WO1994024648A1

WO1994024648A1 - A digitizing system and methods for its operation

Info

Publication number: WO1994024648A1
Application number: PCT/US1993/003626
Authority: WO
Inventors: Shyun Dii Du; Chi Yuan Wang; Muh Yu
Original assignee: Micropen Computer Corporation
Priority date: 1993-04-19
Filing date: 1993-04-19
Publication date: 1994-10-27

Abstract

The invention is a digitizing system (20) that includes a tablet (32) on which there is present an electrically conductive resistive layer (34). The system (20) provides unidirectional flows of electric current across the layer (34) parallel either to the tablet's Y-axis (56) or to its X-axis (66). The system (20) also includes an electrically conductive stylus (38) having a point (44) for contacting the layer (34). The flow of electric current across the layer (34) establishes a gradient in electrical potential across the layer (34) for measuring coordinates of a location at which the point (44) contacts the layer (34). The tablet (32) and the layer (34) are preferably transparent and juxtaposed with a transparent LCD (36) so an image of a path traced by a point (44) may be presented on the LCD (36).

Description

A DIGITIZING SYSTEM AND METHODS FOR ITS OPERATION

Technical Field

The present invention relates generally to digital computer input devices that facilitate an operator's manual entry into a computer of two dimensional X-axis and Y-axis digital coordinate data by the operator's positioning of a stylus at locations on a data entry tablet, and, more particularly, to such devices that are transparent and are juxtaposed with a display screen to facilitate displaying an image of a path traced by the stylus.

Background Art

Various different devices and techniques are known for facilitating an operator's entry of two dimensional X-Y coordi- nate data into a digital computer by positioning a stylus at locations on a data entry tablet. Devices for entering coordi¬ nate data in this way are commonly referred to as "digitizers." One class of known digitizers apply an electrical excitation signal to an electrically conductive resistive layer present on an exposed surface of the digitizer's tablet. Some techniques for obtaining digitized coordinate data using such a tablet are disclosed in a series of patents issued to Scriptel Corporation, i.e., U.S. Patent No.s 4,456,787, 4,600,807, 4,649,232, and 4,650,926. These patents all disclose applying an alternating current ("AC") excitation to a transparent resistive layer of a digitizing tablet.

Scriptel Corporation's U.S. Patent No. 4,456,787 discloses supplying this AC excitation to the resistive layer by emission from the point at which the stylus contacts the layer. This patent discloses that electrical connections to parallel horizon¬ tal and vertical edges of the tablet supply the AC signal first from one pair and then from the other pair of the tablet's parallel edges through analog switch arrays to a digitizing circuit. Conversely, the other three patents assigned to Scriptel Corporation, i.e. U.S. Patent Nos. 4,600,807, 4,659,232, and 4,650,926, all disclose applying the AC excitation through analog switch arrays alternatively either to horizontal or vertical edges of the tablet's resistive layer. These later Scriptel patents disclose digitization of the AC signal received by the stylus to obtain X-Y coordinate data.

Another U.S. Patent No. 4,435,616 also discloses the use of arrays of analog switches to supply bipolar driving signals to a tablet's resistive layer. This patent discloses that measuring a stylus' contact location along one coordinate axis requires digitization of the signal present on the stylus while an electric current flows first in one direction across the resistive layer and then in the opposite direction across the resistive layer.

U.S. Patent No. 4,055,726 discloses using arrays of semicon¬ ductor switches for supplying bipolar, time-displaced, sawtooth- shaped excitation signals alternatively to opposite parallel edges of a tablet's resistive layer. This patent discloses that the bipolar, time-displaced, sawtooth-shaped excitation signals combine in the resistive layer to sweep a line of zero voltage potential across the resistive layer from one of its parallel edges to the other. Observing the time relative to the sawtooth waveforms at which the stylus receives the zero voltage potential measures the X-axis and Y-axis coordinates at which the stylus contacts the resistive layer.

