Difficulty: Easy
Correct Answer: Tie unused AND/NAND inputs to VCC (via about 1 kΩ); tie unused OR/NOR inputs to ground.
Explanation:
Introduction / Context:
Floating TTL inputs are susceptible to noise pickup and may bias internal transistors in their linear region, increasing power consumption and causing spurious switching. Properly terminating unused inputs ensures defined logic levels and improves noise immunity and reliability in the field.
Given Data / Assumptions:
Concept / Approach:
For functions that are active when inputs are HIGH (e.g., NAND/AND), tying unused inputs HIGH prevents unintended activation; for NOR/OR, tying unused inputs LOW prevents spurious outputs. Although TTL inputs often default “HIGH-ish,” relying on that behavior invites noise and extra current consumption.
Step-by-Step Solution:
Verification / Alternative check:
Datasheets and logic design guides consistently recommend defined biasing of unused inputs. Some TTL series allow direct ties; using a small resistor adds protection against shorts during rework.
Why Other Options Are Wrong:
Leaving disconnected or floating is unsafe; tying to an unused output is undefined and may drive illegal states; tying everything to VCC ignores gate function differences.
Common Pitfalls:
Assuming TTL’s default bias is sufficient; mixing TTL and CMOS rules (CMOS gates should not be left floating either but have different input structures).
Final Answer:
Tie unused AND/NAND inputs to VCC (via about 1 kΩ); tie unused OR/NOR inputs to ground.
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