Appendix
D/
X-Pad
Data Interpretation
The X-Pad
is one of the
easiest
peripherals
to
use. It
simply
outputs
the X- and
Y-coordinate positions
and Pen Status
information
consisting ofproximity, Pen
down, and
margin
conditions.
The
data
is represented by
numbers that
vary from a
value
of at
the
upper-left corner ofthe
surface (the
Origin) to
their
maximum values
as
the Pen is moved to the
lower-right corner.
Each data
output corresponds
directly with
a
position on
the
Screen.
Position numbers
created by the X-Pad are the
same as
those that
are
manually keyed into the Computer
to
select
a
desired
display location.
The Pen Status
information provides
additional
capabilities
in the use ofthe
X-Pad.
Pen proximity provides
a
means of
disabling
data
when the Pen is out of
range
ofthe
Writing Surface. X- and
Y-margins tell when the
Pen
is
beyond
the
7"
x
9"
Display Area
and defines left, top, and
right
margin
areas
that can be used
for
Menu or other
purposes.
A lower
margin
is not defined.
However, a lower
margin can
be
easily created in software. It
should
be
noted
that a
Menu
can be created
in
the
Display
Area,
as
well
as
in
the
Margin Area.
See Table
2 earlier in this manual for a
description of
Pen
Status
.
The
Computer reads
the X-Pad data
and
status
information
from three memory
addresses
—
X
=
PEEK(65376), Y
=
PEEK(65377),S
=
PEEK(65378).
The X-Pad
is
fully buffered to
allow the Computer
to
simply
input
or
read the data
and status
information in a random or
asynchronous
fashion. When the X- data
is read
by
the
Computer,
the Y-data
and
status
word
updates are
inhibited to
maintain a grouping ofthe
coordinate
data
pair
and status.
This is to
insure that the
data
and status are all
read
at a single
time
—
so
they
represent the Pen at a
single
location
while
the
Pen is
moving.
The
status
word
can be
read
at any time, for
example,
while
waiting for
a Pen-down
condition.
X- data,
Y-data,
and status
can then
be read in
order.
Data
Format
1st Word 2nd Word
3rd Word
Description
Output
X-data
Y-data
Status
Address 65376
65377
65378 The
Interface
is
fully
buffered
to
allow
the Computer
to read the
data
and
status
information.
Bits
When the
X-data
DO
O(LSB)
O(LSB) Pen
Down
word is
read by the
D1 1 1
Proximity
Computer, the Y-data
D2
2 2
X-margin
and
status word
D3
3 3
Y-margin
updates are
D4 4
4
Logic inhibited
to main-
tain
a
grouping.
D5
5 5
Logic
D6
6 6
Logic The
status
word
can
D7 7 7
Logic
be read
at
any
time.
47
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