Service manual
DOUBLE-DENSITY
DISK
CONTROLLER
--'1
3
:-
1
Circuit Description
Refer to
the
Block Diagram (fold-out from Page 4-1)
while
you read
the
following Circuit
Descriptions.
The
board's function is to translate
the
instructions
of
the
processor to
the
disk
drives.
Assume that
the
processor wants
to
write on
disk
drive
number
one. First,
the
processor
sends
the
proper
enabling
and
control signals over
the
control lines.
These
signals are
made
compatible
with
the
1797
disk
controller by
the
control decoder
and
inverter.
The
controller
then
blocks all
interrupts
to
the
processor (except its own) by
sending
a
"block
interrupts"
signal
to
the
interrupt
control ICs.
This
prevents
another
peripheral
from
interfering
with
the
transfer of data
between
the
processor
and
the
disk
drive.
The
controller also signals
the
buffer direction control
IC
to
allow
input
from
the
processorto
pass
through
the
data
buffer to
the
controller.
The
interface control latch
then
starts
the
motor of disk drive
number
one by translating
the
drive control signals from
the
processor.
The
datafrom
the
processor is
now
sent
over
the
data bus,
through
the
1797,
the
support
logic
and
disk interface logic,
and
to drive
number
one over
the
disk
serial data
line
(the
support
logic
and
disk interface logic help
the
1797 disk controller
communicate
with
the
drive electronics).
The
support
logic
consists ofICs
uaB,
U13, U14A,
and
U15.
The
interface logic consists
ofU19,
U20, U21,
and
U22. These are all explained below.
The read process is similar to
the
write
process. First,
the
processor
sends
the
proper
enabling
and
control signals over
the
control lines, just as before (of course this
time
the
signals enable
the
disk
controller board to read instead of write). The signals are
made
compatible
with
the
1797 disk controller
IC
by
the
control decoder
and
inverter. The
controller
again
blocks all
interrupts
to
the
processor (except its own) by
sending
a
"block
interrupts"
signal to
the
interrupt
control ICs, preventing
another
peripheral
from interfering
with
the
transfer of data.
The
controller also signals
the
buffer direction
control
IC
to allow
output
to
the
processor to pass
through
the
data buffer from
the
controller.
The
interface control latch
then
starts
the
motor of disk drive
number
one
by
translating
the
drive control signals from
the
processor.
The
data from
the
disk
is
now
sent over
the
drive serial line,
through
the
disk
interface logic,
the
support
logic
and
the
1797 disk controller,
and
to
the
processor via the data buffer
and
data bus.
The
phase
lock loop (PLL),
which
is
part
of
the
support
logic,
and
the variable control
oscillator
(VCO)
together track
the
frequency of data read from the disk.
This
tracking
generates a read clock
(RCLK)
signal
that
tells
the
disk
controller
how
fast to
read
the
data. (The frequency of
the
incoming
read data changes
due
to variations
in
the
rotating
speed of
the
disk
and
the
position of
the
data on
the
disk.)