adapter that the command has been completed. When data is being transferred between the controller and the adapter,
the controller will cause signal DREQ* to go true whenever the controller wants to transfer a byte. The OUT* signal is
used to inform the controller of the direction of data transfer.
Device Electronics PCB. The Device Electronics PCB contains the power-up and power-down
sequencing circuitry and the head-position control logic. It accepts commands from the Disk Controller PCB and, along
with the Motor Control PCB, controls the storage and retrieval of data to and from the disk. The electronics PCB uses a
microprocessor to supervise, control, and monitor all drive operations.
The electronics PCB receives its instructions from the controller PCB over an 8-bit, unidirectional bus (BUS0-BUS7).
Two other bits from the controller, BA0 and BA1, are decoded by the electronics PCB to determine if the byte is a
command, a cylinder or sector address, or a seek.
A command byte can be any of the following commands: Offset P (positive), Offset N (negative), Write Enable, and
Fault Reset. The commands are decoded by discrete logic sent to the Head Disk Assembly (HDA) for implementation.
If the byte is a cylinder address, sector address, or seek, motion of the rotary arm actuator to move the heads is required.
The microprocessor interprets these commands and causes the motion of the actuator.
The microprocessor uses the output of a "bad track" PROM, located on the Commutator PCB in the HDA, and the
cylinder address to determine if the track being requested is bad. If so, the actual track address on the disk for the
requested cylinder address is generated. The distance, in number of tracks, from the current head position and the
direction of movement is also calculated by the microprocessor.
The servo controller that controls the motion of the actuator is a combination of circuitry on both the Device Electronics
PCB and Motor Control PCB.
Movement of the actuator is initiated by the microprocessor. Based upon the distance the heads must move, a velocity
profile is determined. That is, the heads are not moved with a constant velocity; rather a current is applied to the voice
coil in such a manner as to cause the actuator to increase in velocity in a ramp fashion. The actuator may or may not
reach maximum velocity, depending upon the distance that must be traveled. As the heads approach the desired track,
the current to the voice coil is ramped down such that the actuator movement is slowed gradually, finally stopping when
the heads are over the desired track.
Based upon the velocity profile, the microprocessor generates velocity reference signals VR0-VR3. The signals are sent
to the Motor Control PCB where the Velocity Digital-to-Analog Converter (DAC) converts them to an analog current that
is used to drive the actuator's voice coil.
Feedback to the servo system is from the servo head, which generates signal Track Pulse. The positioner PROM uses
this signal and other signals related to actuator movement to generate the signals S0-S3. These signals are sent to the
Motor Control PCB where the Velocity-Synthesis Circuit uses them to generate a velocity feedback signal.