The Motor Control PCB receives velocity reference signals VR0-VR7 from the electronics PCB. These signals are
converted to an analog signal by the Velocity Digital-to-Analog Converter (DAC). The resulting analog signal is used to
generate signal SA. SA is used by the servo power amplifier to generate signals VC1 and VC2 (Voice Coil 1 and 2),
which drive the voice coil of the rotary actuator.
The signal POS, from the electronics PCB, is used to correct the position of the heads when they are on track, but drifting
slightly. POS along with the synthesized velocity signals, S0-S3, from the electronics PCB are converted to an analog
signal, VEL, by the velocity synthesis circuit. The signals VEL and POS are used, along with VR0-VR3, to generate the
signal SA and cause actuator movement.
If power to the disk unit is lost, relay K1 will deenergize. The contacts of the relay will disconnect VC1 and VC2 from the
voice coil and connect a capacitor across it. The capacitor, in the HDA, is charged by +5V and polarized to force the
positioner to move to the outer stop, a safe position away from the data zone.
Indicator Panel. The indicator panel, located on the front panel, provides controls and status indicators
for the disk unit.
The panel has six light-emitting diode (LED) indicators and three switches. The function of the indicators and switches is
explained in table 1-3. Each of the six LEDs has a current limiting resistor connected to its cathode. The other ends of
the six resistors are connected to +5V. The anode of each LED is connected, through the connector, to a logic signal
within the disk units circuitry. When the logic level goes true (low level) the appropriate LED will illuminate.
The anodes of the LED are also connected, each through an isolation diode, to switch S2, LED Test. When this switch is
raised, the anodes of the six isolation diodes are grounded, simulating a true signal to the six LEDs; all six LEDs are then
Switch S1 generates the signal SUB SYS TEST. When the switch is raised, the signal causes the adapter to perform its
GO/NO GO test. The thumbwheel switch generates signals DEV SEL 1 and DEV SEL 2 to assign a unit number to the
Head Disk Assembly. The Head Disk Assembly (HDA) contains the disk itself and the electronics
necessary to supply drive pulses to the disk drive motor. A major component of the HDA is a large die-cast aluminum
casting. All other components of the HDA mount on this casting.
Interior of the HDA Subassembly. The lower half of the HDA Subassembly consists of a large cavity
which is sealed by a bottom cover. Within this cavity are the recording media, read/write heads, and rotary arm actuator.
The recording media consists of three 8-inch diameter, rigid, Winchester-technology disk platters. Five of the six
surfaces are used for data storage. The sixth surface, the bottom side of the lower disk platter, has prerecorded servo
information, which is used by the servo system to position the read/write heads on the desired cylinder.