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GENERAL
Each 2250-3, under control of a 2840-2,
generates
images on the 12-inch
by
12-inch usable display
area of
a 21-inch
cathode-ray tube (CRT).
An image
can
be
composed of straight lines (vectors) , points,
standard characters
(in
two sizes), and special char-
acters formed with vectors and points (Figure 2).
A visible
display is produced
when
an
el
ectron
beam in the CRT strikes the phosphor-coated CRT
screen, causing the portion of the coating struck by
the beam to glow briefly. Normally, the glow fades
within
a fraction
of a
second,
too
soon
for the human
eye
to carefully perceive and identify the image. For
this reason, the display must be redrawn
continuously
Figure 2. Example of
a
2250 Display
2250 OPERATIONS
(regenerated)
at a rate
that
will
cause the display to
appear
steady
and
stationary to the observer. Re-
generation
is performed
automatically,
under control
of
a
program
in
the
2840.
The 2840
accomplishes
regeneration by continuously retransmitting control
and display data to the 2250; this data can
be
modified
during
regeneration
by the 2840, as directed
by
the
buffer program and/or the CPU program, to update
or change the display. The 2250 also
performs
various
nondisplay services for the user by
providing
the
interface between the user and the problem
pro-
gram with
the following devices:
1. Programmed function
keyboard
.
Provides
keys and overlays for user communication
2250 Operations
7
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