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10. Standard Digital Codes

The two previous chapters discussed the physical characteristics of the media and how digital logic levels are represented on the media. This chapter looks at how information that we, as human beings, understand such as alphanumeric characters are represented digitally.

Computers understand numbers, plain and simply. They process information in binary using only the numbers 1 and 0. Unfortunately, humans do not work in binary but in alpha-numberics (numbers and alphabets). Characters such as numbers and the letters of the alphabet are assigned a 7 or 8 bit code to identify each character. This 7 or 8 bit code is a number that the computer can work with. The types of characters stored in a computer include:

There are 2 major translation codes (called character sets) that exist today:

1. ASCII (pronounced ah-skee)
2. EBCDIC (pronounced eb-ce-dic)

The job of these character sets is to translate from the human world (where we use alpha-numeric characters) to the computer world (which uses binary numbers 1s and 0s).

10a. EBCDIC - Extended Binary Coded Decimal Interchange Code

Table 18-1 shows the EBCDIC translation table. Computers speak in binary code which is 1s and 0s. The computers do not know what the letter "A" is. Instead they speak of the letter "A" as the binary number 1100 0001. It is not easy for humans to remember binary numbers such as 1100 0001 but it is easier to remember the hexadecimal number C1. The hexadecimal number C1 is equal to the binary number 1100 0001 where C = 1100 in binary and 1 = 0001 in binary.

In addition the hexadecimal number C1 is equal to the decimal number 193. The table 18-1 shows both the decimal (dec) number and the hexadecimal (hex) number for the capital letter "A". Lower case "a" is represented by the EBCDIC decimal code 129 or hexadecimal code 81.

Besides character codes such as the previous letter "A", the EBCDIC code also defines control characters. These are characters that have special meaning. For example, the control character FF stands for Form Feed and is used by printers to advance one page or to eject a page. The decimal code for FF is 12 and the hexadecimal code is C.

Both hexadecimal and decimal codes are indicated because many times, a program or interface will report the EBCDIC code in one or the other formats. You may have to use Table 18-1 to translate from the numerical code to the actual character.

Note: Some EBCDIC codes are not defined and have no name.

Introduction to Data Communications
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