# Design an 8-to-1 MUX using a 3-to-8 decoder and AND gates and one OR gate.

Discussion in 'Homework Help' started by Anony1234, Oct 28, 2013.

1. ### Anony1234 Thread Starter New Member

Oct 28, 2013
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This isn't homework, but it is a question I have had since the beginning of the semester.

I did this by putting each output of the 3-to-8 decoder going into a 2-input AND gate, each of which has an input I0 thru I7 going into it as well, and then connecting all of these to an OR gate at the end.

This is meant to simulate the multiplexer equation of: output Z=I0S0'S1'S2' + ... + I7S0S1S2.

However, I kind of didn't answer the question correctly because I used those additional inputs into the AND gates, of I0 thru I7. It isn't specified whether this is allowed (it seems not to be the way the question is phrased), but I couldn't think of any way to do it otherwise.

Is there any way to do this other than the way that I specified?

2. ### WBahn Moderator

Mar 31, 2012
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Sounds fine. In practice you would use eight 2-input NAND gates and one 8-input NAND gate plus the 3:8 decoder.

Since the output HAS to depend on I0 through I7, you had to use them at some point.

3. ### absf Senior Member

Dec 29, 2010
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If you must use 8x AND gates to design your MUX, then you'd have to invert the inputs before entering the AND gates. The 8-input OR gate also has to be replaced with a NOR gate to invert the input back, so the output would be correct. See the attached schematic for reference.

It would be more elegant to design with NAND gates as suggested by WBahn...

Allen

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4. ### Anony1234 Thread Starter New Member

Oct 28, 2013
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THanks all. I'll consider the other methods as well.

5. ### WBahn Moderator

Mar 31, 2012
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I think a generic 3:8 Decoder (i.e., no specific part number) would be an active-HI output (i.e., only the decoded line is HI, the rest are LO).

absf likes this.
6. ### absf Senior Member

Dec 29, 2010
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I think so. But 74138 is more commonly used than 74238 in real life.

Allen

7. ### ScottWang Moderator

Aug 23, 2012
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If the frequency is not too high, then it can be using CD4028 to replacing the 74138 and no need to use the 74HC04.

8. ### WBahn Moderator

Mar 31, 2012
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But we aren't talking real life. It appears to me that he is working with generic paper-only logic constructs and not a family of logic parts at all.

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9. ### Danm1 Member

Jul 19, 2010
55
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A lot of the 7400 series logic was used is collage course work, but has steadily been used less in actual products if the complex combinational logic can be put in FPGAs and CPLDs etc.

10. ### WBahn Moderator

Mar 31, 2012
18,085
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That may be. But nothing the OP has said indicates one way or the other what is being used, if anything. This may be a simulated design or it may be a purely paper/pencil design using generic, conceptual logic blocks. The latter seems the more likely at this point. Only the OP can clear this point up.