Best practice for unused TTL gate inputs What should be done with unused inputs on TTL logic gates to ensure reliable operation and prevent noise or extra power draw?

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:

  • Technology: TTL (not CMOS). TTL inputs source small currents and should not be left floating.
  • Goal: Force a solid logic HIGH for AND/NAND unused inputs and a solid logic LOW for OR/NOR unused inputs.
  • Use modest resistors (around 1 kΩ–10 kΩ) if direct tying is undesirable.


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:

Identify gate family and unused input type.For AND/NAND: tie to VCC (often via ~1 kΩ to limit fault current).For OR/NOR: tie to ground to hold them inactive.Verify that tying does not conflict with any intended future re-use or test points.


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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