CONTROL CODES

Referring to table 12-1, if you want to print the word Navy, you would send the decimal codes 78, 97, 118, and 121 to the printer. An 8-bit code can represent decimal values from 0 through 255 making it possible to represent 256 different codes with one character set. ASCII only defines the first 128 codes. The other 128 codes are used by software developers and printer manufacturers for additional characters. The additional characters are called the alternate character set. Alternate Character Sets With the development of printers capable of printing graphics and nonstandard characters, the second half of the ASCII character set (128 through 255) became available to define additional special characters and features. Because software programmers found that having only one character set severely limited the capabilities of graphic capable printers, additional character sets were developed. Today, it is not unusual to find programs with eight or more complete character sets. These additional character sets may contain math symbols, foreign alphabets such as Greek, Russian, or Japanese, and other special symbols. To print the characters in an additional character set, you must have a graphics-capable printer and the program must specify the character set as well as the character code. Therefore, the characters printed are a combination of hardware and software capability. To make all this work together, software programmers must write a routine called a printer driver that performs several functions. A printer driver is written for each printer the software will support. The driver tells the software what the capabilities of the printer being used are and tells the printer how to print each character in the character set or sets. CONTROL CODES To make a printer print, the computer must have a method to control the printer. Printer control is accomplished with control codes. The original ASCII code contains 32 control codes. However, additional codes are needed to control the special features in modern printers. Most printers use a combination of the ASCII control codes and escape codes to enable and disable printer functions. Table 12-2.—Selected ASCII Control Codes ASCII Control Codes Table 12-2 shows examples of the ASCII control codes. Some of them you will recognize, such as carriage return and line feed. When printing, if the printer reaches the end of a line, the software must send a carriage return and a line-feed code. Without the line-feed code, the printer would overstrike the data just printed. The start-of-text tells the printer that all the codes following are data codes to be printed. Some printers have a selectable option that will automatically generate a line feed for every carriage return. Escape Control Codes The technology of printers has grown so that the original ASCII control codes can no longer support all the capabilities of most modern printers. Escape control codes are used to enhance printer operations beyond the limitations of the ASCII codes. Escape refers to the ASCII code 027, or the code generated by the ESCAPE key of the keyboard. Escape control codes can be used to change the style of print, the size of the print, whether the print is bold, and various other features of the printer. Escape codes are not standard and are defined by the printer manufacturer. 12-3