Category: Combinational Logic

Consider the following diagram for a DMUX

where the unselected output wire is set to zero. Would a Dmux still be a useful chip if this value was always set to 1? If yes, why, if not, why not?

Question

Category: Combinational Logic

Consider the following diagram for a DMUX

where the unselected output wire is set to zero. Would a Dmux still be a useful chip if this value was always set to 1? If yes, why, if not, why not?

BlackTom AI · Accepted Answer

Category: Combinational Logic

First, restating the scenario in my own words helps set the stage: we’re looking at a DMUX (demultiplexer) diagram where the unselected output wire is normally set to zero. The question asks whether the DMUX would still be useful if that unselected output wire were always set to 1, and to justify either why it would remain useful or why not.

Option 1: No. It would render the dmux useless because all of the rest of the machine would be expecting a zero in this place rather than a one and there is nothing we could do to fix it.
This choice argues that forcing the unused output to 1 breaks the expectations of downstream circuitry, which typically assumes a 0 on the unused path to indicate an inactive or non-selected channel. The claim that there is “nothing we could do to fix it” is a strong denial, but it is not entirely accurate: you could invert the signal, or reinterpret the meaning of that wire, or adjust enable logic elsewhere. The statement correctly identifies that a constant 1 on the unused line would propagate a continuous nonzero condition in places that expect a 0, potentially causing incorrect operation unless compensations are introduced. The flaw is the absolute pessimism about fixable workarounds; real-world design often provides a way to adapt signals or add conditioning.

Option 2: Yes. It doesn’t matter what is on this output.
This option claims the unused output has no impact on system behavior. In practice, a constant 1 on the unselected line would be misinterpreted by downstream components that rely on a defined inactive state (often 0). If the rest of the machine expects a 0 when a select is inactive, forcing 1 would lead to a mismatch, combinational conflicts, or unintended activation. Therefore this choice is incorrect because the signal on the unused line typically carries semantic meaning (inactive vs active) that matters for correct operation.

Option 3: Yes. You might have to invert this signal or change the expected interpretation of this signal.
This option acknowledges the functional impact of the unused line and proposes a corrective approach: invert the signal or reinterpret its meaning. If the unused output is forced to 1, you could use an inverter in downstream logic to restore the expected 0 when nothing is selected, or adjust the design assumption so that a 1 on that line is treated as the inactive condition. This demonstrates a practical design mindset: adapt the surrounding logic to accommodate a changed signal convention rather than discarding the DMUX’s usefulness altogether. This option is plausible and valid in many design contexts because it recognizes a compensatory solution rather than an intrinsic failure.

Option 4: No. The ones make the output of the dmux unpredictable.
This choice argues that 1s on the unused line render the DMUX output unpredictable. Predictability matters in digital design, but the claim here overgeneralizes the issue: whether the DMUX output becomes unpredictable depends on how the rest of the circuit interprets the line and whether proper conditioning or re-interpretation is applied. In many schemes, designers explicitly handle the unused line (e.g., treat it as a known constant or route it through an inverter) to maintain predictability. Therefore, while partly reasonable, this option is overly absolute and does not consider possible fixes.

In summary, each option presents a distinct viewpoint about the effect of hard-wiring the unselected DMUX output to 1 and how to deal with it. The reasoning highlights why an absolute dismissal (Option 1) or an overly permissive claim (Option 2) is flawed, while recognizing that both a corrective approach (Option 3) and a nuanced take on predictability with proper conditioning (Option 4) are viable considerations, albeit with caveats. The practical takeaway is that the impact depends on downstream expectations and whether you implement inversion or a reinterpretation strategy to preserve correct behavior.

4520_COMP_SCI_X_0005 Practice Exam

Category: Combinational Logic Consider the following diagram for a DMUX where the unselected output wire is set to zero. Would a Dmux still be a useful chip if this value was always set to 1? If yes, why, if not, why not?

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