*Question E8C09:
Which digital code allows only one bit to change between sequential code values?*  

*Correct Answer is  D: Gray code*  

### AI Response: (may contain errors)  

The correct answer to the question is **Gray code**. Here's why:

### Why Gray Code is Correct:
- **Gray code** is a binary numeral system where two successive values differ in only one bit. This property is particularly useful in applications such as rotary encoders and error correction in digital communications, as it minimizes the chance of errors during transitions between values.

### Why the Other Choices are Incorrect:
- **A: Binary Coded Decimal Code (BCD)**  
  - BCD represents decimal numbers using four bits for each digit. For example, the decimal number 25 is represented as 0010 0101 in BCD. In BCD, multiple bits can change between sequential values (e.g., from 0010 to 0011).

- **B: Extended Binary Coded Decimal Interchange Code (EBCDIC)**  
  - EBCDIC is an 8-bit character encoding used mainly on IBM mainframe and midrange computer systems. It does not have the property of changing only one bit between sequential values.

- **C: Extended ASCII**  
  - Extended ASCII is a character encoding that uses 8 bits to represent characters, allowing for 256 different symbols. Like EBCDIC, it does not ensure that only one bit changes between sequential values.

### Summary:
- **Gray code** is unique in that it changes only one bit at a time, making it ideal for reducing errors in digital systems. The other options do not share this property, as they can change multiple bits between values.

This understanding of Gray code and its applications can help you in both the exam and practical scenarios in amateur radio and digital communications.