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small hysteresis between the upper and lower thresholds is maintained.
Overvoltage conditions are not detected.
After POR is released, the
drive is recalibrated, typically taking six seconds.
Circuit Description
See the schematics in SECTION 3 for the POR circuit.
Comparators U6
(output pins 13 and 14) compare the supply voltages to voltage refer-
ence CR12 to produce the signals -PORA, -PORB, and +POR.
C24 sets the
11 to 60 ms delay.
When the voltage across C24 rises above the CR12
reference, the U6 (output pin 1) comparator deasserts the three POR
signals.
Pin 56 (-RESET CAP) on the 5080 is an input if a POR is initiated at
the Q250/Q280: the 5080 is reset by the POR circuit either through the
RO plug or through Q28.
On the other hand, when the host asserts -RST on the SCSI bus, the
5080 pulls the -RESET CAP pin (now an output) low.
If the RO plug is
installed, -POR A is also pulled low and a cold reset is triggered.
If the RO plug is not installed, Q28 remains turned off, and the POR
circuit is unaffected.
However, a warm reset now follows, controlled
by the microprocessor, which has been triggered by an interrupt from
the 5080.
At the start of a warm reset, flip-flop U29 (Q
=
pin 9) is toggled on
by -RESET CAP, U29 then deasserts the SCSI lines -MSD, -C/D, -I/O via
gates U24 and U26.
Later, the microprocessor clears the flip-flop and
enables the lines via DICEY pin AUX 6.
2.3.2
Motor Control Circuit
Refer to the schematic in SECTION 3 for the circuit diagram.
The
motor control IC (U8) senses the differential output signals from the
three Hall-effect devices mounted around the motor, and drives cur-
rents through the three delta-wound motor coils.
The Hall signals
replace a commutator by indicating when to commutate current to the
coils.
Speed is sensed from a Hall signal and compared in U8 to a 2
MHz clock from FYLO.
Starting current is inversely proportional to
R97.
Loop gain and bandwidth of the op-amp current integrator are
controlled by R95, R96, C35, and C42.
C36 filters the integrator
output.
C39, C40, and C4l reduce ringing.
COIL A to COIL B voltage is processed by comparator U6 (output pin 2)
to generate MOTOR POSITION, which the microprocessor checks to deter-
mine that the motor is up to speed.
The signal is two cycles per
revolution.
2-20
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