Graphics
Note
that
if
a
parameter
byte
is
lost
or
garbled
in
transmission,
all
following
end
points
will
be
improperlyread.
To
minimize
data
errors
causedby
the
loss
of
a
data
byte,
any
command
can
be
used
to
reset
the
parameter
count
and
restore synchronization. Nops (z),
redundant
format,
or
pen
down commands
can
also be
inserted
to
insure
synchronization
if
necessary.
Graphicssequences
can
extend
indefinitely.
In
general, longersequences
are
preferred
as
they
minimize
the
overhead
necessary
for a plot sequence.
Ec
• p <
for
ma
t >
must
be
sent
for
each
series
of
vectors. As
the
sequence
length
decreases,
the
percentage
of
preamble
characters
increases,
and
the
vector
drawing
rate
goes down.
The
worst possible case would be to
send
Ec
• p <f ormat >for
each
vector; approximately50/
of
the
characters
sent
would
be
overhead,
reducing
vector speed
by
a factor
of
2.
The
general
format
for
an
absolute
plotting
sequence is:
Ec
•
pia
<byte1)
<byte2> <byte3> <byte4>
(z)
<byte1> <byte2> <byte3> <byte4>
...
. . . <byte1> <byte2> <byte3> <byte4>Z (or
any
upper
case
command)
Each
block
of
4
bytes
specifies a single point.
The
« i" indicates
that
absolute
format
is
to be
used.
The
«a"
raises
the
pen
before
it
is
moved to
the
point
specified by
the
next
four
bytes
and
lowered.A
HOP
(z)
can
be
added
to
insure
synchronization,
if
necessary.
The
lowered
pen
draws
a vector
as
it
moves
to
the
next
point,
and
so on.
The
upper
case
«2"
terminates
the
plotting
sequence.
The
vector
end
point
formats
allow
the
pen
to be moved completely off
the
screen
(an
absolute
coordinate
of
1000, for example).
The
actual
range
of
the
pen
position
can
be from -16384 to
16383. Vectors
that
extend
beyond
the
screen
are
clipped so
that
they
will
not
wrap
around.
Pen Control
The
terminal
uses
the
concept
of
a «pen"
in
drawing
vector
data.
The
pen
can
be
lifted
or
lowered
as
well
as
be positioned
using
absolute
or
relative
coordinates.
For
example,
the
pen
is
lifted,
moved
to
a
starting
coordinate, lowered
and
moved to
an
endpoint
to
draw
a line.
The
pen
is
initially
down
and
positioned
at
absolute coordinates 0,0 following power
up
or
a full reset.
If
the
pen
is
raised
and
coordinates given,
the
pen
is moved
to
the
coordinates
and
then
lowered.
The
pen
is
normally
left
in
the
down position.
Raise Pen:
Lower Pen:
Ec
• p a
Ec
• p b
This
command lowers
the
imaginary
plotting
pen
to
the
drawing
surface.
The
pen
draws
a
line
as
it
moves from
its
current
position.
Use Cursor As Next Data Point
Ec
• pc
This
command
causes
the
position of
the
graphics
cursor to be used
as
the
next
data
point.
D-27
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