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A MAGNETIC DEVICE FOR COMPUTER GRAPHIC INPUT
M. H. Lewin
RCA Laboratories, Radio Corporation of America
Princeton, New I ersey
not insert between the CRT face and the pen any
material (such as a sheet of paper) which will prevent light transmission.
The Rand tablet consists of a thin Mylar sheet
containing on one side, an array of etched copper
lines in the X direction and, on the other side, a
similar array of fine lines in the Y direction. By
means of capacitor encoding networks, also etched
on the same sheet, a unique voltage pulse train is
applied to each X and Y line from a common pulse
pattern generator. The pen in this case is merely a
metallic electrostatic pickup connected to a high
input-impedance amplifier. The pulse train picked
up by the pen depends on the X and Y lines nearest
to its tip. This serial pulse pattern (in Gray code to
eliminate errors) is converted into a parallel binary
address with appropriate peripheral logic, which
includes a shift register and a code converter. The
system is entirely digital and the tablet is relatively
inexpensive. In addition, thin paper sheets can be
inserted between the tablet surface and the pen for
tracing maps and curves .
Both of the approaches described above utilize
the pen as the signal pickup device and the writing
surface as the signal generator. While the Rand tablet system materially simplifies the writing surface
used and reduces the complexity of the peripheral
INTRODUCTION
Recent work on systems to facilitate the input of
graphical information to a computer has resulted in
the development of the light penl and the Rand
tablet. 2 Both of these devices allow a user to
"write" on a flat surface with a special, hand-held
electronic pen. Periodically, the pen position is de.:.
tected and converted into a machine-readable address. In this way, the pattern which is traced out
by the pen is directly converted into binary code
and stored in the machine. Devices such as these
promote the easy input of graphical data such as
curves, maps, diagrams, and other drawings. They
should also be of interest to many researchers concerned with character and pattern recognition.
The light pen is normally used in conjunction
with a cathode-ray tube as the writing surface. A
light-sensitive element in the pen generates a signal when the flying spot on the tube face reaches
the pen tip. The timing of this signal, relative to
the timing of the scanning pattern, establishes the
.pen position. Appropriate digital and analog peripheral circuits are necessary to convert this signal
into an equivalent binary address for storage. Clearly, the speed of movement of the pen is limited by
the scanning frame rate of the CRT. Also, one can-
831
From the collection of the Computer History Museum (www.computerhistory.org)
832
PROCEEDINGS -
FALL JOINT COMPUTER CONFERENCE,
electronics required for a given pen position resolution, the amount of circuitry needed, for the generation of the appropriate pulse sequences and for the
conversion of detected pulse sequences into parallel
binary addresses, is not negligible. *
The work on which this paper is based was initiated to develop a graphic input device which
would require a minimum of associated circuits
while maintaining simplicity in the construction of
the writing surface. The system to be described utilizes the pen as the signal generator and the writing
surface as the address detector. The pen contains in
its tip a small magnetic head which periodically
generates a localized magnetic field pulse. (Since
the coupling is magnetic, it is not shielded by most
materials placed between the pen and the tablet.)
The writing surface contains a number of thin winding layers in a laminated structure. Each winding
layer consists of a single, continuous wire pattern
designed to detect one of the pen address bits.
Thus, there are as many layers as there are address
bits, each developing a positive or negative induced
voltage as a function of the pen position. All layers
generate output pulses in parallel and these signals
are of sufficient magnitude to set a register directly.
WRITING SURFACE WINDING PATTERN
The magnetic head in the pen tip consists of a
small, linear ferrite core with an air gap and winding as indicated in Fig. 1. The coil is periodically
driven with a voltage pulse as shown. Any wire,
brought in the vicinity of the air gap and oriented
so as not to be perpendicular to the air slot, will
link some of the magnetic flux generated and will
thus develop an induced voltage pulse whose shape
is similar to that of the drive signal. The polarity of
the induced voltage is determined by the familiar
right-hand rule. Its magnitude is greatest when the
wire is parallel to the slot.
face is divided into two sets of areas or sectors
which may be labeled "even" and "odd." When th~
pen tip is positioned over anyone of the even sectors, a positive voltage pulse (binary "one") is induced across the two winding terminals. When the
pen is over any of the odd sectors, a negative outConsider a wire winding pattern in a plane surface over which the pen is "writing." The pen tip is
*T.he authors state that the system "contains some 400
tranSIstors and about 220 diodes; however, little attempt
has been made to minimize the number of components."
1965
I
Jl
PERIODIC DRIVE
VOLTAGE PU LSE
LINEAR
FERRITE
CORE WITH
A I R GAP
I
MAGNETIC~
FIELD
PATTERN
/
I
/INDUCED
", VOLTAGE
.......... -..........
