Device usage: The 74HC283 (a 4-bit binary adder with carry) can be used to implement which kind of adder directly?
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A4-bit BCD
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B8-bit BCD
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C4-bit full
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D8-bit full
Answer
Correct Answer: 4-bit full
Explanation
Introduction / Context:The 74HC283 is a standard CMOS implementation of a 4-bit parallel binary full adder with carry-in and carry-out. Recognizing its native capability helps in choosing additional logic when building wider adders or BCD adders.
Given Data / Assumptions:
- Part: 74HC283.
- Features: 4-bit addition, carry-in, carry-out.
- Target: identify which adder type it directly realizes.
Concept / Approach:A full adder accepts two operand bits and a carry-in, producing a sum bit and carry-out. The 74HC283 integrates four such stages for 4-bit vectors, supporting ripple-carry cascading for wider adders. BCD addition requires decimal correction logic (add 6 when needed), which is not built into the 74HC283, so it is not a direct BCD adder.
Step-by-Step Solution:
Use A[3:0], B[3:0], and Cin as inputs.Obtain S[3:0] and Cout as outputs.For 8-bit addition, cascade two 74HC283s via Cout→Cin (not a single device).For BCD addition, append correction logic around the 74HC283.Verification / Alternative check:Databook descriptions and reference schematics label the 74HC283 as a 4-bit binary full adder with carry look-ahead compatible cascading.
Why Other Options Are Wrong:
- 4-bit BCD / 8-bit BCD: Need decimal-correction circuits (not provided internally).
- 8-bit full: Requires cascading two chips; one 74HC283 alone is 4-bit.
Common Pitfalls:Assuming a single chip covers wider bit-widths; overlooking decimal-correction requirements for BCD.
Final Answer:4-bit full