For a 1-of-10 BCD (binary-coded decimal) decoder that selects one decimal output line (0–9), how many binary input lines are required?

Introduction / Context:
BCD represents decimal digits 0–9 using 4-bit binary words. A 1-of-10 BCD decoder asserts exactly one of ten outputs according to the BCD input. Determining the number of input lines helps with pin budgeting and logic design for display and selection circuits.

Given Data / Assumptions:

• Digits 0 through 9 are encoded in BCD.
• Each BCD digit requires four bits (0000 to 1001).
• Six combinations (1010 to 1111) are invalid and typically blanked or treated as don’t-care.

Concept / Approach:
Since BCD uses 4 bits per digit, the decoder must accept four input lines in order to uniquely represent the ten valid decimal values. The decoder then activates one corresponding output line among ten, often with additional blanking or error handling for invalid codes.

Step-by-Step Solution:
Recognize BCD width: 4 bits.Map valid codes: 0000..1001 → outputs 0..9.Provide four input pins to convey these bit values.Hence, the required inputs = 4.

Verification / Alternative check:
Standard ICs such as the 7442/74LS42 (BCD-to-decimal decoder) accept four inputs and produce ten active-low outputs, confirming the four-input requirement.

Why Other Options Are Wrong:

• 8 or 10 inputs: Unnecessary for encoding a single decimal digit.
• 1 input: Insufficient to represent ten possibilities.
• 5 inputs: Would represent up to 32 values, exceeding BCD needs.

Common Pitfalls:

• Treating invalid BCD inputs as valid; proper designs blank displays or signal error on 1010..1111.
• Confusing BCD with straight binary for 0–15 which would be a 4-to-16 decoder.

Final Answer:
4