### Communities

tag:snake search within a tag
user:xxxx search by author id
score:0.5 posts with 0.5+ score
"snake oil" exact phrase
created:<1w created < 1 week ago
post_type:xxxx type of post
Q&A

# 3+3 doesn’t show 6

+3
−0

I have a problem which I don’t understand how to solve. I have this 2-bit adder, but when I select A1AO to be (11)2 3 and B1BO to be (11)2 3 and run it on Logisim it doesn’t show me 6. Instead it shows me 4.

How can I fix it?

Why does this post require moderator attention?
You might want to add some details to your flag.
Why should this post be closed?

#### 2 comment threads

A more common full-adder implementation (1 comment)
Free schematics editor (1 comment)

You are accessing this answer with a direct link, so it's being shown above all other answers regardless of its score. You can return to the normal view.

+2
−0

The correct 1 bit full adder is this:

Why does this post require moderator attention?
You might want to add some details to your flag.

#### 0 comment threads

+8
−0

For the benefit of later readers, I thought I would explain why the original circuit "didn't work" and how exactly the "correct" circuit differs from the original circuit.

The issue is the 3-input XOR gate in the original circuit. The behavior of an XOR gate with more than 2 inputs is not precisely defined. Some people assume that a 3-input gate acts as F = A xor B xor C, which is the implementation in your "correct" circuit. However, some people define the XOR function as "1 and only 1", regardless of the number of inputs. If you used Logisim to simulate this circuit you discovered that Logisim defaults to the latter definition. As I recall, you can change the behavior of the gate in Logisim, but the default is "1 and only 1".

So, when all three inputs are 1 the Logisim default XOR will provide an output of 0. If you use two 2-input XOR gates the output will be 1, which is what you want in this case.

Why does this post require moderator attention?
You might want to add some details to your flag.

#### 2 comment threads

Logisim option (1 comment)

This community is part of the Codidact network. We have other communities too — take a look!

You can also join us in chat!

Want to advertise this community? Use our templates!

Like what we're doing? Support us!