U.S. Patent No. 3,699,439 discloses using arrays of diodes electrically connected to a resistive layer for supplying electric currents to a tablet. This patent discloses that the diodes connected to the tablets edges are all oriented in the same direction with respect to the resistive layer for supplying an electric current into that layer. Transistor switches individually connected in series with each array of diodes connected to the respective edges of the resistive layer supply independently controllable electric currents through the diode arrays into the resistive layer. If a stylus contacts the resistive layer, an electric current flows from an edge of the tablet, across the resistive layer to the stylus' point, and then out of the tablet via the stylus. This patent discloses that measuring the ratio of the electric currents concurrently flowing into opposite sides of the resistive layer permits determining the X-Y coordinates at which the stylus contacts the tablet. While all of the techniques described above permit measuring X-Y coordinates of a location at which a stylus contacts a resistive layer, their respective electronic circuits and the signals for controlling their operation are comparatively complicated. Moreover, the electronic circuits that sense either voltage or current to determine X-Y coordinates are comparatively complicated either because they must measure an AC potential either to digitize it, measure multiple alternative voltages, measure the instant in time at which the potential passes through zero (0) volts, or they must independently measure two electric currents and determine a ratio of those currents.

Disclosure of Invention

An object of the present invention is to provide a si pli- fied digitizing system that includes a display screen.

Another object of the present invention is to provide a digitizer that simplifies determining Y-axis and X-axis coordi¬ nates of a location at which a point of a stylus contacts a resistive layer on a tablet included in the digitizer. Another object of the present invention is to provide a digitizer adapted for displaying with an overhead projector an image present on the digitizer's display screen.

Another object of the present invention is to provide a digitizing system that is economical to manufacture. Yet another object of the present invention is to provide a cost effective digitizing system.

Briefly the present invention is a digitizing system that includes a plate-shaped tablet preferably having a top edge and a bottom edge that are respectively disposed normal to a Y-axis of the tablet. The tablet also preferably has a left-hand edge and a right-hand edge that are respectively disposed normal to a X-axis of the tablet. The tablet also has a planar surface that is surrounded by these edges and on which there is present an electrically conductive resistive layer. The digitizing system includes a means for providing a unidirectional flow of electric current across the resistive layer substantially parallel to the tablet's Y-axis between its top and bottom edges. If an electric current flows between the top and bottom edges of the tablet, it establishes an electrical potential across the resistive layer for sensing Y-axis position. The digitizing system also includes a means for providing a unidirectional flow of electric current across the resistive layer substantially parallel to the tablet's X-axis between its left-hand and right-hand edges. If this electric current flows between the left-hand and right-hand edges it establishes an electrical potential across the resistive layer for sensing X-axis position. The digitizing system further includes a means for alternatively and repetitively causing either one or the other of these electric currents to flow through the resistive layer either along the tablet's Y-axis or along its X-axis.

The digitizing system also includes an electrically conduc¬ tive stylus having a point for contacting the tablet's resistive layer. Contacting the stylus's point to the tablet's resistive layer at some location between the top and bottom edges and between the left-hand and right-hand edges while either the Y-axis or X-axis currents flow across the layer applies either the Y-axis or X-axis electrical potential at that location to the stylus. The digitizing system includes a means for receiving and measuring the electrical potential from the stylus while the stylus contacts the tablet. Measuring the potential on the stylus while the Y-axis current flows through the resistive layer determines the Y-axis coordinate of the location at which the stylus contacts the resistive layer. Measuring the potential on the stylus while the X-axis current flows through the layer determines the X-axis coordinate of the location at which the stylus contacts the tablet.

