Encoder I/O count – for a decimal-to-BCD encoder, how many separate data input lines exist to represent digits 0 through 9 before encoding them into 4-bit BCD?

Introduction / Context: An encoder converts one-of-N active inputs into a binary code. The decimal-to-BCD encoder is a classic example found in learning laboratories and MSI catalogs. It accepts decimal digit lines and outputs a 4-bit Binary Coded Decimal representation. This question asks you to recall how many input lines such an encoder must have to cover digits 0–9.

Given Data / Assumptions:

• Decimal digits: 0,1,2,3,4,5,6,7,8,9 (ten distinct symbols).
• BCD code width: 4 bits (0000 to 1001 used for 0–9).
• Only one input line is active at a time in the basic (non-priority) encoder.

Concept / Approach: Each decimal symbol must have a unique input wire in a simple one-hot interface. Therefore, a decimal-to-BCD encoder requires ten data inputs, each corresponding to a digit. The internal logic maps whichever input is asserted to its 4-bit BCD code on the outputs. Priority encoders extend this idea by resolving conflicts if multiple inputs are asserted simultaneously, typically choosing the highest-priority (largest) digit and producing a valid BCD code plus a “valid” flag.

Step-by-Step Solution:

Verification / Alternative check: Check any standard truth table for a decimal-to-BCD encoder: exactly ten inputs are listed, with corresponding 4-bit outputs and often an “enable” and “valid” pin.

Why Other Options Are Wrong: Two, four, five, and sixteen do not match the count of decimal symbols; sixteen would be appropriate for hexadecimal encoders, not decimal.

Common Pitfalls: Confusing encoders with decoders (decoders have one code input and many outputs); mixing decimal and hexadecimal contexts.

Final Answer: ten