THE   3.5-INCH   FLOPPY   DISK   CON- STRUCTION— The 3.5-inch floppy disk is in a hard plastic case. The media access hole is covered by a metal  spring  loaded  shutter.  Write  protection  is provided by a slide switch on the bottom of the case. High density, 3.5-inch floppy disks have a media indicator hole in the disk. A disk without this hole cannot be formatted as a high density disk. FLOPPY  DISK  DRIVE  OPERATION—  Several components are common to all floppy disk drives. The spindle assembly/drive motor turns the disk at the proper speed. The drive circuit board controls the reading and writing of data on the disk. Connectors and cables connect the disk drive to the disk controller. The read/write heads actually read data from a disk and write data on a disk. DENSITY AND COERCIVITY— Density is the term that describes how much data can be stored on a disk.   Coercivity   is   how   much   magnetic   force, measured in oersteds, is required to properly write data on a disk. The density and coercivity of a disk is directly related to the magnetic media of the disk. USING LOW-DENSITY DISKS IN HIGH- DENSITY DRIVES— Avoid using low-density disks in high-density drives, especially in 5.25-inch drives. This is because of the difference in the size of the tracks that   high-density   drives   use.   Never   format   a low-density 5.25-inch disk as a high-density disk. The 3.5-inch disk drives do not have these problems because the media indicator hole in the disk case prevents using a low-density disk in a high-density format. FLOPPY DISK DRIVE INSTALLATION AND CONFIGURATION—   When installing a floppy disk, you have to determine how the disk is to be configured. You have to set the drive select jumper. Drive selection is  also  dependent  on  the  type  of  drive-to-controller cable  used.  You  must  also  determine  the  correct  setting for  the  terminating  resistor,  the  diskette  change line/ready jumper, and the media sensor jumper. FLOPPY DISK CARE AND HANDLING— Taking care of floppy disks will improve the reliability of the data stored on the disk. It is important to be aware of all potential sources of stray magnetic fields when storing your disks. DISK MEMORY SET— The disk memory set is also commonly referred to as a disk file unit or mass memory  storage  unit. These  devices  have  large removable disk packs and are mainly for use with mainframe computers. MAGNETIC DISK PACKS— Magnetic disk packs are hard platters coated with a magnetic oxide. They range is size from just 1 disk to over 14 disks. Many disk packs have a servo surface that contains permanently  recorded  data  used  for  positioning  the heads. DISK  FILE  UNIT  CONTROLS  AND  IN- DICATORS (DISK UNIT)— The disk memory set’s controls   and   indicators   allow   the   operator   and technician to set operating modes and monitor the operation of the disk memory set. DISK  MEMORY  SET  CONTROLLER—  The disk memory set’s controller manages the operation of the disk memory set. It has six main functional areas: the   controller   intercommunications   bus, microprocessor,  buffer  memory,  controller  to  disk  drive interface, the data bus control unit, and the CDS channel interface. DISK DRIVE UNIT—  The disk drive unit controls the  rotation  of  the  disk  pack,  the  positioning  of  the read/write heads, and the reading and writing of data on the  disk. DISK MEMORY SET OPERATIONS— Disk memory set operations include disk formatting, write operations, and read operations. CARE AND HANDLING OF MAGNETIC DISK PACKS—  Properly taking care of the disk packs can prevent major head crashes and data loss. FIXED HARD DISK SYSTEMS— Fixed  hard disk systems are also commonly referred to as hard disks.  They  are  common  in  minicomputers  and personal computers. FIXED HARD DISK DRIVE CONSTRUC- TION—  Fixed hard drives consist of one or more disk platters in a sealed head/drive assembly (HDA). The HDA also contains the read/write heads and the head actuator assembly. The head actuator assembly can be a stepper motor or voice coil. It controls the movement of the heads. The spindle motor is mounted outside of the HDA. The spindle motor shaft penetrates the HDA and turns the disk. The logic board of a fixed disk drive controls  the  position  of  the  heads  and  read/write operations. DATA ENCODING METHODS—  Methods for encoding  data  on  disks  were  developed  to  increase  data reliability and keep the controller synchronized with the drive. The two most common encoding methods in use are modified frequency modulation (MFM) and run length limited (RLL). 10-32