To initiate determination of Y-axis and X-axis coordinates of a location at which the stylus's point contacts the resistive layer, the system preferably includes a means for inactivating the application of the Y-axis and X-axis electric currents while applying a stylus contact sensing electrical potential to the tablet's resistive layer. If the stylus receives the contact sensing electrical potential, then the digitizing system removes that potential from the resistive layer, and applies first the Y-axis electric current and then the X-axis electric current across the resistive layer. The digitizing system's tablet including its electrically conductive resistive layer are preferably transparent. The tablet is preferably juxtaposed with a transparent, plate-shaped display screen so an image of a path traced by a point of the stylus may be presented on the display. A case surrounds all edges both of the tablet and of the display screen and has a solid back wall which encloses a surface of the display screen furthest from the tablet. The case's front wall immediately adjacent to the tablet's resistive layer is pierced by an aperture having edges that surround a central region of the layer. The case's solid back wall is formed with a removable panel. Disengaging the panel from the back wall provides a second aperture through the case thereby permitting transmission of light through the second aperture, the display screen, the tablet, and the first aperture.

These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures.

Brief Description of Drawings

FIG. 1 is a perspective illustration of a digitizing system in accordance with the present invention in conjunction with a personal computer that is portrayed using dashed lines. FIG. 1 illustrates a computer cable coupling the system's digitizer and display screen to an interface card, together with a computer cable coupling the interface card to an optional, conventional cathode ray tube ("CRT") computer display;

FIG. 2 is a perspective illustration of the digitizer of FIG. 1 that depicts its plate-shaped tablet, stylus and plate- shaped display screen with the digitizer's case portrayed in dashed lines;

FIG. 3 is a schematic diagram illustrating an electronic circuit for the tablet and the stylus together with an electronic circuit that provides unidirectional flows of electrical current across a resistive layer present on the tablet, and also with an electronic circuit that receives and processes an electrical signal from the stylus that is used in determining the location on the tablet at which the stylus' point contacts the resistive layer;

FIG. 4 is timing diagram depicting signals for controlling the operation of the tablet's electronic circuit together with corresponding signals from the tablet and stylus; and

FIG. 5 is a perspective illustration of a back wall of the digitizer's case that depicts disengagement of a removable panel from the back wall.

Best Mode for Carrying Out the Invention

FIG. 1 is a perspective view of a digitizing system in accordance with the present invention referred to by the general reference character 20. The digitizing system 20 includes an interface card 22 adapted for installation into an Industry Standard Architecture ("ISA") bus of a personal computer 24 portrayed in FIG. 1 with dashed lines. A computer cable 26 connects the interface card 22 to a digitizer-display 28. The digitizer-display 28 includes a transparent, plate-shaped tablet 32 having a resistive layer 34 present on one of its surfaces. The tablet 32 is preferably a sheet of coated glass type no. PD5013.2IR manufactured by Donnelly Corporation of Holland, Michigan. Juxtaposed with the tablet 32 in the digitizer-display 28 is a plate-shaped, back-lit liquid crystal display ("LCD") 34, illustrated in FIG. 2, whose length and width are slightly less than that of the tablet 32. The LCD 36 is preferably a model no. LM64P722 manufactured by Sharp Corporation of Japan. The digitizing system 20 also includes a stylus 38 that is connected by an elongated electrically conductive stylus cable 42 to the digitizer-display 28. The resistive layer 34 of the tablet 32 is disposed on the surface of the tablet 32 furthest from the LCD 36 so that the layer 34 faces outward from the digitizer-display 28 to be contacted by a point 44 of the stylus 38. Optionally, the interface card 22 of the digitizing system 20 may also be connected by a second computer cable 46 to a conventional VGA computer display 48 for presenting thereon the same images as appear on the LCD 36.

Referring now to FIG. 2, the tablet 32 has a top edge 52 and a bottom edge 54 that are parallel to each other and disposed normal to a Y-axis 56 of the digitizer-display 28. The tablet 32 also has a left-hand edge 62 and a right-hand edge 64 that are parallel to each other and disposed normal to a X-axis 66. The edges 52, 54, 62 and 64 surround the tablet 32 and its resistive layer 34. Respectively secured to the resistive layer 34 along each of the edges 52, 54, 62 and 64 of the tablet 32 by electri¬ cally conductive adhesive are elongated printed circuit boards ("PCBs") 72, 74, 76 and 78. The PCBs 72, 74, 76 and 78 support electronic components included in an electronic circuit that provides unidirectional flows of electrical current across the resistive layer 34 either along the Y-axis 56 or along the X-axis 66.

