1-of-10 BCD Decoder — Required Number of Inputs A 1-of-10 BCD decoder activates exactly one of ten outputs (0–9). How many input lines are required to represent the BCD value applied to the decoder?

Introduction / Context:
Binary-Coded Decimal (BCD) represents decimal digits 0–9 using a 4-bit binary pattern. A 1-of-10 decoder uses this 4-bit code to select one of ten output lines. Understanding the required number of inputs is key when interfacing counters and displays.

Given Data / Assumptions:

• Target device: 1-of-10 decoder (e.g., 74xx42 family).
• Valid inputs: decimal 0 through 9 only.
• Input coding: standard 8421 BCD.

Concept / Approach:
BCD uses 4 bits per decimal digit. Therefore, a BCD decoder that uniquely identifies digits 0–9 requires four input lines (A, B, C, D). Illegal codes 1010–1111 are typically treated as don’t-care or invalid.

Step-by-Step Solution:
Recognize 8421 BCD → 4 bits.Map 4-bit combinations 0000–1001 to outputs 0–9.Ignore 1010–1111 (not used) or handle per datasheet.Hence, the decoder needs 4 input lines.

Verification / Alternative check:
Examine datasheets for BCD decoders; pin listings show four data inputs plus enable pins.

Why Other Options Are Wrong:

• 8: Would be used for two BCD digits (8 bits).
• 10: Misinterprets outputs as inputs.
• 1: Cannot represent 10 distinct values.

Common Pitfalls:
Confusing the number of outputs (10) with the number of input lines (4 for BCD).

Final Answer:
4