Handling unused TTL NAND inputs In practical circuit design, what should be done with unused inputs on a TTL NAND gate to ensure stable, low-noise operation?

Introduction / Context:
Unused inputs can cause excess power, oscillation, or susceptibility to noise. TTL and CMOS families behave differently when inputs are left open; correct biasing depends on the logic technology.

Given Data / Assumptions:

• Gate family: TTL (bipolar), device type: NAND.
• Goal: avoid floating inputs and undefined internal states.
• Use recommended bias practice from TTL datasheets.

Concept / Approach:
TTL inputs tend to float HIGH due to internal biasing but can pick up noise and switch unpredictably. Best practice is to tie unused TTL inputs to a defined logic level. For a NAND gate, tying unused inputs HIGH keeps them logically neutral, letting the used inputs control the gate output without added propagation or current issues.

Step-by-Step Solution:

Verification / Alternative check:
Application notes recommend tying unused TTL inputs to HIGH; for CMOS, unused inputs must also be tied to defined levels (often via pull-ups/pull-downs).

Why Other Options Are Wrong:

• Let them float: risks oscillation and increased supply current.
• Tie LOW: For NAND, a LOW on any input forces output HIGH, potentially altering logic or increasing static current.

Common Pitfalls:
Confusing TTL and CMOS practices; while both should not float, the current behavior and preferred tie level can differ by gate function.

Final Answer:
tie them HIGH