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22. Standard Digital Codes

Computers process information in digital form. Characters are assigned a 7 or 8 bit code to indicate which character it is. This 7 or 8 bit code becomes a number (usually hexadecimal) that the computer can work with. The characters stored in a computer include:

Lower case letters:		a - z
Upper case letters:		A - Z
Digits:		0 - 9
Punctuation Marks:		. , ; : ! ? etc...
Unit Symbols:		# $ % & * etc...
Control Codes:		EOF, etc..

There are 2 major codes existing today: ASCII (pronounced ah-skee) and EBCDIC (pronounced eb-ce-dic).

22a. EBCDIC - Extended Binary Coded Decimal Interchange Code

EBCDIC is used mainly by IBM mainframes and compatibles. It is not common in the PC LAN world unless you are connecting to the IBM mainframe world. In order to connect, you would require either an IBM 3270 terminal emulation program or a device called a gateway.

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.

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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