HP 7925D Service Manual page 204

7925
for the heads to settle, the ACRY signal will become active
(ACRY
=
0). The drive ready flip-flop is not affected. It
remains set from the initial head load
operation~
When the ACRY signal becomes active (ACRY
=
0), it
cancels the 1667 millisecond timeout cycle; clocks the
ACRY attention flip-flop set; and enables future seek,
recalibrate, or write operations. The state of the ACRY
signal can be observed at the test point on drive control
PCA-A4 labeled "ACRY".
The set output from the ACRY attention flip-flop causes
the attention status bit to be active (status bit 8
=
1). This
will notify the controller that the disc drive has correctly
positioned the heads over the home position (cylinder 0).
This status bit can be selectively cleared by the controller
if it issues a CLA command.
The heads will remain settled over the home position
(cylinder 0) until a seek, set offset, or another recalibrate
command is decoded, or until they are unloaded when the
RUN/STOP switch is set to STOP or a fault condition is
detected.
A-49. EMERGENCY RETRACT OPERATION.
The circuitry used to retract the heads during an
emergency condition is located on PMR PCA-A9. It con-
sists ofthe retract timer, programmable voltage regulator,
.and linear motor relay. An emergency retract operation is
initiated whenever the head positioning servo loop is dis-
abled (SEN
=
1) or a power supply failure is detected
(PSF
=
0). These conditions can be observed at the test
points on PMR PCA-A9 labeled "SEN" and "PSF".
Whenever either of these conditions exists, the linear
motor relay will be de-energized to permit a retract volt-
age to be applied to the linear motor coil. Initially a retract
voltage of approximately 7 volts is applied to the coil for
about 500 milliseconds. The retract voltage is then re-
duced to approximately 4 volts until the carriage is fully
retracted (CRB
=
1) at which time the retract voltage is
removed. During an emergency retract operation, the car-
riage will normally reach its fully retracted position be-
fore the retract voltage is reduced. Sustaining the higher
retract voltage for an excessive period of time can damage
the programmable voltage regulator, therefore, the re-
tract voltage is reduced in the event that the carriage fails
to reach its fully retracted position before the retract timer
times out. The retract timer is designed to accept power
from either the +10'or +36 Vdc supply, thus, if either
supply should fail, the circuit will still function. Further,
if both supplies should fail (as in the loss of mains power),
the rotating spindle will act as a generator to provide
enough power to retract the heads. The emergency retract
voltage can be observed at the test point on PMR PCA-A9
labeled "ERV".
If a timeout or interlock fault should occur during normal
operations (TOF or ILF
=
1
l,
the servo enable flip-flop will
be cleared to disable the head positioning servo loop. This
will de-energize the carriage unlatched solenoid (ECS
=
0
and CSOL = 1) and linear motor relay (SEN = 0
l,
disable
Appendix A
the linear motor power amplifier (LMAE
=
0), and initiate
an emergency retract operation (ER
=
0) after a 60
millisecond delay to ensure closure of the linear motor
relay contacts. The state of the ER signal can be observed
at the test point on PMR PCA - A9 labeled "ER".
The interlock (ILF) line goes through an inverter and
becomes the signal ILFL. If an interlock fault (ILF
=
1,
ILFO
=
0) or a write fault (WFLT
=
0) should occur, head
selection will be terminated. This action prevents the
heads from writing on the disc during an emergency re-
tract operation.
If a failure is detected in one or more of the power supplies
(PSF
=
0), a greater emergency is said
to
exist because it
cannot be assumed that supply voltages are available
to
power the disc drive circuitry. In this case, the linear
motor relay is immediately de-energized (PSF
= Q)
and a
FET switch grounds the ER signal line
(E'R
=
0) to force
an emergency retract operation. In addition, the PSF sig-
nal disables power to the spindle permitting it to coast to a
stop and holds the door unlock solenoid de-energized to
prevent access to the pack chamber until the carriage has
been fully retracted (CRB
=
1), spindle has come to a stop
(SPD
=
1), and the RUN/STOP switch has been set to
STOP (STOP
=
1).
A-50. SECTOR SENSING SYSTEM
The sector sensing system (see figure A-19) consists of
circuits on track follower PCA-A5 and microprocessor
PCA-A2, although all communication between these two
PCA's occurs via motherboard PCA-A7. The purpose of the
sector sensing system is to monitor circumferential head
position by continually monitoring the physical location of
each data sector as it passes beneath the heads. It notifies
the controller when the present sector count equals the
addressed sector. In addition, it enables the read/write
system for a data transfer operation and determines the
response of the controller when the rotational position
sensing (RPS) feature is enabled.
To accomplish this, a sector clock and index pulse are
derived from the servo code which is magnetically re-
corded on the servo surface (see figures A-2 and A-8). The
servo code consists of 6720 di-bits per revolution, although
three of these di-bits are not recorded in the index zone. As
the servo head flies over the servo surface, a voltage is
magnetically induced. The output from the servo head is
directly coupled to the input of the differential
preamplifier stage. This stage consists of two differential
amplifiers coupled together by a filter network. The gain
of the first differential amplifier is controlled by the out-
put from the AGC circuit on track follower PCA-A5. The
differential output is filtered and coupled to a second,
fixed-gain differential amplifier. The output from the dif-
ferential preamplifier stage can be observed at the test
point labeled "PRE". It will be approximately 1.4 volts
peak-to-peak.
A-23