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7.2
CONTROLLER
STEP
DIRECTION
HEAD SELECT
0,1,2
MPU: PC BOARD 20243-XXX
CONTROLLER
SEEK·
COMPLETE
READY
TRACK 0
STEPPER
CIRCUIT
FIGURE 26:
M
PU: PC Board
20243-XXX
INDEX
CIRCUIT
TRACK
0
CIRCUIT
MPU
IWR
MOTOR
ON
STEPPER
MOTOR
WRITE
CIRCUIT
MOTOR
PCB
The microprocessor monitors and controls the internaI drive functions and the
host interface lines. The MPU has only three active modes: Initializing, Seek-
ing, Waiting.
At power-on the MPU initializes the stepper circuit to phase A and resets aIl
interface lines under its control. Power is then applied to the Motor Control
PCB and the Index line is monitored for spindle rotation. If rotation is not sensed
within 15 seconds, the MPU removes power.
As the drive spins-up, 660 revolutions are counted before the MPU assumes that
the drive is at nominal speed. The drive then initiates recalibration. If the heads
are not over Track 0, the drive will step at 3 msec./track. When both mechani-
cal Track 0 and electrical phase A are present, the heads are at Track 0 and
stepping will cease.
Within 20 msec. both SEEK COMPLETE, Track 0 and READY will go true
and the drive is then ready to accept inputs from the controller.
When in the idle mode, the MPU loops, waiting for step pulses. The MPU also
monitors power and, should the power drop by approximately 20070, it will revert
to the initialization routine described above.
Upon receiving a step pulse, the MPU pauses for 500 p,sec., to allow for addi-
tional pulses, before executing the seek operation. Every incoming pulse will
reset the 500 p,sec. timer. The seek will not beginuntil the last pulse is received.
When seeking, the MPU counts the number of tracks to be covered and searches
the PROM for the optimum acceleration/deceleration algorithim.
Steps are accomplished by setting IC lA to sequential phases. It provides bath
a current source and sink with the motor windings. Near the end of a seek, ICs
4A and 4B are set to provide additional current to offset hysteresis.
Upon completion of a seek operation settling time is calculated, based upon
the length of the seek, and the interface line is activated in the specified time
periode With SEEK COMPLETE, the MPU returns to the idle mode.
Whenever a seek operation crosses the write current boundary switch at Track
128, the MPU reduces the write current to the heads to eliminate pulse crowd-
ing duringwrite operations.
30
ST212 PRODUCT MANUAL/Rev. B
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