Any
single
key
or
combination
of
key
closures can
be
uniquely identified by
these
codes.
To
set
up
the second matrix
sow,
the
driver
outputs
I
1101
(IDH)
and
again
inputs
the
receiver data
to
check
for
key
closures. This
is
repeated
for
the
remaining
lines.
At
the
conclusion
of
the scan, the
CPU
"knows"
what
keys
are
being
held.
Normally, it then
finds
the number of
the
lowest
key
(for
example
DO
is
31,
transposes
it
up
an
octave
by
adding
12
to
it,
and
saves this
number
to
send
to
the
DAC.
The
MOUE,
ARPEGGIATE,
and
SEQUENCER
switches connected
to
P21
are
scanned
in
the same
way.
The
diodes wired throughout the
matrix
allow n-key rollover, which
is
the simultaneous pressing
of
any
number
of
switches and
keys.
They prevent
switched
bits
from
returning through
other
closed switches on the same column,
which
would
activate other rows
in
addition
to
the
one
the
CPU
thinks
it
is
scanning.
If
it
is
desired to change the
ARPEGGIATE
UP/DOWN
function to
DOWN
9,
add
D103.
When
troubleshooting the keyboard,
check
schematic for
connections
between problem
keys.
ltDead"
notes,
if
not
causld
by
broken
J-wires
or oxidation,
usually
occur in
groups which
belong
to
the
same
matrix row
or
column.
As
you
can
see,
all that
is
in the keyboard circuitry
are
the few keyboard control
switches,
some
diodes,
and
U113 Microcomputer, Occasionally, physical
shock
can
disconnect the keyboard cable. But
if
the
keyboard
is connected correctly
and
still
doesn't work,
it
is
a
good bet
to replace
the
Microcomputer.
If
the
GATE
is stuck
on,
the problem could
be
a
stuck
key
(but
check
also
the
DRONE
switch).
The
easiest
way
to
check
for
a
stuck
key
is
to
disconnect the
keyboard
cable.
The
GATE
should
go
off.
2-12
DAC
U112
DAC
is
an
Analog
Devices
AD558
8-bit
unit,
but
for best accuracy
only
the
most-significant
6
bits
are
used.
By
no
accident, the
DAC
is scaled
to
provide the
exact
voltages
which
the
VCOs
and
VCF
require
to
piay
sernitones, which
is
83
mV
(1/12~)/step.
If
given the number 01, the
DAC
output is
83
mV
(1/12V),
if
given
the
number
1101
(decimal
131,
the
output is
1.083V
(1
3/12V),
and
so
on.
As
mentioned above, the actual binary
numbers
sent to
the
DAC
are
the
key numbers
(1
through
37)
pIus
12
units. This pre-addition
allows
the sequencer transpose
function.
Since
the
data
bus cannot
Itgo
negative,"
it
would
only
be
possible
to
transpose-down
the lowest
octave
recorded
by
subtracting
(up
to
twelve units) from the recorded
key
number. This
means
the
untransposed
3-V
CV
range
from
the
DAC
normally
ranges
from
1
to
4V,
Thus
when
a
maximum
downward
transposition
is
required, the
DAC
output
will
be
0
to
3V.
On
the
other hand, maximum
upward
transposition
will
raise the
output
range
from
2
to
5V.
The
DAC
has
its
own internal latch
which
is
clocked
by
tve
positive-going
DAC
EN
pulse
from
U113-26
(P20).
Since,
as
mentioned above, the dhta driver outputs
a
series
of
different
numbers
when
scanning the keyboard, the
DA~
EN
signal
is
needed
to
tell
the
DAC
when
data
output is specifically
intended
for
it
(rather than for the
matrix),
The
CPU
only
refreshes
the
DAC
when the
CV
is
to
change.
(In
other words,
when
a
note
is repeated
the
DAC
is
not
refreshed.)
To
refresh, the
microcomputer
briefly
pulls
DAC
EN
low.
The
QAC
latches the
data
on
the
rising edge
of
this
pulse.
R1181
DAC
SCALE
provides adjustment
by
altering the
gain
of
the
DAC1s
internal
amplifier,
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