# 4 to 1 Multiplexor Circuit Help

Joined Jan 30, 2018
6
This is my 4 to 1 Multiplexor circuit but I need help because it isn't right. Does anyone know what I'm doing wrong? It has to be in Simcir.

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#### dl324

Joined Mar 30, 2015
16,732
Welcome to AAC!

Is this homework?

Joined Jan 30, 2018
6
Welcome to AAC!

Is this homework?
No, it's not. I'm self-teaching about Computer Science through a book and I can't seem to figure out the MUX circuit in Simcir.

#### crutschow

Joined Mar 14, 2008
34,074
Please draw it as a normal schematic with right angle connections and a layout that minimizes the length of the connections.
That drawing too convoluted to readily follow.

#### WBahn

Joined Mar 31, 2012
29,885
You might ask if your connections make sense, even from a symmetry standpoint. You S0 and So' signals each go to two of the channels. Wouldn't is seem reasonable for the same to be true for your S1 and S1' channels?

Drawing the schematic in a neat, orderly fashion often reveals things like this because things just look unbalanced.

#### dl324

Joined Mar 30, 2015
16,732
No, it's not. I'm self-teaching about Computer Science through a book and I can't seem to figure out the MUX circuit in Simcir.
The first thing you need to learn is that neatness counts. It makes the circuit easier for us to read and easier for you to troubleshoot connectivity problems.

Look at the symmetry, or lack thereof, of the select signals.

Joined Jan 30, 2018
6
The first thing you need to learn is that neatness counts. It makes the circuit easier for us to read and easier for you to troubleshoot connectivity problems.

Look at the symmetry, or lack thereof, of the select signals.
Please draw it as a normal schematic with right angle connections and a layout that minimizes the length of the connections.
That drawing too convoluted to readily follow.

Joined Jan 30, 2018
6
Is this easier to read? I'm not exactly sure the schematics of how to organize the circuit. Thanks to all for the help.

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#### WBahn

Joined Mar 31, 2012
29,885
Is this easier to read? I'm not exactly sure the schematics of how to organize the circuit. Thanks to all for the help.
That's better, but see if that software lets you make straight lines and 90° bends in wires.

Consider the following observations:

Joined Jan 30, 2018
6
So I should make it symmetrical? I'm having trouble figuring out which switches connect to which gates.

#### dl324

Joined Mar 30, 2015
16,732
Is this easier to read? I'm not exactly sure the schematics of how to organize the circuit. Thanks to all for the help.
Not much better. One place to learn how the professionals do it is to look at datasheets.

#### crutschow

Joined Mar 14, 2008
34,074
So I should make it symmetrical? I'm having trouble figuring out which switches connect to which gates.
Yes.
And make all the lines orthogonal, not slanted (also called rat's nest).

#### WBahn

Joined Mar 31, 2012
29,885
So I should make it symmetrical? I'm having trouble figuring out which switches connect to which gates.
How did you come up with the connections that you have?

Think about how you would describe the logic in words.

You want the output signal to equal D0 if S0 is NOT pressed AND S1 is NOT pressed.

You want the output signal to equal D2 if So is NOT pressed AND S1 is pressed.

What would the corresponding statements be for D1 and D3?

#### WBahn

Joined Mar 31, 2012
29,885
Not much better. One place to learn how the professionals do it is to look at datasheets.
As near as I can tell, this Simcir application only does rat's nest connectivity. So the best she can probably do is to position components so as to reduce the chaos as much as she can.

#### Bordodynov

Joined May 20, 2015
3,168
See

#### dl324

Joined Mar 30, 2015
16,732
How is this relevant to what's wrong with the OP's circuit?

#### dl324

Joined Mar 30, 2015
16,732
I'm self-teaching about Computer Science through a book and I can't seem to figure out the MUX circuit in Simcir.
Simcir doesn't let you draw clean schematics. If you're self-teaching, why do you want to use it? There are better free logic simulators.

#### Bordodynov

Joined May 20, 2015
3,168
How is this relevant to what's wrong with the OP's circuit?
I showed that the electronic circuit must consist of inverters and 2-N-HE logic elements. It is also better to break the circuit into smaller blocks. Then it is more understandable. I also demonstrated the functioning.