PULSE
JL
Figure 1. Magnetic head in pen tip.
in close proximity to the winding layer. It is desired to arrange the winding pattern so that the surput signal (binary "zero") is obtained. A winding
configuration which will satisfy these requirements
is shown in Figs. 2 and 3.
Assume the plane surface is divided into m sectors
(m an even number), half even and half odd. The
odd and even sectors alternate and can be labeled 1
2, . . . m as shown in Fig. 2. Each sector consist~
of n winding "stripes" or wires, each of which is a
segment of the total length of wire used in the winding. Let the stripe ij be the jth stripe of the ith sector,
where 1 ~i~m and 1 ~j~n.
The procedure for laying out the winding pattern
is shown in Fig. 3. In making a given winding
"pass" over the surface, from left to right, one
winds the wire vertically up in a specified stripe
position, then continues the winding horizontally to
the right to the next designated stripe position, then
winds the wire vertically down, then horizontally to
the right, then vertically up, ... etc., until the right
end of the .plane is reached. The wire is then returned horizontally from right to left and another
pass is started from left to right. The procedure indicated in Fig. 3 is summarized in Table 1. As a
simple example, the pattern for four sectors, each
containing four stripes, is given in Fig. 4. An examination of this winding configuration reveals that
j
From the collection of the Computer History Museum (www.computerhistory.org)
833
A MAGNETIC DEVICE FOR COMPUTER GRAPHIC INPUT
WINDING
PLANE
n WINDING
STRIPES PER
SECTOR
SECTOR
ODD
.u
EVEN
WINDING
TERMINALS
.J..L.L..LJ..J.J...I.J..---...I....--- . . . . J . - - - - -
2
NUMBER·
m-I
3
m
Figure 2. Partitioning of a winding plane.
r-;:-
-
"ACTIVE
PLANE
WIDTH
r-
---
1---
--
r-;::--~-
II
---
SEC TOR:
_.0---
AL
CONTIN UE .-/
THIS
PATTERN
---
---
--
4
,
~,
START
TERMIN
~
--
-
~-
I
2
3
1---.-
1---
f--.-
/v
)
- - -m-3
1------ - - -
'--
m-2
~-
m-I
f..---
m
-- -
.-
_ n-I"R-L"RETURN PATHS
r
~
-·-FINISH TE:"RMI NAL ~
...
Figure 3. Winding layer pattern.
From the collection of the Computer History Museum (www.computerhistory.org)
834
PROCEEDINGS -
r+-
r+t - l- f - - ~- f - - f - - f -
"ACTIVE'
PLANE
WIDTH
FALL JOINT COMPUTER CONFERENCE,
f-
f-
f- f - 1--
all stripes in the odd sectors have the same sense,
opposite to that of the stripes in the even sectors.
Figure 5 shows a side view of two adjacent sectors, with three possible head positions indicated.
The field pattern for position A causes a given polarity signal to be developed across the winding terminals. For position C, because the sense of the
windings is reversed, the opposite polarity signal
will be induced. If the pen tip is in the immediate
vicinity of the boundary between the sectors (position B), very little signal will be generated since
positive and negative components will cancel. Thus,
the output pulse polarity determines whether the
pen is over an odd or an even sector. The number
of stripes (n ) required in a sector depends on the
desired output pulse magnitude. As n increases, for
a given sector width, the induced signal increases.
The winding pattern shown in Fig. 3 is interesting because it contains no wire crossovers. It can
therefore be photo etched on a thin, conductor-clad
insulator sheet, such as copper-clad Mylar, or otherwise deposited via screening or evaporation techniques on a thin insulator substrate. Two patterns
can be placed on either side of a given sheet.
-
,
STAR T
r--
t- t-- t-- t-- 1-- f-- f-- f--- f-- f-- 1--1-
~
,
FINISH :>
Figure 4. Configuration for m
= 4, n = 4.
B
A
FIELD PATTERN\
WINDING----'-- ® ®
STRIPES
1965
C
IJ
®:~~~~®
~
SECTOR
----.,.....-SECTOR i + 1---"-1
Figure 5. Side view of winding stripes with three possible
head positions.
MULTILAYER TABLET
The writing surface is constructed by stacking or
laminating as many thin winding layers as there are
pen address bits. Thus, for a tablet to resolv~ any
one of 1024 X 1024 locations, ten-double-sided
sheets are required. Half of the winding layers are
oriented in the X direction, half in the Y direction.
The total thickness of the system can be kept small
by using sheets only a few mils thick. Each X layer
has an identical companion Y layer oriented orthogonal to it.