The tablet 32 together with its PCBs 72, 74, 76 and 78 and the LCD 36 are enclosed within a case 82 portrayed with dashed lines in FIG. 2. The case 82 surrounds the LCD 36, all the edges 52, 54, 62 and 64 of the tablet 32, and the PCBs 72, 74, 76 and 78 secured thereto. The case 82 includes a front wall 84 immediately adjacent to the resistive layer 34. The front wall 84 is pierced by a first aperture 86 formed by edges 88a, 88b, 88c and 88d that surround a central region 92 of the resistive layer 34 that is also encompassed by the PCBs 72, 74, 76 and 78. Located at the corners of the case 82 immediately adjacent to the vertex of the edges 54 and 62, and also immediately adjacent to the vertex of the edges 54 and 64 are sockets 94 only one of which is visible in the illustration of FIG. 2. One or the other of the sockets 94 receives a plug (not depicted in any of the figures) that is attached at the end of the stylus cable 42 furthest from the stylus 38. The stylus cable 42 may be plugged into either one or the other of the sockets 94 to readily adapt the digitizer-display 28 for use either by right-handed or by left-handed individuals. The case 82 also encloses an elongated LCD power PCB 96 that is located immediately beneath the PCB 76, and that extends parallel to the entire width of the immediately adjacent LCD 36. Secured to the LCD power PCB 96 are components of an electronic circuit that provides electrical power for illuminating the LCD 36. Also secured to the LCD power PCB 96 is a connector 98 with which one end of the computer cable 26 mates. Referring now to FIG. 3, depicted there is a schematic diagram that illustrates an electronic circuit for the tablet 32 and the stylus 38 together with an electronic circuit that provides unidirectional flows of electrical current across resistive layer 34. FIG. 3 also depicts an electronic circuit that receives an electrical signal from the stylus 38 for use in determining the location on the tablet at which the point 44 of the stylus 38 contacts the resistive layer 34. As depicted in FIG. 3, secured to the PCB 78 at the bottom edge 54 of the tablet 32 are a plurality of Y-axis diodes 102. Preferably, the Y-axis diodes 102 may be either MR816 or 1N4148 diodes. The electrical¬ ly conductive adhesive that secures the PCB 78 to the tablet 32 also electrically interconnects anode terminals 104 of the Y-axis diodes 102 to the resistive layer 34 at locations that are spaced apart from each other along the bottom edge 54 of the tablet 32. Cathode terminals 106 of the Y-axis diodes 102 are electrically connected together by a diode common line 108 that extends from the digitizer-display 28 through the computer cable 26 (illus¬ trated by dashed-line ellipses in FIG. 3) to the interface card 22. The PCB 72 at the left-hand edge 62 of the tablet 32 supports a plurality of X-axis diodes 112. The X-axis diodes 112 may also preferably be either MR816 or 1N4148 diodes. The X-axis diodes 112 have cathode terminals 114 that are electrically connected to the resistive layer 34 by the electrically conduc- tive adhesive at locations that are spaced apart from each other along the left-hand edge 62 of the tablet 32. Anode terminals 116 of the X-axis diodes 112 are electrically connected together with each other and with the cathode terminals 106 of the Y-axis diodes 102 by the diode common line 108. The Y-axis diodes 102 are all oriented for conducting electricity in one direction with respect to the resistive layer 34 while the X-axis diodes 112 are all oriented for conducting electricity in the opposite direc¬ tion. Accordingly, the Y-axis diodes 102 permit an electric current to flow only out of the resistive layer 34 into the diode common line 108, while X-axis diodes 112 permit an electric current to flow only into the resistive layer 34 from the diode common line 108. Supported on the PCB 76 at the top edge 52 of the tablet 32 are a plurality of Y-axis Field Effect Transistor ("FET") switches 122. The Y-axis FET switches 122 preferably are VN10LF N-channel FET transistors. The electrically conductive adhesive securing the PCB 76 to the tablet 32 connects source conduction electrodes 124 of each of the Y-axis FET switches 122 to the resistive layer 34 at locations that are spaced apart from each other along the top edge 52 of the tablet 32. A sensing current source line 126 electrically connects together drain conduction electrodes 128 of all the Y-axis FET switches 122. The sensing current source line 126, that extends from the digitizer-display 28 through the computer cable 26 to the interface card 22, connects the drain conduction electrodes 128 of the Y-axis FET switches 122 to a source of electric current (not illustrated in any of the FIGs.) that is a positive pole of a +5 volt ("V") power supply. Thus connected between the sensing current source line 126 and the resistive layer 34, the Y-axis FET switches 122 are oriented for conducting electricity across the resistive layer 34 in a direction which corresponds with the direction of electric current flow through the Y-axis diodes 102, and the resistive layer 34 connects the Y-axis FET switches 122 and the Y-axis diodes 102 into a series circuit.

