# Multiplexer - same logic circuits?

Thread Starter

#### rpx89

Joined Nov 13, 2022
6
Hi,

I am new to digital circuits and got a pretty basic question I think.

Did I understand correctly that a multiplexer is just a combination of circuits ( sum of products) and these are all the same independently of anything else?

As in, the boolean function that culminates in its digital circuits is always the below?

What differs is the gates that connect to it for example:

F (A, B, C, D) = Σm (0, 1, 3, 4, 8, 9, 15)

With the above I just need to find the selection lines that go into I0, I1, I2 and I3 respectively?

Thanks

#### ericgibbs

Joined Jan 29, 2010
18,614
hi rpx.
Welcome to AAC.
Is this a Homework or a College assignment question.?
E

#### WBahn

Joined Mar 31, 2012
29,827
It's virtually impossible (for me, anyway) to decipher what you are saying as it seems to jump all over the place and to pull stuff seemingly out of nowhere.

Yes, a multiplexer is just a combinatorial circuit (as opposed to a sequential one), but WHAT is the same independent of WHAT else?

What differs is WHAT gates that connect to WHAT?

You have a function of four inputs, A, B, C, and D, and a sum of (presumably) minterms over those four inputs. But what are A, B, C, and D? How do they relate to your 4:1 multiplexer logic, which is a function of SIX inputs?

What does it mean to find selection lines that "go into" I0 through I3?

Thread Starter

#### rpx89

Joined Nov 13, 2022
6
hi rpx.
Welcome to AAC.
Is this a Homework or a College assignment question.?
E
Hi, I will have a homework yes on it, but at the moment just studying. Unfortunately do not have much support so thought I'd ask here. I basically just want to know that the same multiplexer ( same logic circuits) is used for all boolean functions. Lets say I have a function f = 'ab + 'ac and all I need to do is to figure out which inputs go into the multiplexer, this being an already "made" device the logic circuits referred in my question's image.

Thank you

#### WBahn

Joined Mar 31, 2012
29,827
Hi, I will have a homework yes on it, but at the moment just studying. Unfortunately do not have much support so thought I'd ask here. I basically just want to know that the same multiplexer ( same logic circuits) is used for all boolean functions. Lets say I have a function f = 'ab + 'ac and all I need to do is to figure out which inputs go into the multiplexer, this being an already "made" device the logic circuits referred in my question's image.

Thank you
Again, I don't know what you mean by the same multiplexer being used for all boolean functions.

A multiplexer is an example of one specific boolean logic function out of the literally infinite number of possible boolean logic functions.

#### Irving

Joined Jan 30, 2016
3,791
Maybe making it more concrete will help... here is a 4:1 mux - such devices are available in almost every logic family and are based on simple combinatorial logic. The output Z takes the state of one of the 4 data inputs A, B, C, D, depending on the state of the selector inputs S0, S1. Thus we can write the truth table as shown, where X = don't care., and the logic function is Z = A.S0'.S1' + B.S0.S1' + C.S0'.S1 + D.S0.S1 as per your original post.

What's not clear is your second assertion: F (A, B, C, D) = Σm (0, 1, 3, 4, 8, 9, 15)? I think you're trying to say is that the output is a function of the inputs ABCD and S0S1 where these relate to some physical pins on a specific chip? Well of course that is true and that will be defined by the datasheet of the specific chip.

#### MisterBill2

Joined Jan 23, 2018
17,729
A multiplexer is a logic level controlled switch. The function is to select signal paths to a common port in a controlled manner. Usually the control signal is a multiple bit binary logic level signal, but that is not part of the definition. A digital multiplexer usually connects more than one digital signal to a common output.
For a useful explanation look up a CD4053, which is a dual 4 input multiplexer.

Thread Starter

#### rpx89

Joined Nov 13, 2022
6
Maybe making it more concrete will help... here is a 4:1 mux - such devices are available in almost every logic family and are based on simple combinatorial logic. The output Z takes the state of one of the 4 data inputs A, B, C, D, depending on the state of the selector inputs S0, S1. Thus we can write the truth table as shown, where X = don't care., and the logic function is Z = A.S0'.S1' + B.S0.S1' + C.S0'.S1 + D.S0.S1 as per your original post.

What's not clear is your second assertion: F (A, B, C, D) = Σm (0, 1, 3, 4, 8, 9, 15)? I think you're trying to say is that the output is a function of the inputs ABCD and S0S1 where these relate to some physical pins on a specific chip? Well of course that is true and that will be defined by the datasheet of the specific chip.

View attachment 280674View attachment 280676
Thank you so much, you understood correctly, this is what I needed.

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