The layout of a winding pattern to detect a given
address bit is, of course, a function of the position-to-address coding scheme used. By using a
closed, cyclic code, such as Gray code, one is as-
From the collection of the Computer History Museum (www.computerhistory.org)
A MAGNETIC DEVICE FOR COMPUTER GRAPHIC INPUT
sured that no more than one address bit in a given
coordinate direction can be undecided. That is, for
any pen position, the head can be located over, at
most, one boundary between sectors. For these reasons, it would appear that a conventional binary
coding scheme should not be used because the pen
point may be positioned over more than one indecision boundary. However, the addition of a small
amount of external logic, no more complicated than
that requ.ired for a parallel Gray-to-binary conversion, may allow a conventional binary code to be
X2 OUTPUT
:
835
used. (The indecision correction algorithm involved
is described in the next section.) Assuming such a
code, winding patterns can be laid out as illustrated
by the simple 8 X 8 example shown in Fig. 6. The
"most significant" X or Y layer has only two sectors, the next four, then eight, etc. The total number of X or Y winding layers (address bits) depends on the resolution required in the location of
the pen tip. The "least significant" layer (the one
with the largest number of sectors) may have only
one stripe per sector (i.e., n = 1).
XI OUTPUT
xoOUTPUT
II
I
I
I
I
I
I
I
I
1
X2: TWO SECTORS
XI: FOUR SECTORS
Xo : EIGHT SECTORS
----------
bbY OUTPUT
Y2:TWO SECTORS
2
YI: FOUR SECTORS
YI OUTPUT
Yo OUTPUT
Yo: EIGHT SECTORS
Figure 6. Six winding layers for a 64 (8 X 8) position array
using conventional binary coding.
All of the winding layers must be close to the
pen point to allow the generation of sufficiently
large output signals. The output voltage induced in
a more significant layer (one with many stripes per
sector), when the pen is over a given sector, is the
sum of the voltages induced in all the stripes of that
sector (refer to Fig. 5). This integrating effect allows one to locate a more significant layer at a distance from the pen tip which is larger than that for
a less significant layer. Thus, the tablet is laminated
with the most significant winding layers at the bottom and the least significant layers nearest to the
surface.
Note that, in order to detect approximately equal
magnitUde signals at the outputs of the X and Y
layers, the air gap in the magnetic head must be or-
iented at 45 0 to the X or Y orthogonal stripes.
Thus, the pen must be marked or shaped appropriately to insure that it is held roughly in the correct
orientation. Small variations from 45 0 will not
change the induced signals appreciably.
Other more sophisticated head designs, which
allow the system to operate indep~ndently of pen
orientation, are possible. For example, one can use
more than one air gap in the pen tip. Using two orthogonal gaps (pulsed at different times) * as the
pen orientation changes, the magnitude of the induced signal from one gap increases while that from
the other decreases. Orthogonal gaps can also be
used to generate a rotating magnetic field. Another
methodt involves using two or more air gaps to
*Suggested by J. A. Rajchman.
tDue to J. Avins.
From the collection of the Computer History Museum (www.computerhistory.org)
836
PROCEEDINGS -
FALL JOINT COMPUTER CONFERENCE,
1965
them one, X and zero) available from each layer.
One and zero are acceptable signals (positive and
negative pulses). X represents almost no output
pulse-an undecided bit. An examination of the
one, zero transitions, when counting in conventional
binary code, will show that if one follows the following simple rules, errors at multiple-transition
boundaries can be resolved and the conventional
binary pattern can be used:
Detect the most significant bit which is undecided (i.e., the most significant X output). Arbitrarily decide this bit to be one or zero.
Force all less significant bits to be the complement of the bit chosen above. *
The addition of a very small amount of external
logic will allow this procedure to be used. For example, in the circuit shown in Fig. 7, an undecided
output is arbitrarily decided as a zero and all less
significant outputs are forced to be one.
generate a number of discrete field orientations
(say, three), one of which will always be acceptable
for any pen orientation. Periodically, these orientations are sequentially tested and the acceptable one
is chosen. This testing may involve the use of an
additional test winding layer whose stripes are all
oriented at 45 0 to the X and Y stripes. Each of
these arrangements, however, increases the complexity not only of the magnetic head but also of
the peripheral electronics. At this stage, the requirement of proper pen orientation, which allows the
system to be very simple, does not appear to be a
very severe user restriction. If necessary, some simple mechanical approach, such as housing the head
at the end of a flexible shaft (similar to that used
in speedometer cable), would permit the sleeve of
the pen to rotate while the head orientation stays
relatively fixed.
INDECISION CORRECTION ALGORITHM
*This method will work provided that the winding patterns are designed such that, for any two adjacent address
bits having a transition boundary in the same position, the
"zone of indecision" for the more significant bit overlaps
that of the less significant bit.
For a system such as the one described above,
there are actually three possible output signals (call
OUTPUT
A
... ,
A i + 10
_-~._ _ _ 1
A i + 2 _O---------J
---J
Ak
p.
