Selecting an input on the 74151 (8-to-1) multiplexer For a 74151 eight-line multiplexer, what must the select inputs S0, S1, and S2 be set to so that the output Y equals input I5 (assume standard select ordering)?

Introduction / Context:
The 74151 is a classic 8-to-1 data selector/multiplexer. It routes one of eight data inputs (I0–I7) to its output Y based on a 3-bit select code on S2, S1, S0. Correctly mapping the desired input index to the select lines is a routine task in digital design and troubleshooting.

Given Data / Assumptions:

• 74151 select lines are ordered S2 (MSB), S1, S0 (LSB).
• Desired input: I5.
• Assume enable is active and no inversion options are altering the basic selection.

Concept / Approach:
To select input Ik, load the binary representation of k on the select lines, with S2 as the most significant bit and S0 as the least significant bit. For k = 5, the binary code is 101, so S2 = 1, S1 = 0, S0 = 1. Some datasheets present the order as S2 S1 S0; the question provides the tuple in the order S0, S1, S2, so we assign accordingly.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check against the truth table in a 74151 datasheet: Y = Ii where i = S2S1S0 in binary when the enable pin allows selection.

Why Other Options Are Wrong:

• 010, 001, 110: These select I2, I1, and I6 respectively, not I5.

Common Pitfalls:
Confusing bit order (S2 vs S0) or forgetting an active-LOW enable that forces Y to a fixed state regardless of S2–S0.

Final Answer:
S0 = 1, S1 = 0, S2 = 1