Definition in TTL Terminology: What is a “floating” TTL input?

Introduction / Context:
Correct handling of unused inputs matters for noise immunity and power consumption. In TTL, the term “floating” specifically refers to an input pin left unconnected—something generally discouraged in robust designs.

Given Data / Assumptions:

• TTL inputs have internal biasing that tends to interpret a floating input as HIGH, but with poor noise immunity.
• Leaving inputs unconnected can lead to oscillations, increased power, or spurious switching due to noise pickup.
• Good practice ties unused inputs to a defined logic level via a direct connection or resistor, as appropriate.

Concept / Approach:
The definition question is about terminology. A “floating” TTL input is one that is not connected to a defined logic level or another driven signal—i.e., it is left open. The best practice is to avoid floating inputs by tying them HIGH or LOW in line with device recommendations.

Step-by-Step Solution:
Identify the definition → “floating” means not connected.Check alternatives: tied to VCC, tied to GND, or tied to other used inputs are not floating.Therefore, the correct definition is “unused input that is not connected.”

Verification / Alternative check:
Data sheets and application notes recommend avoiding floating TTL inputs and specify pull-up or pull-down methods depending on the gate type and desired default logic state.

Why Other Options Are Wrong:

• a: This is a defined HIGH via a resistor, so not floating.
• b: Tying to a used input defines a level; not floating.
• c: Tied to ground is a defined LOW; not floating.

Common Pitfalls:
Assuming that because TTL “floats HIGH,” leaving it open is acceptable; noise and EMI can still cause unpredictable behavior.

Final Answer:
unused input that is not connected.