I
0
.... - - - - - - - - '
OJ
L A . (TO LESS SIGNIFICANT
I
POSIT IONS)
r-l) -
k+1 = NUMBER OF ADDRESS
BITS IN ONE COORDI NATE DIRECTION
o~ j ~ k
,..---'----'---,
POSITIVE
PULSE
DETECTOR
NEGATIVE Pj=IIF POSITIVE PULSE IS DETECTED
PULSE
Qj=1 IF NEGATIVE PULSE IS DETECTED
DETECTOR Aj=1 IF NO PULSE IS DETECTED
~
TABLET LAYER j
OUTPUT PU LSE
Figure 7. Mechanization of indecision correction algorithm
for conventional binary address coding.
From the collection of the Computer History Museum (www.computerhistory.org)
A MAGNETIC DEVICE FOR COMPUTER GRAPHIC INPUT
EXPERIMENTAL MODEL
An initial experimental model consisting of a 32
32 array, using ten winding layers (five X and
five Y) to resolve anyone of 1024 pen positions,
has been constructed and is operating as described
above. A photograph of the pen and tablet is shown
in Fig. 8. The winding layers for this model were
wound by hand using conventional No. 33 insulated
coil wire. Each of the layers follows the configuraX
837
tion given in Fig. 3. A conventional binary code was
used. The windings were potted with an epoxy resin
to allow the tablet to present a flat surface to the
pen. The stripes are lJ8" apart, and the total thickness of the ten-layer system is approximately 0.1".
Clearly, a much higher stripe density is achievable
using present photoetching techniques. Also, one can
easily laminate a ten double-sided sheet system,
required for a 1024 X 10.24 array, and· obtain a
total thickness less than 0.1".
Figure 8. Experimental 32 X 32 tablet with pen.
The pen tip contains a linear ferrite core, 3/16"
O.D. and lJ8" J.D., wound with 30 turns, and driven
from a conventional General Radio pulse generator.
Approximately 100 volts is developed across the
head winding during the pulse peak. The core has a
15 mil air gap. Little attempt was made to optimize
the core drive circuit so as to obtain optimum output signals. Each of the winding layers is terminated in 100 ohms. This value was chosen to criti-
cally damp output ringing. A photograph of a typical
output impulse is shown in Fig. 9. The reverse polarity signal has the same shape. The waveform is clean
and has sufficient amplitude to set a flip-flop. It can
no doubt be made larger with appropriate pen drive
circuit design. The timing indicated shows that one
need not be concerned with the speed of movement
of the pen. The pen is marked to permit proper
From the collection of the Computer History Museum (www.computerhistory.org)
838
PROCEEDINGS -
FALL JOINT COMPUTER CONFERENCE,
1965
position of a number of patterns, each of which is
designed to detect one of the pen address bits directly, the amount of associated circuitry is minimized. Although the initial artwork involves the laying out of as many patterns as there are address bits
in one coordinate direction, subsequent fabrication
of a number of tablets should be simple and inexpensive.
ACKNOWLEDGMENTS
Figure 9. Typical winding output pulse across 100 ohms. Vertical scale: 0.1 volt/div. Horizontal scale: 0.2
,usec/div.
The author wishes to express his appreciation to
H. Schnitzler, who constructed the experimental devices and who assisted in many of the tests.
orientation of the air gap with respect to the winding stripes.
A set of ten peripheral circuits, which includes
the logic given in Fig. 7 and which also contains
digital-to-analog converters, is used to demonstrate the operation of the tablet by permitting the
position of the pen to be displayed as a spot on a
CRT face.
REFERENCES
1. B. M. Gurley and C. E. Woodward, "LightPen Links Computer to Operator," Electronics,
pp. 85-87 (Nov. 20, 1959).
2. M. R. Davis and T. O. Ellis, "The Rand Tablet: A Man-Machine Graphical Communication
Device," Proc. 1964 Fall Joint Computer Conference.
CONCLUSIONS
By constructing the writing surface as the super-
Table 1. Winding procedure.
I!U pI! IN STRIPE
'''DOWN'' IN STRIPE
POSITION NUMBER POSITION NUMBER
L...,R PASS NO.1 (START)
II
31
51
I
I
I
----
------.,..
.-:::::::----
---
ETC.
(m-I) I
2n
4n
6n
I
I
I
mn
RETURN R-L
L-R PASS NO.2
12
2 (n-I)
32
4(n-l)
I
I
I
(m-I) 2
I
I
I
m(n-I)
RETURN R-L
t
CONTI NUE TH I S PATTERN
t
L-R PASS NO. n
In
3n
I
I
I
(m-I) n
21
41
I
I
I
m I (FIN ISH)
From the collection of the Computer History Museum (www.computerhistory.org)