Introduction / Context:
The 74154 is a classic TTL part used for address decoding and selection tasks. Knowing its polarity and fan-out is critical when integrating with memory maps or enabling one of many devices.
Given Data / Assumptions:
- 74154 has two enable inputs and 16 outputs.
- Outputs are active-LOW, one at a time, corresponding to the 4-bit input.
- It is a decoder/demultiplexer, not an encoder or display driver.
Concept / Approach:
A 4-bit binary input selects exactly one of 16 outputs to go LOW, with all others remaining HIGH. This is the definition of a 1-of-16 active-LOW decoder and is the key feature used in chip-select trees, memory decoding, or resource selection.
Step-by-Step Solution:
Recognize device function: decode 4-bit value → 1-of-16 line active.Note polarity: outputs are active-LOW (LOW = selected).Verify enablers: enable pins must be correctly asserted for outputs to respond.Therefore, statement A is correct.
Verification / Alternative check:
Typical datasheets label outputs Y0–Y15, active-LOW, with two enables (often one active-LOW and one active-HIGH).
Why Other Options Are Wrong:
1-of-8, 3-bit, active-HIGH: That describes a different part (e.g., 74138 family).BCD to seven-segment: Requires a dedicated decoder/driver (e.g., 7447/4511), not 74154.Priority encoder: Opposite function; encoders compress inputs into codes.
Common Pitfalls:
Confusing 74154 with 74138 or display decoder drivers.
Final Answer:
It is a 1-of-16 decoder that accepts a 4-bit binary input and provides 16 active-LOW outputs
Discussion & Comments