ad hoc notes about the operation of a single bit in the adder. what you need to do is think about how you would string together two of these single bit adders into a two bit adder.

i don't know what is a two bit adder
for me a have the basic operation of full adder
to do that

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a single full adder adds one bit (the "nth bit") from two binary numbers together with the Carry In bit (so the sum of these 3 bits can be 0, 1, 2, or 3 or 00, 01, 10, or 11 in binary). the result is the two bits i shown to the left. the left bit is the Carry Out bit and the right bit is the nth bit of the sum.

what do you do with the Carry Out the nth bit of a full adder?

i was told previosly that the final carry out is
the next bit the third place

## 1. How do I build a circuit using 2 full adders?

To build a circuit using 2 full adders, you will need to first understand the basic principles of a full adder. A full adder is a digital circuit that performs addition of two binary numbers and takes into account a carry from the previous bit. To create a circuit using 2 full adders, you will need to connect the outputs of the first full adder to the inputs of the second full adder, and then connect the output of the second full adder to the final output of the circuit.

## 2. What is the purpose of using 2 full adders in a circuit?

The use of 2 full adders in a circuit allows for the addition of two 4-bit binary numbers. This is because each full adder can handle two bits of data, allowing for a total of 4 bits to be added together. Using multiple full adders in a circuit allows for the addition of larger binary numbers.

## 3. How do I determine the inputs and outputs of a circuit using 2 full adders?

The inputs for a circuit using 2 full adders will be the two binary numbers that you wish to add together. The outputs will be the sum of these two numbers and a carry bit, if necessary. The first full adder will take in the least significant bits of the two numbers, while the second full adder will take in the carry bit and the next least significant bits.

## 4. Can I use more than 2 full adders in a circuit?

Yes, you can use as many full adders as needed in a circuit, depending on the size of the binary numbers you wish to add. For example, if you want to add two 8-bit binary numbers, you will need to use 4 full adders in the circuit.

## 5. Are there any other components needed to build a circuit using 2 full adders?

In addition to the 2 full adders, you will also need logic gates such as AND, OR, and XOR gates to connect the inputs and outputs of the full adders. These gates will help to determine the carry bit and the sum of the two binary numbers. You may also need other components such as resistors and capacitors for proper circuit functioning.

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