74LS151 (8-to-1 multiplexer) select code: To route data input I1 to the output of a 74151/74LS151 eight-line multiplexer, what 3-bit select input code must be applied (assume the active-low enable /G is asserted low)?

Introduction / Context:
The 74151 (also known as 74LS151) is an eight-line data selector/multiplexer. It selects one of eight data inputs I0–I7 and routes it to the output under the control of three select inputs, commonly labeled S2 (MSB), S1, and S0 (LSB). Understanding the select-code mapping is essential when interfacing microprocessors, counters, or address lines to multiplexed signals.

Given Data / Assumptions:

• Device: 74151/74LS151 8:1 MUX.
• Inputs: I0 through I7.
• Select lines: S2 (MSB), S1, S0 (LSB).
• Enable: active-low /G must be 0 to enable the outputs.
• Goal: select I1.

Concept / Approach:
The MUX chooses input Ix where x is the binary value present on S2 S1 S0, interpreted as a 3-bit number with S2 as the most significant bit. Therefore, to select I1, we must place the binary value 001 on S2 S1 S0, while ensuring /G is low so the device is enabled. Any other select pattern will point to a different input, and a deasserted enable will inhibit the outputs regardless of the select code.

Step-by-Step Solution:

Verification / Alternative check:
Cross-check against standard truth tables for the 74151 family: each input number equals the binary value of S2 S1 S0. Simulation or bench testing will show Y (and /Y) follow I1 only when S2 S1 S0 = 001 and /G = 0.

Why Other Options Are Wrong:

• S2 S1 S0 = 010: Selects I2, not I1.
• S2 S1 S0 = 100: Selects I4, not I1.
• S2 S1 S0 = 011: Selects I3, not I1.

Common Pitfalls:
Confusing bit order (treating S0 as MSB), forgetting to assert /G low, or assuming Gray-code selection (the 74151 uses straight binary).

Final Answer:
S2 S1 S0 = 001 (with /G = 0)