Handling unused inputs on NAND gates (TTL/CMOS best practices): Evaluate the statement: “An unused NAND input can be left unconnected, or just pulled high, or tied together with another input without affecting the logic output.”

Introduction / Context:
Proper termination of unused inputs is critical for noise immunity and predictable behavior. Floating inputs can assume indeterminate levels, inject noise, and increase power consumption, especially in CMOS where input gates have very high impedance.

Given Data / Assumptions:

• NAND gate inputs must be at defined logic levels.
• TTL floating inputs may default high but are not guaranteed and are poor practice.
• CMOS floating inputs are especially problematic due to high input impedance and can oscillate or pick up noise.

Concept / Approach:
The safe approach is to tie unused inputs to a valid logic level: for NAND, tying to logic 1 through a proper connection (direct for CMOS with ESD safeguards or via resistor where recommended) maintains intended function. “Leaving unconnected” is never advised. Tying inputs together is acceptable only when both are driven by the same defined source—not to excuse leaving them floating.

Step-by-Step Solution:

Verification / Alternative check:
Datasheets and logic design guides consistently warn against floating inputs; application notes specify pull-ups/pull-downs or direct ties to rails as appropriate.

Why Other Options Are Wrong:

Common Pitfalls:
Relying on TTL’s default-high behavior; ignoring input protection and ESD structures; forgetting that floating inputs can inject switching noise into the system.

Final Answer:
Incorrect