A Y-axis control signal line 132, that extends from the digitizer-display 28 through the computer cable 26 to the interface card 22, electrically connects together in common gate control electrodes 134 of all of the Y-axis FET switches 122. If the electrical potential on the Y-axis control signal line 132 is higher than the electrical potential on the sensing current source line 126, then the Y-axis FET switches 122 turn off and no electric current flows through the Y-axis FET switches 122 into the resistive layer 34. If the electrical potential on the Y-axis control signal line 132 is significantly less than the electrical potential on the sensing current source line 126, then the Y-axis FET switches 122 turn on. Moreover, if the potential on the Y-axis control signal line 132 turns on the Y-axis FET switches 122 and if the electrical potential on the diode common line 108 connected to the cathode terminals 106 of the Y-axis diodes 102 is significantly less than the potential on the sensing current source line 126, e.g. at least a few volts less, then an electric current flows from the source conduction electrodes 124 of the Y-axis FET switches 122 into the resistive layer 34, across the resistive layer 34 from the top edge 52 to the bottom edge 54 substantially parallel to the Y-axis 56, and out of the resistive layer 34 into the anode terminals 104 of the Y-axis diodes 102. This unidirectional flow of electricity across the tablet 32 establishes a voltage gradient across the resistive layer 34 with the highest potential being along the top edge 52, and with the lowest potential being along the bottom edge 54. The presence of this gradient in electrical potential across the resistive layer 34 permits sensing a Y-axis coordinate of a location at which the point 44 of the stylus 38 contacts the tablet 32. Supported on the PCB 74 at the right-hand edge 64 of the tablet 32 are a plurality of X-axis FET switches 142. The X-axis FET switches 142 are also preferably VN10LF N-channel FET transistors. The electrically conductive adhesive securing the PCB 74 to the tablet 32 connects drain conduction electrodes 144 of each of the X-axis FET switches 142 to the resistive layer 34 at locations that are spaced apart from each other along the right-hand edge 64 of the tablet 32. A sensing current drain line 146 electrically connects together source conduction electrodes 148 of all the X-axis FET switches 142. The sensing current drain line 146, that extends from the digitizer-display 28 through the computer cable 26 to the interface card 22, connects the source conduction electrodes 148 of the X-axis FET switches 142 to a sink of electric current (not illustrated in any of the FIGs.) that is a ground potential pole of the +5 V power supply. Thus connected between the sensing current drain line 146 and the resistive layer 34, the X-axis FET switches 142 are oriented for conducting electricity across the resistive layer 34 in a direction which corresponds with the direction of electric current flow through the X-axis diodes 112, and the resistive layer 34 connects the X-axis FET switches 142 and the X-axis diodes 112 into a series circuit.

