8 to 1 MUX w/ two 4 to 1 MUX and one 2 to 4 BIN/DEC decoder

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Discussion Overview

The discussion revolves around the implementation of an 8 to 1 multiplexer using two 4 to 1 multiplexers and a 2 to 4 binary to decimal decoder. Participants explore the roles of the decoder and the necessary select lines for the multiplexers.

Discussion Character

  • Homework-related
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants note the need for an additional select line (S2) for the 8 to 1 multiplexer, which requires three select lines.
  • There is confusion regarding the term "2 to 4 binary to decimal decoder," with some participants questioning if it is a typo and seeking clarification on its function.
  • One participant describes the decoder as having 2 inputs and 4 outputs, suggesting it operates in positive logic without inverters.
  • Another participant asserts that a demultiplexer is distinct from a decoder, emphasizing that a decoder does not have select lines.
  • Some participants express uncertainty about the problem statement, particularly regarding the mention of "decimal" in the context of the decoder.
  • There is a suggestion to consider the decoder as a regular decoder, with the "binary to decimal" phrasing serving as a conceptual aid.
  • Participants discuss the potential for the 4:1 MUX chips to have output enables and how the decoder might control these enables, raising questions about the number of outputs relative to the number of multiplexers.
  • One participant proposes using only one input to the 2:4 decoder to generate enable signals for the multiplexers.

Areas of Agreement / Disagreement

Participants express differing views on the terminology and function of the decoder, with no consensus reached on the correctness of the problem statement or the role of the decoder in the circuit.

Contextual Notes

There are unresolved questions regarding the definitions and roles of the components discussed, particularly the decoder's function and the implications of the "binary to decimal" terminology.

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Homework Statement



Draw a diagram to show how to implement a 8 to 1 multiplexer with two 4 to 1 multiplexers and a 2 to 4 binary to decimal decoder.

Homework Equations



/

The Attempt at a Solution



I know that I'm going to need another select line (S2) since an 8 to 3 multiplexer has 3 select lines but I've been struggling with the decoder. I have no idea what purpose the decoder should serve. Any help would be appreciated :).
 
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whoareyou said:

Homework Statement



Draw a diagram to show how to implement a 8 to 1 multiplexer with two 4 to 1 multiplexers and a 2 to 4 binary to decimal decoder.

Homework Equations



/

The Attempt at a Solution



I know that I'm going to need another select line (S2) since an 8 to 3 multiplexer has 3 select lines but I've been struggling with the decoder. I have no idea what purpose the decoder should serve. Any help would be appreciated :).

What is a "2 to 4 binary to decimal decoder"? Is that a typo? 2 binary bits to a 4-bit decimal representation? Can you post a schematic of the logic arrangement of this decoder?
 
berkeman said:
2 binary bits to a 4-bit decimal representation?

Yes, exactly!

Like this, except with 2 inputs and 4 outputs, and I guess we could also assume positive logic (so no inverters on the output side). The decoder could also have a chip select / enable line but I'm still not seeing how to implement it.

1FD8MCe.png
 
What you are describing is a 2:4 DeMux, which has nothing to do with decimal. Are you sure you have the problem statement correct?
 
Umm, no. A demultiplexer is the opposite of a multiplexer. A decoder is just like the picture I attached above. For example, if all the inputs are high except A2 then in binary that's 11 so pin 11 on the output side would be high (ie. without those inverters on the chip in the picture above.) This decoder has no select lines, but a demxer would.
 
whoareyou said:
Umm, no. A demultiplexer is the opposite of a multiplexer. A decoder is just like the picture I attached above. For example, if all the inputs are high except A2 then in binary that's 11 so pin 11 on the output side would be high (ie. without those inverters on the chip in the picture above.) This decoder has no select lines, but a demxer would.

Ah, I guess you're right. It's a decoder function, not a demux (routing) function.

Still, there is no "decimal" component to the decode function, which makes me wonder if the problem statement is correct...
 
You could think of it as a regular decoder then, 2 inputs and 4 outputs. I guess the "binary to decimal" thing is just a way for us to understand how the decoder works.
 
Do the 4:1 MUX chips have output enables?
 
They could, yeah. So the decoder would control whether or not the multiplexer is enabled? I tried thinking about that but then the decoder has 4 outputs but there are only be 2 enables (1 for each multiplexer) ...
 
  • #10
whoareyou said:
They could, yeah. So the decoder would control whether or not the multiplexer is enabled? I tried thinking about that but then the decoder has 4 outputs but there are only be 2 enables (1 for each multiplexer) ...

Think about using only one input to the 2:4 decoder and tie off the other input. Use the one input as a High/Low MUX select line, which generates two OE~ signals at the output of the 2:4 decoder...
 

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