A BCD digit represents decimal 0–9 using four binary bits (8421 code). Therefore a 1-of-10 BCD decoder needs 4 input lines and produces ten decoded output lines.
Formula / Key Relation
BCD digit = 4 bits
2⁴ = 16 possible codes; 10 are valid decimal digits
Detailed Explanation
Concept & Reasoning
A decoder with n binary inputs can represent up to 2^n input combinations. Four bits provide 16 combinations, of which the ten combinations 0000 through 1001 are valid BCD digits. The remaining six are invalid BCD codes. Thus the decoder still requires four input bits.
How to Solve It in the Exam
BCD = 8421 = four input bits.
Important Exam Point
Do not confuse number of outputs (10) with number of inputs (4).
Related Revision Path
This question sits in the revision path Digital Electronics → Combinational Logic → Decoders .
Quick Trick
BCD = 8421 = four input bits.
Common Mistake
Answering 10 because it is a 1-of-10 decoder.
Exam Tip
Do not confuse number of outputs (10) with number of inputs (4).
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