TABLE OF CONTENTS This chapter describes the most commonly used binary codes. In particular, we will discuss the Binary Coded Decimal (BCD), the Excess 3 Code, the 2*421 Code, the Gray Code, and the American Standard Code for Information Interchange (ASCII) code. We will also discuss parity bits.
In general, encoding is the process of putting a message into a code. In this text, encoding will refer to a process where the decimal numbering system as well as other alphanumerics (combinations of numbers and letters) are represented by the digits of the binary numbering system, that is 0 and 1. In this section we will introduce four different codes where 4 bits are used to represent a single decimal character.
The Binary Coded Decimal (BCD) uses 4 bits to represent the decimal numbers 0 through 9 and these are shown in the Table 4.1. This code is also known as 8421 code because the digits 8,4, 2, and 1 represent the weight of its bits position. In other words, the BCD is a weighted code.
| Decimal | BCD |
|---|---|
| 0 | 0000 |
| 1 | 0001 |
| 2 | 0010 |
| 3 | 0011 |
| 4 | 0100 |
| 5 | 0101 |
| 6 | 0110 |
| 7 | 0111 |
| 8 | 1000 |
| 9 | 1001 |
We observe that only ten of the sixteen (2 4) possible combinations are used in BCD; the remaining six combinations 1010, 1011, 1100, 1101, 1110, and 1111 are invalid in BCD. The designations A through F to represent these numbers are...
