E8C09: Gray code: Digital code allows only one bit to

Q: Which digital code allows only one bit to change between sequential code values?

The correct answer is D: Gray code. The correct answer is D: Gray code. Gray code is a digital code that allows only one bit to change between sequential code values. This property minimizes errors when values are changing, because only one bit changes at a time. In Gray code, adjacent values differ by only one bit. For example, in 3-bit Gray code: 000, 001, 011, 010, 110, 111, 101, 100. Each step changes only one bit. This is different from binary code where multiple bits can change simultaneously (like 011 to 100, where all three bits change). Gray code is used in applications where multiple bits changing simultaneously could cause errors, such as in analog-to-digital converters, rotary encoders, and some digital communication systems.

Updated: Dec 9, 2025 | Source: 2024-2028 Question Pool | Topic: E8C

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Which digital code allows only one bit to change between sequential code values?

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Deep Dive: E8C09

The correct answer is D: Gray code. Gray code is a digital code that allows only one bit to change between sequential code values. This property minimizes errors when values are changing, because only one bit changes at a time. In Gray code, adjacent values differ by only one bit. For example, in 3-bit Gray code: 000, 001, 011, 010, 110, 111, 101, 100. Each step changes only one bit. This is different from binary code where multiple bits can change simultaneously (like 011 to 100, where all three bits change). Gray code is used in applications where multiple bits changing simultaneously could cause errors, such as in analog-to-digital converters, rotary encoders, and some digital communication systems.

Why Other Answers Are Wrong

Option A: Incorrect. Binary Coded Decimal (BCD) doesn't have the single-bit-change property. BCD encodes decimal digits in binary, and adjacent values can have multiple bit changes. Option B: Incorrect. Extended Binary Coded Decimal Interchange Code (EBCDIC) doesn't have the single-bit-change property. It's a character encoding, not designed for single-bit transitions. Option C: Incorrect. Extended ASCII doesn't have the single-bit-change property. ASCII is a character encoding where adjacent characters can have multiple bit changes.

Exam Tip

Single bit change = Gray code. Remember: Gray code is the digital code where only one bit changes between sequential values. This minimizes errors during transitions.

Memory Aid

**G**ray **C**ode = **O**ne **B**it **C**hange (think 'GC = OBC')

Real-World Example

You're using a rotary encoder that outputs Gray code. As you rotate the encoder, the output changes from one value to the next, but only one bit changes at a time. This prevents errors that could occur if multiple bits changed simultaneously (which might be misread during the transition). Gray code is ideal for this application.

Source & Coverage

Question Pool: 2024-2028 Question Pool

Subelement: E8C

Reference: FCC Part 97.3

Key Concepts

Gray code Single bit change Digital code Error minimization

Verified Content

Question from the official FCC Extra Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the E8C topic.