Theory
of
Operation 2-5
Seek
Operation
Before reading or writing a sector of
data,
the
controller must position
the
read/write
head
of
the
disc drive
over
the
desired sector. To
do
this, the controller
sends
signals to the disc
drive
to
step
the
head
to
the
desired track.
The
controller
then
begins
to
read
the
disc. Track
and
sector information
is
stored
as a
preamble
to
each
sector,
and
the disc
is
formatted
in a
manner
to allow
the
controller to
detect
the
beginning of a
preamble.
On
finding a
pream-
ble, the Z80A first checks
that
the
track
number
is
correct.
If
not, it
will
continue
to
step
the
head
until
the
correct track
is
found.
The
sector
number
is
then
compared.
If
correct,
the
Z80A waits for
the
mark
to indicate
the
beginning of the
data
field
and
then
begin
reading
or
writing
the
data field. If incorrect, it
passes
over
this
sector
and
looks for
the
next
until it finds
the
desired sector.
If
it
can't
find
the
sector
after two revolutions of
the
disc,
an
error
condition
is
indicated
to
the
host
system.
Drive Theory
of
Operation
Introduction
This section provides a general description of
the
drive
module,
part
number
09895-69430.
Since this
module
is still
being
modified, this description
will
only
cover
those
areas
common
to all versions.
General Description
The
basic function of
the
drive
is
to indicate to
the
controller
when
it
is
ready
to
operate,
and
respond
to the
commands
of the controller to:
• Receive
and
generate
control signals,
• Position
the
read/write
heads
to selected tracks,
• Read
or
write
data
on
the
disc
when
selected.
Signals received
and
transmitted by
the
drive
are
shown
in Table 2-1.
Some
signals received
by the drive
are
gated
with drive select
so
that
no
stepping,
reading
or
writing
can
be
performed
on
an
unselected
drive. Also,
some
signals
generated
within
the
drive are
gated
with drive select
so
that
they
can't
be
transmitted from
an
unselected
drive.
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