Restore operation. The Reverse Slow Mode is used for unloading the heads and
for
performing the first portion of a Restore operation. When operating in the Slow Velocity
Mode, the velocity reference developed from the Address Difference
lines is not used and
therefore Velocity Reference Enable (NLVREG) is not activated.
The
Error Check Logic performs two types
of
checks concerned with operation of the
positioner. The
first
check determines
if
the positioner has completed a seek within the
maximum
allowable time. This check is done
by
the Seek Time Error (NLSTEG) check
cirCUitry and is a gross type of check to determine simply that the positioner has not
become stalled due to a fault. Although each seek
is
checked by this circuit, it does not
verify that the time for the specific distance moved was compatible with the specific time
associated with that length
of
seek. Rather,
it
determines that the positioner has not
become
stalled while attempting a seek.
The other error check performed
Is to determine that the positioner has not travelled
outside
of
the legal range of travel. This is performed by the position
limit
monitor
circuitry which generates a Position Limit Error (NLPLEG) signal
if
the positioner exceeds
the normal range
of
travel. This check is performed only during the time that the heads are
loaded onto the disk.
4.7.5
SECTOR
ELECTRONICS
Figure 4-12*
is
a functional block diagram
of
the Sector Electronics logic and should be
referred
to
in conjunction with the following discussion.
The major function of the
Sector Electronics logic is to provide Sector pulses at the
I/O
interface which electrically subdivide the disk storage surface into a number
of
sectors
for
the purpose
of
addressing data stored on the disk. Additionally, the specific number of the
sector passing under the Read/Write heads
is
transmitted to the
I/O
interface
on
the
Sector Count lines. These lines specify the sector count presented
in
a binary format. The
count indicates the particular segment
of
the disk surface currently under the Read/Write
heads.
In
addition to the Sector Pulse
and
Sector Count, the Index Pulse is provided as
an
output
of the
Sector Electronics. This line provides a signal which is a pulse occurring once per
revolution of the
disk
and may be utilized
to
define the sector reference, i.e., sector zero.
Referring to Figure
4-12, it can
be
seen
that the inputs to the Sector Electronics logic are a
signal from the Upper
Sector/Index Sensor, a signal from the Lower Phasellndex Sensor,
Clock
Signals obtained from the clock countdown portion of the Start/Stop Control logic
and the Drive Motor Enable (LDMEG) signal also obtained from the
Start/Stop Control
logic.
The
removable cartridge may
be
sectored either electronically
or
mechanically while the
lower (fixed) platter in dual disk machines is sectored electronically.
Electronic sectoring can
be
in one
of
two configurations since the removable cartridge
may
be
one of two configurations,
I.e.,
an
index notch only,
or
with sector slots
and
an
index slot. The associated types of electronic sectoring
are
provided from
an
index-only
type
of
cartridge and from a sector-plus-index-slot type
of
cartridge.
The normal top-loading cartridge has one slot in the armature plate which is referred to
as
the index notch. This is the standard top-loading cartridge arrangement. Some specially
modified top-loading cartridges have additional notches used for mechanical sectoring
purposes.
*Foldout drawing,
see
end
of this section.
4-27
630D
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