IC decoders — typical output polarity when a valid input is applied In most integrated-circuit (IC) decoders used for display driving or address decoding (for example, 74xx138 or BCD-to-7-segment drivers), when the correct input code is present the corresponding output line typically switches to which active state?

Introduction / Context:
IC decoders translate binary codes into one-of-N outputs and are common in address decoding and seven-segment display driving. Many standard TTL/CMOS decoder families use outputs that are active-LOW, especially when their outputs are intended to sink current from LEDs or enable lines with negative-true logic. Recognizing the usual polarity prevents wiring mistakes and simplifies troubleshooting.

Given Data / Assumptions:

• Generic decoder devices as used in displays and address selection.
• Outputs intended to select one line or illuminate LED segments.
• Typical TTL open-collector or CMOS sink-capable outputs.

Concept / Approach:
In an active-LOW decoder, the asserted state is logic 0 at the output. This is advantageous for driving common-anode displays and for wire-OR configurations with open-collector outputs. When the correct input code is applied, the decoder activates the matching output by pulling it LOW, while all others remain HIGH (or inactive).

Step-by-Step Solution:

Verification / Alternative check:
Examine typical devices such as 74xx138 (3-to-8 decoder) and 74xx47 (BCD-to-7-segment for common-anode displays). Datasheets label outputs with overbars or specify “active-LOW”; bench tests confirm that outputs go LOW when selected, sinking current through the load.

Why Other Options Are Wrong:

• active-HIGH: common for some drivers (e.g., common-cathode displays) but not the default for many decoder ICs aimed at sinking loads.
• high impedance / open: pertains to tri-state or open outputs when inactive, not the asserted state.
• toggle: decoders do not toggle; they select a single line deterministically.

Common Pitfalls:

• Mismatching display polarity (common-anode vs common-cathode) with decoder output polarity.
• Omitting current-limiting resistors when driving LEDs.

Final Answer:
active-LOW