Hi everybody, I'm new to the forum and got on here because I'm looking for help with making a circuit that calculates the twos compliment of a 4 bit binary number. I'm using logisim and I understand you would invert the numbers and add one although for me to visualize what that would mean circuit wise is hard at this point. I know learning is key but I can't seem to find this anywhere, help would be greatly appreciated!

 

Is this homework, a design exercise, or a real circuit to be implemented?

 

its a homework assignment.

Is this homework, a design exercise, or a real circuit to be implemented?

 

As you say, you would invert the bits and add one.

Can you make a circuit that would add 1 to an arbitrary 4-bit binary number?

Can you make a circuit that would invert all of the bits in a 4-bit binary number?

Are you familiar with a 4-bit binary adder made from Full Adders?

You need to show your best attempt to do as much as you can.

 

As you say, you would invert the bits and add one.

Can you make a circuit that would add 1 to an arbitrary 4-bit binary number?

Can you make a circuit that would invert all of the bits in a 4-bit binary number?

Are you familiar with a 4-bit binary adder made from Full Adders?

You need to show your best attempt to do as much as you can.

heres what I have so far

 

You are close, but think about what you are doing with that circuit.

You 4-bit adder's output is

S = A+B+Cin

If your input is V, then

S = -V = ~V + 1

But you are setting A and B both to ~V, so you are getting

S = ~V + ~V + 1

S = (~V + 1) + (~V + 1) - 1

S = 2(-V) - 1

S = -(2V + 1)

You input is V = 3, so you have

S = -7

The 4-bit two's complement representation of -7 is 1001, which is what you circuit is producing.