FLOPPY DISK DRIVE UNITS (INPUT/OUTPUT)

Floppy disk drive units are physically smaller than magnetic disk drive units and are typically used with personal (desktop) computers (fig. 2-22). The unit consists of a disk drive in which the disk rotates and a controller containing the electronic circuitry that feeds signals onto and from the disk. The disk (diskette) is a thin, flexible platter (floppy disk) coated with magnetic material so characters can be recorded on the surface in the form of magnetized spots. Floppy disks come in several sizes from 3 to 8 inches in diameter. The most common are the 8-inch disk, the 5 1/4-inch disk, and the 3 1/2-inch disk.

Figure 2-22. - Floppy disk drive unit.

PRINTERS (OUTPUT)

Printers are widely used output devices that express coded characters on hard (paper document) copy (fig. 2-23). They print out computer program results as numbers, letters, words, symbols, graphics, or drawings. Printers range from electronic typewriters to high-speed printers. High-speed printers are usually used on mainframes and minis to prepare supply requisitions, pay checks, inventory, or financial reports at 10 lines per second and faster. The types of printers we'll discuss are daisy-wheel, dot matrix, ink jet, and laser. These are the ones commonly used with personal computers.

Figure 2-23. - Printer.

Daisy-Wheel Printers

Daisy-wheel printers have the most professional-looking, pleasing-to-the-eye print of all the printers in the character-at-a-time impact printer class. Daisy-wheel printers are often used in an office or word processing environment, where crisp, sharp, high-quality (letter quality) characters are a must. The daisy-wheel printer uses a round disk, with embossed characters located at the end of each petal-like projection (one character per petal), similar to the petals of a daisy, as shown in figure 2-24. A drive motor spins the wheel at a high rate of speed. When the desired character spins to the correct position, the print hammer strikes that character causing it to be printed on the paper. Once printed, the daisy wheel continues to move, searching out the next character to be printed, until the line is completed. The speeds of daisy-wheel printers range from 30 to 60 characters per second (cps).

Figure 2-24. - A daisy-wheel print wheel.

Dot-Matrix Printers

Dot-matrix printers, (also known as the wire matrix printers) create characters in much the same way you see numbers on the scoreboard at a baseball or football game. In contrast to the daisy-wheel printers, dot-matrix printers use an arrangement of tiny pins or hammers, called a dot matrix, to generate characters a dot-at-a-time. A dot-matrix print head builds characters out of the dots created by the pins in the matrix. Figure 2-25, view A, shows what dot matrix characters look like when printed.

Figure 2-25. - Dot-matrix printing.

The dot matrix is defined in terms of rows and columns of dots. A 5 by 7 matrix uses up to five vertical columns of seven dots to create a character. An example of a 5 by 7 matrix printing the letter H is shown in figure 2-25, view D. The size of dot matrixes varies from a 5 by 7 matrix to as large as a 58 by 18 matrix. A number of dot-matrix printers use a single vertical column of pins to print characters, as shown in figure 2-25, view B. The characters are printed by moving (stepping) the print head a small amount and printing the vertical columns one at a time until the character is printed as shown in figure 2-25, views C and D.

The size of the matrix determines the quality of the printed character. In other words, the more dots used to print a character, the better the character is filled in and the higher its print quality. Dot-matrix printers are faster than the daisy-wheel printers with speeds ranging from 60 to 350 cps, but their print quality is not as good.

Ink Jet Printers

Ink jet printers employ a technique very similar to the way we use a can of spray paint and a stencil. A spray of electrically charged ink is shot (under pressure) toward the paper. Before reaching the paper, the ink is passed through an electrical field which forms the letters in a matrix form. The print resulting from this process consists of easy to read, high-quality characters. Some manufacturers use large droplets of ink for faster printing, while others use small droplets for better clarity but with slightly reduced printing speeds. These printers can print up to 300 cps.

Laser Printers

Laser printers direct a beam of light through a rotating disk containing the full range of print characters. The appropriate character image is directed onto photographic paper, which is then put through a toner, developed, and used to make additional copies. The print resulting from this process consists of sharp, clean images that are easy on the eyes. These printers can print up to 20,000 plus lines per minute, or 26,666 cps (characters per second).

KEYBOARDS (INPUT)

A keyboard is nothing more than an array of switches called keyswitches. Keyboards are designed to input a code to the computer when a keyswitch is depressed. Each keyswitch, or key, on the keyboard is assigned a particular code value; and it is usually imprinted with a legend to identify its function. Figure 2-26 shows a keyboard combined with a crt on a microcomputer.

Figure 2-26. - Keyboard combined with a crt and microcomputer.

The primary purpose of a keyboard is to enter or input alphanumeric (numbers, letters, and special characters) character codes. The major grouping of keyswitches on a keyboard will be in one of the two styles of a typewriter keyboard arrangement (QWERTY or DVORAK).

The typewriter keyswitches are arranged in 4 rows of 10 or more switches. The keyboard arrangement shown in figure 2-27 is QWERTY. The rows are usually offset to the row above to make it easier to reach all the keys when typing. The tops of the individual keyswitches are sculptured to conform to the shape of the human finger.

Figure 2-27. - Keyboard layout.

Other groupings of keyswitches are used for special purposes, such as number entry (calculator) keypads, special function switches (F1-F12), and cursor control keys. The special function switches allow an operator to use the special functions designed in the software. For example, in a word processing program, you can use them to spell check a document, search for a particular portion of text, move text from one place to another, and to print hard copies of a document. These are but a few of the functions allowed; however, as you become more familiar with computers you will learn them all. The cursor control key allows you to move to different locations on the screen.

The design of keyboards varies from device to device and is dependent on the requirements of the system in which the keyboards are installed.

Keyboards are generally used with nontactical computer systems. However, the newer tactical display system consoles have optional keyboards for data entry. A keyboard may be built into the display device, or it may be a separate component connected only by a communication cable.