HARD DISKS AND HARD DISK DRIVES.\ Although most microcomputer systems you will be working on use some type of floppy disk drive, you should also be aware that all micros also contain a small sealed unit called a Winchester disk or hard disk drive. Unless the disk drive is an external type unit, you might never know it was there. Floppy disk drives are easy to spot; all you have to look for is the drive's door, that open slot where you insert the diskette. Hard disk drives, on the other hand, are normally sealed

Figure 1-20.\A floppy disk drive being removed from system unit.

Figure 1-21.\Internal hard disk drive mounted on disk drive chassis.

units that can be tucked away just about anywhere. Generally, you will find them inside the system unit (figure 1-21) in the space occupied by one of the floppy drives.

Hard disk drives provide you with many timesaving features not available nor possible with the floppy disk drives. These include increased access speeds, greater storage capacity (up to 8 gigabytes [GB] of storage), and overall convenience. Working with hard disk is much easier because you can quickly end one program and start another, all through the operating system.

Hard disk units consist of rigid metal platters that are tiered or stacked. In most cases, the disks themselves are not removable, and for this reason can be hermetically sealed in the storage unit along with the access mechanism that contains the read/write heads. Because this type of disk drive is completely sealed and free from air contamination, the read-write heads can be positioned to within 20 millionths of an inch from the surface of the disk. This also allows the disk to be rotated at a high rate of speed-typically, 3600 revolutions per minute. Hard disks also comes in two sizes (diameters): 5.25 inches, and 3.50 inches, with 3.50 being the most popular. Their storage capacities range from 500 megabytes to 8 gigabytes, with the majority in the 1.2- to 2-gigabyte range.

As mentioned earlier, hard disks can be placed internally within the system unit, like the one shown in figure 1-21, or externally as a detachable unit, as shown in figure 1-22. You may have also noticed that the disk drive shown in figure 1-22 is a portable hard drive in that the hard disk is removable. The hard disks and read/write mechanism are enclosed within a rugged case. The result is a self-contained hard disk you can insert like a floppy and remove for reasons of security.

Figure 1-22.\External hard disk drive with removable disk cartridge.

Printers

Most printers you are likely to use while working on microcomputers are the low-speed varieties. They are able to print text at rates of 200 to 800 characters per second and usually output this information one character-at-a-time. After close examination, you will find that basically the only major difference between printers designed for micros and those used on larger mainframes is their size, and the speed at which they print. Other than that, they are very much alike in terms of setup, operation, and operational features.

Micro printers, like mainframe printers, are either impact or nonimpact. At least six printing technologies are used to produce hundreds of microcomputer printer models. These technologies include dot-matrix (which forms characters using a series of dots), xerographic, electrostatic, ink jet, and laser.

DOT-MATRIX PRINTER.\ The dot-matrix printer uses a print head made up of pins. It creates the characters by using these pins to generate characters a dot-at-a-time. Dot-matrix printers have speeds that range from 60 to 350 cps.