Service manual
3-
4
1l....
DOUBLE-DENSITY
DISK
CONTROLLER
U14
RAW READ LATCH
U14lengthens
the
raw
read
pulse
from
the
disk
drive electronics to a size
readable
by
the
1797 (U12).
U15
DRIVE BUFFER
U15 buffers
the
control
and
data
signals to
the
two
drive interfaces.
U16,
U17,
U18,
Q1, AND Q2 PHASE LOCK LOOP TRACKING
AND PRECOMPENSA
nON
U16
supplies
U17
with
either
a
pull-up
(PU) or
pull-down
(PD)
signal.
This
signal
is
then
filtered by C29
and
R13
and
then
called FC - frequency control. Voltage control oscil-
lator U1?
changes
its frequency,
higher
with
a PU signal
and
lower
with
a PD signal.
The
change
in
frequency
produces
a
corresponding
change
in
rotational
speed
of
the
disk.
Variable resistor R10 biases
the
PU
and
PD
output
at 1.4 volts (this
means
that
PU/PD are
1.4 volts
when
the
phase
lock is inactive). This allows
the
phase
lock to
more
quickly
lock
on
to
the
data
being
read. Variable resistorR17 adjusts
the
VCO's
center
frequency to
2 MHz.
The
VCO's
output
is
divided
down
by U16. This signal
then
becomes
the
read
clock
(RCLK)
signal,
which
is
used
by
the
1797 to separate
disk
data
and
disk
clock signals.
The
U16 also strobes
the
early
and
late signals for
data
precompensation.
These
inter-
nally latched signals
determine
which
phase
of a four-phase clock generator, U18, will
be
used
for
the
precompensation
process. All
phases
are
identical
in
pulse
width
(+
or
-50
nsec).
The
phase
delay
time
is set by R20.
Precompensation,
used
for 80-track
double-density
disk
write
operations, places
data
properly
on
the
disk
so
that
it
can
be
read
back
with
minimum
error. (Error
may
be
introduced
by
the
shifting of old
data
as
new
data
is
written
- as
data
is
written,
data
ad
jacent to
the
new
data
is shifted over
because
of
the
nature
of
the
magnetic
medium
of
the
disk.)
U18,
precompensation
clock generator,
provides
the
clock signals
needed
for precom-
pensation
of write signals.
U19, U20, U21, AND
U22
DISK DRIVE INTERFACE
The
disk
drive interface lCs buffer all
signals
to
and
from
the
disk
drive electronics by
means
of
open
collector drivers.
U23 VOLTAGE REGULATOR
U23
supplies
+5
V
regulated
toU1?, isolating
the
U17 from
theZ-89
+5
V
power
supply.
u