In the first three entries of the truth table of a multiplexer whatever the value (0 or 1) of D0,D1,D2 it does not have any impact on the output because when the signals go through the AND gate the output of the it is 0 in the first three entries. Why am I wrong?

Let me explain with an example:

When D0 is 0, output (F) is equal to 0
When D0 is 1 still the output (F) is 0
So D0 is not equal to F (the output)

What am I missing here?








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hi s,
Your question is a little vague.
I assume you have understood the purpose of the S1and S2 Select lines on the multiplexer.

The Select lines select the D1, or D2 or D3 Data Sources.
The Input Data to Selected source will be output on F unchanged, ie: if High input, then High output and vice versa.

Do you follow that OK.?

E

 

What am I missing here?

The function of S1, S2 and their associated inverters in enabling/disabling the gates.

 

The default assumption for ALL combinatorial logic is that ALL inputs matter.

 

The error in this question is that the effect of the data inputs on the output is not shown, only sort of implied. The circuit is correct but the effect of each data input is not displayed directly, only suggested.
Both posts and 3 are correct, the flaw is in the truth table, which is incomplete. It must actually show the outputs for both of the conditions for a data input. Instead, a common convention has been used and so what would usually be understood iis instead misunderstood.
The shortcut is that with each input selected the "F" value equals the value of the input selected.