designing 4x1 mux with 2x1 muxes and logic gates

Thread Starter

puzzle

Joined Oct 30, 2016
53
Hello,

Can someone please explain me how to design a logic circuit of 4x1 mux using 2x1 muxes and logic gates ?

the truth table of 4x1 mux is :

s0 s1 y

0 0 x0
0 1 x1
1 0 x2
1 1 x3

hence y = x0*s0'*s1'+x1*s0'*s1+x2*s0*s1'+x3*s0*s1

I know how to implement it just with logic gates but i must use also 2x1 muxes.
I will also appreciate an explanation

thanks!
 

Thread Starter

puzzle

Joined Oct 30, 2016
53
I'm familiar with this answer :

upload_2016-11-1_15-13-58.png

but I will be glad to hear another different solutions using also logic gates
 

GopherT

Joined Nov 23, 2012
8,009
I'm familiar with this answer :

View attachment 114588

but I will be glad to hear another different solutions using also logic gates
It cannot be done unless your multiplexers have another "disable" pin. Otherwise one channel on each will always be on. I am assuming these are analog multiplexers (passing an analog signal) so a digital logic front end cannot be used.

If they are digital multiplexers, then let me know.
 

Thread Starter

puzzle

Joined Oct 30, 2016
53
It cannot be done unless your multiplexers have another "disable" pin
You are probably talking about this circuit :
upload_2016-11-1_15-57-17.png

It cannot be done unless your multiplexers have another "disable" pin. Otherwise one channel on each will always be on. I am assuming these are analog multiplexers (passing an analog signal) so a digital logic front end cannot be used.

If they are digital multiplexers, then let me know.

All the signals moving through the muxes are digital i.e : '1' / '0'
 

AnalogKid

Joined Aug 1, 2013
10,986
The circuits in posts #2 and #4 are fine. Are you sure about the question? It might be that what is wanted is a 4:1 digital mux with *only* logic gates; that is, not using mux chips.

ak
 

WBahn

Joined Mar 31, 2012
29,976
I'm familiar with this answer :

View attachment 114588

but I will be glad to hear another different solutions using also logic gates
So why isn't that answer sufficient? Was a limit placed on how many 2:1 muxes you could use?

If you want to also use logic gates, then you could simply replace the last mux with a logic-gate implementation. But what would the point be?
 

WBahn

Joined Mar 31, 2012
29,976
It cannot be done unless your multiplexers have another "disable" pin. Otherwise one channel on each will always be on. I am assuming these are analog multiplexers (passing an analog signal) so a digital logic front end cannot be used.

If they are digital multiplexers, then let me know.
????

Why won't the circuit he showed work?

Why assume that these are analog signals with the original post provided a Boolean expression for the output in terms of the input signals?
 

WBahn

Joined Mar 31, 2012
29,976
I will also appreciate an explanation
What kind of explanation are you hoping for? You provided a circuit using three 2:1 muxes fifteen minutes after the original post saying that you were familiar with it. So what is it that you need an explanation of?
 

GopherT

Joined Nov 23, 2012
8,009
????

Why won't the circuit he showed work?

Why assume that these are analog signals with the original post provided a Boolean expression for the output in terms of the input signals?
Why assume when the OP is available to answer the question?
The truth table determines the Channel selection, not the contents of the channel. You teach this stuff? Really?
 

WBahn

Joined Mar 31, 2012
29,976
Why assume when the OP is available to answer the question?
YOU are the one that is assuming -- "I am assuming these are analog multiplexers..."

The truth table determines the Channel selection, not the contents of the channel. You teach this stuff? Really?
Where did I say anything about the truth table?

I said that the original post provided a Boolean expression for the output in terms of the input signals.

y = x0*s0'*s1'+x1*s0'*s1+x2*s0*s1'+x3*s0*s1" requires that x1, x2, x3 be Boolean values.
 

GopherT

Joined Nov 23, 2012
8,009
YOU are the one that is assuming -- "I am assuming these are analog multiplexers..."



Where did I say anything about the truth table?

I said that the original post provided a Boolean expression for the output in terms of the input signals.

y = x0*s0'*s1'+x1*s0'*s1+x2*s0*s1'+x3*s0*s1" requires that x1, x2, x3 be Boolean values.
Yeah, but making myself clear that I am making an assumption is a way to clarify communication with someone (except you). That way, the OP knows he left out some information, and, if my assumption was incorrect, I encourage him to reply back for additional info - did you see that part? Or were you sooo exited to point out an error that you just scrambled to your keyboard to document it?
 

WBahn

Joined Mar 31, 2012
29,976
What information was left out?

People that understand that signals in Boolean expressions are not analog had no problem understanding that he is talking about a digital circuit and therefore he wasn't talking about an analog multiplexer.

While you're whining and pouting because you made an unreasonable assumption and got called on it, perhaps you should also consider that it might be a hydraulic multiplexer since he didn't say it wasn't and ask him to clarify that, too.
 

GopherT

Joined Nov 23, 2012
8,009
...perhaps you should also consider that it might be a hydraulic multiplexer since he didn't say it wasn't and ask him to clarify that, too.


You are really schizophrenic - one minute you say it is obviously digital and the next you are concerned it might be hydraulic, but you claim it can't be analog? Weird.
 

AnalogKid

Joined Aug 1, 2013
10,986
It cannot be done unless your multiplexers have another "disable" pin.
Yes, it can, as in post #2.
Otherwise one channel on each will always be on.
True (without a disable pin), but there is nothing in post #1 about this being a not allowed condition.
I am assuming these are analog multiplexers (passing an analog signal) so a digital logic front end cannot be used.
Post #1 says logic circuit, so I think inferring digital muxes is more valid than 'assuming' analog muxes. However, analog muxes can pass digital signals, so there really is no conflict there.

ak
 

AnalogKid

Joined Aug 1, 2013
10,986
You are really schizophrenic - one minute you say it is obviously digital and the next you are concerned it might be hydraulic, but you claim it can't be analog? Weird.
Not concern, just sarcasm. Like suggesting the multiplexers be built out of solenoids (240 Vac coils, opto-isolated zero-crossing triac drivers) moving magnets near hall effect sensors.

He didn't claim anything, he just pointed out that the wording and the context of post #1 strongly imply an all digital logic solution. How can you not see that? Myopia?

And even if your characterization of his responses were correct (and it is not), they are nothing close to schizophrenic in nature.

ak
 
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