A X-axis control signal line 152, that extends from the digitizer-display 28 through the computer cable 26 to the interface card 22, electrically connects together in common gate control electrodes 154 of all of the X-axis FET switches 142. If the electrical potential on the X-axis control signal line 152 is higher than the electrical potential on the sensing current source line 126, then the X-axis FET switches 142 turn off and no electric current flows through the X-axis FET switches 142 out of the resistive layer 34. If the electrical potential on the X-axis control signal line 152 is at or near the ground electri¬ cal potential present on the sensing current drain line 146, then the X-axis FET switches 142 turn on. Moreover, if the X-axis FET switches 142 turn on and if the electrical potential on the diode common line 108 connected to the anode terminals 116 of the X-axis diodes 112 is significantly higher than the potential on the sensing current drain line 146, e.g. at least a few volts higher, then an electric current flows from the cathode terminals 114 of the X-axis diodes 112 into the resistive layer 34, across the resistive layer 34 from the left-hand edge 62 to the right-hand edge 64 substantially parallel to the X-axis 66, and out of the resistive layer 34 into the drain conduction elec- trodes 144 of the X-axis FET switches 142. This unidirectional flow of electricity across the tablet 32 establishes a voltage gradient across the resistive layer 34 with the highest potential being along the left-hand edge 62 and with the lowest potential being along the right-hand edge 64. The presence of this gradient in electrical potential across the resistive layer 34 permits sensing a X-axis coordinate of a location at which the point 44 of the stylus 38 contacts the tablet 32.

To provide electrical control signals that alternatively either activate the Y-axis FET switches 122 and the Y-axis diodes 102 for providing a unidirectional flow of electric current between the top edge 52 and the bottom edge 54 of the tablet 32, or activate the X-axis diodes 112 and the X-axis FET switches 142 for providing a unidirectional flow of electric current between the left-hand edge 62 and the right-hand edge 64 of the tablet 32, the interface card 22 includes a pair of driver electronic circuits illustrated at the top of FIG. 3. The interface card 22 supplies these control signals to the digitizer-display 28 via the diode common line 108, the Y-axis control signal line 132, and the X-axis control signal line 152 included in the computer cable 26.

The electronic circuit included in the interface card 22 that supplies the control signal to the diode common line 108 includes a PNP transistor 162, and a NPN transistor 164. The PNP transistor 162 is preferably a 2N4403 bipolar transistor and the NPN transistor 164 is preferably a 2N4401 bipolar transistor. An emitter conduction electrode 166 of the PNP transistor 162 connects to the positive pole of the +5 V power supply via the sensing current source line 126, while an emitter conduction electrode 168 of the NPN transistor 164 connects to the ground potential pole of the +5 V power supply. Base control electrodes 172 and 174 of the transistors 162 and 164 respectively connect through 10 K ohm resistors 176 and 178 to a diode control signal output terminal 180 of a model 8051 microprocessor 182 that is included in the interface card 22 and illustrated in FIG. 1. Collector conduction electrodes 184 and 186 of the transistors 162 and 164 both respectively connect through 5.1 ohm resistors 192 and 194 to the diode common line 108. A twenty-seven picofarad capacitor 198 connects in parallel with a reverse- biased diode 202 between the diode common line 108 and the ground potential pole of the +5 V power supply. A reverse-biased diode 204 connects between the diode common line 108 and positive pole of the +5 V power supply via the sensing current source line 126. The diodes 202 and 204, which preferably are either MR816 or 1N4148 diodes, protect the transistors 162 and 164 against static electricity discharges.

