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PROGRAMMING
(cont.)
4023
Users
"prompt"
character
from
the
computer. With
the
receipt
of
the
prompt
character,
another
line
is
transmitted to the
computer. Standard
prompt
character
is
the
code
for
the
ASCII
upper-case
"P"
(80
10
)-
The
standard
4023
does not
blink the cursor unless
the
RESET
key
is
pressed.
The
BLINK
CURSOR
strap
on
the
Timing
Card
can be placed
to
make
the cursor blink
continuously.
Features such
as
Position
Cursor
and Read
Cursor
are
standard.
CODE
EFFECT
General
In
order
to offer specialized
functions
and
features to
the
programmer,
several
character
codes
or
code
sequences
are
designated
as controls.
Specialized features
such
as
the
definition
of
fields,
clearing
the
screen, positioning
the
cursor, ringing the
bell,
and
triggering
hard copies allow the
programmer
to
set
up
the
Terminal
in
the
desired
manner.
Code
Set
The
standard
ASCII code
set
is
used
to
provide control
for
data
transmission,
operational
configuration,
and
control over the
specialized
functions
mentioned
in
the
above
paragraph.
Control
Codes
Some
of the control characters used
in
the
Terminal
are
simply designated
as controls.
The
ASCII
Code
Chart
in
Appendix B shows
those
controls.
However,
to
expand
the
number
of
possible
controls,
two-character control
se-
quences
are
used.
The
first
character
of
the
control
sequence
is
ESC
(27
).
It
signals
the
Terminal
that
the
next
character,
regardless of
its
normal
function,
is
to
perform
a
different function.
NOTE
Table 2-2 provides a description of
the
code
controls
and
the effects
they have
on
Terminal
operation.
The
4023
responds
to
or otherwise
uses,
only
those
listed.
memory,
but
are
simply performed. For example,
a
CR
(13,
)
typed on
the
keyboard
will
normally not
alter
Terminal
memory,
but
will
cause the cursor
to position
itself
at
the beginning of the
next
line.
The
operator
may
enter the
CR
character
(or
any
control character) into
memory,
by
holding
down
the
RESET
key while
typing the
control character
desired.
The
screen
will
show
a
block
character
(all
dots
written)
to
signal
that
the
memory
position
contains an otherwise
invisible
control character.
(Only
half
the control characters
are
displayed
as
blocks.
If
BIT
1
of
the control character
is
true,
then
that control
character can
be
displayed.)
However,
for
normal
usage,
control characters
need not be
stored
in
memory.
Stored
control characters (excluding
CR,
NUL,
and
ETX)
can be
transmitted.
Two
control
characters,
NUL
and
ETX
are
normally
stored
in
Terminal
memory
independent
of
the
RESET
key,
as
explained
by
the following:
NUL
A NUL
is
entered
into
the
memory
on
Clear
Page, Erase to
End,
Erase Input, Delete
Line,
and
Delete Character functions.
Nothing
is
displayed
in
a
character
position
if
a
NUL
occupies
that
position
in
Terminal
memory.
NULs
are
not
transmitted.
Thus
the data
string
is
compacted
to
include
only the information
displayed
(or
other-
wise
identified as
being
transmittable).
ETX
When
no
Field
Attribute
Codes
are
stored
in
memory,
pressing
the
ENTER
key
causes an
ETX
code
to store
in
memory
at
the cursor
location.
ETX
is
also
known
as
the
Message Separator Code.
It
marks
the
beginning
or
end
of
text
in
conjunction with
the Enter Function.
With any
Field
Attribute
Code
stored,
the
ENTER
key does
not
store
an
ETX
unless
RESET
is
pressed.
The computer
equivalent of the
ENTER
key
is
the
ETX
code.
By
sending the Terminal an
ETX
(3
10
)
as
the
last
character
of
the
computer
transmission,
the
displayed
computer
data
is
prevented
from
being transmitted
back
to
the
computer
on
subsequent
buffer
entries.
The
computer
originated
ETX
stores regardless of Field
Attribute Codes.
Control Character Performance
Storage.
Control characters
typed by
the
keyboard
or
sent
by
the
computer
are usually
not
stored
in
Terminal
Message
End
Character. This
is
the
last
character
trans-
mitted
from
the
4023
in
a
buffer transmission sequence.
The
standard
End
character
is
CR
(13 10
)-
However,
strap
options
on
the Control
Board permit any ASCII
control
2-2
®
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