assembly,  a  rail  bracket  assembly,  and  a  magnet assembly. The carriage is attached to the voice coil. The carriage  supports  the  head/arm  assemblies  and  provides the vehicle for head/arm positioning. The voice coil moves the carriage in (extended) or out (retracted) as determined by servo logic commands. The  rail  bracket  assembly  provides  a  stable  support and  guide  for  carriage  movement.  The  carriage bearings move along the upper and lower carriage rails as the carriage is extended or retracted by the voice coil. The magnet assembly is a very strong permanent magnet that forms the core of the voice coil and is used to mount components of the velocity transducer. Velocity Transducer The  velocity  transducer  helps  to  control  the acceleration and deceleration of the carriage assembly during seek operations. The transducer coil has a voltage induced in it by the motion of the transducer core attached to the carriage. The voltage polarity and amplitude  are  sensed  by  an  operational  amplifier  and used  to  indicate  the  direction  and  speed  of  carriage assembly movement to the servo circuit logic. Head/Arm Assemblies There are six head/arm assemblies in each disk drive. One of the head/arms holds the read-only servo head. The other five assemblies hold read/write heads. The  servo  head/arm  assembly  and  two  of  the  read/write head/arm  assemblies  are  upper  surface  head/arm assemblies.  The  three  remaining  read/write  head/arm assemblies are lower surface head/arm assemblies. The   read/write   heads   are   mounted   on   cam controlled   head   load   springs.   As   the   head/arm assemblies are loaded (extended) the head load springs apply force (loading force) to the read/write heads to move them toward the rapidly spinning disk surface (3,100 rpm minimum).  The air cushion above the surface of the disk causes the head to float above the recording surface. As the head assemblies are unloaded (retracted), the head spring loading force is restrictedly the cams and the heads are moved away from the recording surface. Servo Circuit The servo circuit is a closed-loop servo system. It is used to move the read/write heads to the desired (addressed)   cylinder   when   commanded   by   the controller. The servo circuit is designed to maintain a NULL or 0 voltage when the heads are in the correct cylinder position.    A position error signal is used to indicate when the heads are not in the proper cylinder location. The position error is fed to the voice coil and results in carriage movement toward the addressed cylinder. A feedback signal is developed using the velocity transducer to oppose the position error and to dampen carriage movement for smoother operation. Track Servo Circuit The track servo circuit is used for maintaining head position over the track centerline. The track servo circuit  positions  the  read/write  heads  based  on information obtained from the servo tracks written on the servo surface of the disk pack. The read-only servo head reads the data written on the servo tracks and is positioned accordingly. The read/write heads mounted above (heads 0 and 1) and below (heads 2, 3, and 4) the servo head are physically aligned to the servo head. By positioning  the  servo  head,  all  read/write  heads  are positioned over the center of the correct track on their respective recording surface of the cylinder. Read/Write Circuits The  read/write  circuits  perform  the  following functions: When  writing,  they  (1)  convert  serial  NRZ signals from the disk control logic to MFM data signals, and (2) generate and control drive current to the write heads for developing the flux fields used to store information on the disk surface. When reading, they (1) detect flux changes from the disk, (2) convert the analog MFM signals to digital MFM data, (3) convert MFM data to NRZ serial pulse train and send it to the disk control logic, and (4) generate the read clock signal. DISK MEMORY SET OPERATIONS The disk memory set receives data from the host computer for storage on the disk and retrieves data from the disk and transfers it to the computer. Because of the relatively fast access time of the disk memory set, the host computer uses the disk as temporary storage of data as well as permanent storage of programs and data. Before a disk can be used, it must first be formatted. 10-21