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13.
10
KEYBOARD
UNIT
The
keyboard
circuit
diagram
is
outlined
in
Fig.
13-43. See Section 13.7.3
for a
description of
exchanges
between
the
keyboard and
CPU
ass'y.
The
key
layout
in this
keyboard
is
shown
in
page 16
and
17.
The
keys
are
connected
to a matrix
consisting
of
YO
thru
Y9
and
X0
thru
X9
(see
Fig.
13-43).
The
character
codes
for
the
PX-7
are
listed in
TAble
13-12.
Note
that
codes not
are
listed in
Table
13-12.
Note
that
codes not
indicated
on
the keyboard have
also
been
defined.
These
characters
are
keyed
in
by combined
use
of
the
SHIFT,
GRAPH,
and
CODE
keys
as indicated
in
Fig.
4-43.
And
when
the
CAPS
LOCK
key
is
pressed,
and
indicator
on
the
left
hand
side
lights
up
to indicate that the
same
characters
keyed
in
by combined
use of the
SHIFT
key
can be
keyed
in.
This
"shift"
mode
is
switched
on
or off
(in-
dicated
by
the
lamp
being switched
on
or
off)
The
above
operations
are
handled by
MSX-
BASIC
software.
When
the
SHIFT
key
is
used
together with the
GRAPH
or
CODE
key
as
indi-
cated
in
the
above
table,
three
keys
must
be
pressed together.
Consider an
example where
the
SHIFT,
GRAPH,
and
[0]
keys
are pressed together.
Current
la
is
passed
via
R8
in
response to the scan
pulse
from
YO,
resulting
in
X0
being
changed
to
L
level
to
acknowledge
that
the
[0]
key
has
been
pressed.
If
the
D1
diode
was
not included
in
the
circuit
shown
in
Fig.
13-42,
however, an image
current
ib
would
be
passed
via
R6, D3,
and
the
GRAPH,
SHIFT,
and
[0] keys, resulting
in
X2
also
being
changed
to
L
level
to
infer
that the
[2]
key
had
also
been
pressed.
Therefore, diodes
D1
thru
D3
are inserted
in
the
SHIFT,
GRAPH,
and
CODE
lines
to
block
reverse currents,
and
thereby prevent
the generation
of image
keys.
ioi
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