Description—4953/4954
The Graphics Tablet functions as an integral part of the
terminal hardware by means Of an interface card con-
nected to the terminal minibus. The minibus is the
terminal's main data routing avenue. Any device con-
nected to the terminal minibus can respond to the Tablet
data sent through that minibus. This allows the operator to
format Graphics Tablet data off-line from acomputer, and
then send that data to the computer when the desired
graphic input has been obtained.
GRAPHICS TABLET COMPONENTS
The four main components of the Graphics Tablet are
the writing surface (Tablet), the Writing Pen, the Power
Module, and the Tablet Control Card which plugs directly
into the minibus of the 4010-Series Computer Display
Terminal.
Tablet. The tablet is aflat writing surface, magnetically
prebiased, on which paper (film, etc.) maybe placed and
then traced to input data to the computer. Under the
surface of the tablet are two grids of magneto-strictive
wires, one set for the Xaxis and one set for the Yaxis. An
acoustic wave is sent along these wires and detected by
the writing pen; the time -between when the wave is sent
and when it is received allows identification of data points
in agrid of 1024 Xpoints by 1024 Ypoints. Identifiable
points are 0.01 inch apart. The 4954 maintains 0.01 inch
resolution by operating with 12 bits of graphic data per
point, or 3840 by 3072 points-
Point identification can be more clearly understood if
one thinks of the writing surface as containing 1024
vertical lines and 1024 horizontal lines. An identifiable
location on the writing surface can be established at the
intersection of any vertical and horizontal line. These
identifiable locations directly correspond to the iden-
tifiable points of the terminal's display screen.
In the optional accessory cursor, the switch function is
performed by the button on top of the cursor.
Power Module. The Power Module contains the power
supply, the tablet pulser and preamp, and the connections
which tie the Graphics Tablet components together. In
addition, the Power Module's front panel contains the
Power Switch and Indicators, The Indicators provide the
following information:
Indicators
READY-"On" indicates that the Tablet is ready to
send another point.
PEN—illuminates when pen pressure is applied.
DATA—Illuminates when data is in the process of being
transferred.
POWER—Illuminates when Power is applied to both
the Graphic Tablet and the terminal.
Tablet Control Card. The Tablet Control card contains
those circuits necessary to convert the pen signal to its
digital equivalent. This card connects directly to the
terminal minibus, and contains circuits for format chang-
ing, data transfer, "header character" data, and other
tablet-related functions.
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Writing Pen. The tip of the writing pen contains a
sensitive pick-up coil which, when moved into the
presence of the tablet, detects the change in magnetic
field caused by the acoustic wave. The detected signal is
then converted to digital information that directly relates
to pen position. Once the specific operating mode has
been established, the writing pen provides sufficient
control to handle all data transfer (stop and go functions)
to the terminal and computer. An optional ballpoint filler
may be used in place of the dry filler for the penfso the
operator can draw the information he is sending to the
computer; otherwise, the standard inkless filler can be
used. The pen can be replaced by an optional accessory
"cursor" if greater accuracy is required (such as tracing a
drawing).
Apen switch is located within the pen, and is activated
by pressing the pen tip against the tablet surface, pressing
the filler slightly into the pen.
TABLET PRESENCE AREA
The Graphics Tablet responds to the pen (In Presence)
whenever the pen is within 0.125 inch of the surface of the
tablet In addition, only acertain portion of the tablet
surface represents the valid data area. The location of this
area is dependent on the biasing of the tablet, but the size
is standard for each of the tablet sizes. On the 4953, this
area is 10.24 inches in each axis; on the 4954, this area is
38.4 inches in the X-axis by 30.72 inches in the Yaxis. This
area is not marked on the tablet surface, since it may move
slightly each time the tablet is biased. It may be located by
the operator by enabling the tablet with an ESC !N, so that
the tablet disarms when the presence area is exceeded. He
may then slowly move the pen to the border of the tablet,
and watch to see when the READY indicator is ex-
tinguished. This marks one presence boundary point. This
point may be marked and the procedure tepeated to find
the complete boundary. An example is shown in Fig. 1-3.
1-2 REV. A, JULY, 1975
