unit. The controller selects the desired head and direct seeks  the  read/write  head  assemblies  to  the  selected cylinder  position. During  a  write  operation,  data  is output from the controller buffer memory to the disk write circuitry and recorded on the disk using the modified frequency modulation (MFM) encoding method.  Modified  frequency  modulation  encoding  is covered in detail later in this chapter. During a read operation  the  drive  recovers  data  from  the  disk  and transfers it to the controller. The disk drive uses a motor driven belt and pulley arrangement to rotate the mounted disk pack at a speed of 3,600 rpm ± 3.5%. The speed of disk pack rotation is monitored by a spur gear and photodcell arrangement. The  read/write  heads,  five  addressable  read/write heads, and one read-only head (servo head) are mounted on arm assemblies controlled by an actuator assembly. The disk pack must be rotating above 3,100 rpm before the actuator assembly will load the heads or move them over the recording surfaces. The heads are designed to float above the disk pack recording surfaces on the air cushion provided by the high-speed rotation. Any contact between the read/write heads and the disk recording surfaces will result in a head crash and damage to both heads and recording surfaces. The heads  are  automatically  unloaded  or  retracted  if  the drive motor power is turned off or the rotation speed of the disk pack drops below 3,100 rpm. The  movement  of  the  read/write  heads  to  the desired cylinder position is controlled by a closed-loop servo  system.  Prerecorded  data  written  on  the  servo surface is used to (1) determine the present position of the read/write heads, (2) control the movement of the read/write heads when seeking a new cylinder, and (3) maintain alignment of the heads to the tracks on the recording surfaces when data is being read or written. The  disk  drive  is  divided  into  the  following electronic  and  electromechanical  assemblies  and functional  areas: Drive motor assembly Spindle assembly Speed sensor Actuator assembly Velocity transducer Head/arm assemblies Servo circuit 10-20   Track servo circuit Read/write circuits Drive Motor Assembly The drive motor, which drives the spindle assembly, is  a  1/2-horsepower  induction  motor.  Power  is transferred to the spindle via a flat, smooth surfaced belt that connects the pulleys of the spindle and drive motor. The speed of the drive motor is sensed by an optical switch and controlled by the motor supply module in the  power  supply. Spindle Assembly The spindle assembly is the physical interface between the disk drive and the disk pack. The surface of the disk pack mounting plate on the spindle mates directly  with  the  center  of  the  disk  pack.  Mating surfaces of the disk pack and spindle are engaged by rotating the cover handle of the disk pack when you install the pack in the drive. When the pack and the spindle  are  fully  engaged,  the  canister  cover  is  released from the disk pack. You can then remove the cover. The spindle is driven by the drive belt, which connects the spindle to the drive motor pulley. A static ground spring is mounted at the lower end of the spindle assembly to protect against the buildup of a static charge. A spur gear is mounted on the lower end of the spindle drive shaft. The teeth of the gear pass through the optical switch and are used as part of the speed sensor. Speed Sensor The speed sensor monitors and controls the rotating speed of the spindle and its attached disk pack. The speed sensor is made up of the spur gear and the speed sensor photocell in the optical switch. The teeth of the spur gear pass through and interrupt the light path between the emitter lamp and photocell. The pulsed output generated by the speed sensor is sent to the power supply module. The power supply varies the drive motor current to control the speed of the drive motor and to maintain spindle speed within the required limits. Actuator Assembly The  actuator  assembly  is  the  mechanism  that supports and moves the head/arm assemblies. The actuator  is  made  up  of  a  carriage  and  voice  coil