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I
;
111
110
101
100
011
010
001
large
000
000
Figure 7.
111
001
last
0512
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Graphic
1
Coordinate
I
I
I
I
I
0205
0205
I
-r
I
I
-r
I
Basic
OIO
011
OIO
011
100
101
110
Character Grid Coordinate System
111
last
Graphic
Coordinate
Line spacing is initiated either by the program
or by the 2250. The program initiates a line space
by specifying a new line (NL) character to the 2840.
The 2840, in turn, decodes the character and sends
resulting signals to the 2250 main deflection section,
which repositions the electron beam to the first char-
acter area center point of a new line. The new line
is 20 or 30 raster units below the previous line, de-
pending on the character size; the first character
area center point of a new line is always at X
=
0000.
Successive NL characters cause successive lines to
be stepped.
If an NL code is not specified, the 2250 displays
characters to the end of a line, automatically steps
to a new line, and continues the display. The 2250
performs automatic line spacing whenever the last
character formed is so near the right boundary of
the display area that character spacing cannot be
12
completed. This occurs when the center point of
the last character formed is to the right of X
=
1, 009
(basic size) or X
=
1,002 (large size).
The 2250 automatically positions the beam for a
new line at the top of the display area
(X
=
0000,
Y
=
1023) only when the last line is so near the lower
boundary that line spacing cannot be completed.
This occurs when the line is below Y
=
0 , 0 20 (basic
size) or Y
=
0, 030 (large size) •
LIGHT PEN
The light pen is a fiber-optic pen-like device (Figure
10). The user communicates with the computer or
the 2840-2 by pointing the light pen at the section of
the displayed image (character, vector, or point)
that he wants to identify to the program. When the
light pen is in the desired position, the user presses
the pen tip against the CRT faceplate
to activate the
tip switch, enabling light-pen operation. The light
pen detects light from the CRT beam when the beam
passes within the field of view of the pen. One de-
tect can occur for each activation of the switch.
Subsequent action is determined by the buffer pro-
gram. This action could be an interrupt of the CPU
program or a logical buffer action such as transfer
to a new buffer address, store X, Y registers, etc.
The buffer program can also cause the light-pen
switch to be bypassed so that the switch open/closed
condition will not affect light-pen detects. When the
light pen is continually activated by the program, a
detect can occur each time the unblanked beam passes
within the field of view of the light pen. This "con-
tinuous detects" mode of operation can be used in
graphic design operations such as light-pen tracking.
In
addition, the buffer program can disable the light
pen as certain information is being displayed, in-
hibiting light-pen detects on that information.
ALPHAMERIC KEYBOARD
This feature provides a typewriter-like keyboard
from which the user can compose and/or modify
messages on the CRT display area. Message areas
on the display can be protected from keyboard action
by the program. A dash-like mark, called a cursor,
is displayed beneath a character or character position
to indicate (to the user) where a character can be
modified or inserted by keyboard action. For ex-
ample, when a cursor is displayed under one charac-
ter in a line of characters, that character can be changed
or blanked by keyboard action. Also, if a cursor is
displayed under a position without a character, a
character can be inserted in that position by key-
board action. A cursor can also appear beneath
a protected character position; however, that posi-
tion cannot be used for character insertion or mod-
ification from the keyboard.
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