Define pull-up and pull-down resistors and their roles with digital inputs.

• A. A pull-up resistor ties an input to a high level when not driven; a pull-down ties it to ground; both prevent floating inputs and define a known state.
• B. A pull-up resistor ties an input to ground; a pull-down ties it to high; both prevent floating inputs.
• C. Pull-up/pull-down have no effect on floating inputs.
• D. Pull-up/pull-down are only used in power supply circuits.

Answer: (A)

When a digital input might not be actively driven, it can float and pick up noise, leading to unpredictable readings. Pull-up and pull-down resistors give that input a defined default state so the system has a reliable starting value.

A pull-up resistor connects the input to a high level (Vcc). If nothing else drives the line, the input reads high. A pull-down resistor connects the input to ground, so when the line isn’t driven, the input reads low. These resistors are chosen with relatively high resistance to minimize current draw, but they’re strong enough to establish a clear logic level when the line would otherwise float.

A common example is a push-button wired to ground with a pull-up: when the button isn’t pressed, the input sits high; when pressed, it’s driven low. This illustrates how pull-ups and pull-downs define a known state and prevent floating.

The other ideas are off because swapping the connections would give the wrong default state, claiming they don’t affect floating inputs ignores their actual purpose, and limiting them only to power-supply circuits misses their central role in digital input stability.