If the diode control signal output terminal 180 of the microprocessor 182 applies a signal to the resistors 176 and 178 that is slightly above ground potential, then the PNP transistor 162 conducts electricity while the NPN transistor 164 does not conduct electricity. If this occurs, the potential on the diode common line 108 will be only slightly less than the +5 V potential that is present on the sensing current source line 126. Conversely, if the diode control signal output terminal 180 applies a signal to the resistors 176 and 178 that is at or near the +5 V potential present on the sensing current source line 126, then the NPN transistor 164 conducts electricity while the PNP transistor 162 does not conduct electricity. If this latter condition occurs, then the potential on the diode common line 108 is only slightly higher than ground potential. Thus, the state of the signal present at the diode control signal output terminal 180 of the microprocessor 182 determines whether the potential on the diode common line 108 is only slightly less than the +5 V potential present on the sensing current source line 126, or is only slightly higher than ground potential.

To provide control signals for turning on either the Y-axis FET switches 122 of the Y-axis control signal line 132, the interface card 22 includes a second driver circuit which includes a PNP transistor 212 and a NPN transistor 214. The PNP transis¬ tor 212 is preferably a 2N4403 bipolar transistor, and the NPN transistor 214 is preferably a 2N4401 bipolar transistor. An emitter conduction electrode 216 of the NPN transistor 214 connects to the ground potential pole of the +5 V power supply. A 10 K ohm resistor 218 connects a FET control signal output terminal 220 of the microprocessor 182 to a base control electrode 222 of the NPN transistor 214. The X-axis control signal line 152 connects to a junction of a collector conduction electrode 224 of the NPN transistor 214 with one terminal of a 39 K ohm resistor 226. The other terminal of the resistor 226 connects to a junction of a base control electrode 228 of the PNP transistor 212 with one terminal of a 4.7 K ohm resistor 232. The other terminal of the resistor 232 connects to a positive pole of a +12 V power supply (not illustrated in any of the FIGs.), as does an emitter conduction electrode 234 of the PNP transistor 212. A collector conduction electrode 236 of the PNP transistor 212 connects to a junction of the Y-axis control signal line 132 with a 1.0 M ohm resistor 238, a cathode terminal of a diode 242, one terminal of a twenty-seven picofarad capacitor 244, and an anode terminal of a diode 246. The second terminal of the resistor 238, an anode terminal of the diode 242, and the second terminal of the capacitor 244 are all connected together in common with the ground potential poles of both the +5 V and +12 V power supplies. A cathode terminal of the diode 246 connects to the positive pole of the +12 V power supply. The reverse biased diodes 242 and 246, which protect the PNP transistor 212 against static electricity discharges, are preferably MR816 or 1N4148 diodes.

Responding to the signal present at the FET control signal output terminal 220 of the microprocessor 182, the collector conduction electrode 224 of the NPN transistor 214 applies a signal to the X-axis control signal line 152 which is the inverse of the signal at the FET control signal output terminal 220. Conversely, the collector conduction electrode 236 of the PNP transistor 212 applies a signal to the Y-axis control signal line 132 which is in-phase with the signal at the FET control signal output terminal 220 of the microprocessor 182. Both the signal on the Y-axis control signal line 132 and the signal on the X-axis control signal line 152 may respectively have a potential either slightly above the potential of the common ground poles of the +5 V and +12 V power supplies, or slightly below the potential at the positive pole of the +12 V power supply.

By applying appropriate signals to the diode control signal output terminal 180 and to the FET control signal output terminal 220, a computer program executed by the microprocessor 182 can control application of an electrical current across the tablet 32 either between the top edge 52 and the bottom edge 54, or between the left-hand edge 62 and the right-hand edge 64 for determining the location at which the point 44 contacts the tablet 32. However, before the computer program executed by the microprocessor 182 applies such position sensing electric currents, it first applies signals to the diode control signal output terminal 180 and to the FET control signal output terminal 220 that cause a stylus contact sensing electrical potential to be applied uniformly across the resistive layer 34 as illustrated in the timing diagram of FIG. 4.

To apply the stylus contact sensing electrical potential across the resistive layer 34 during a time interval T₂ illus¬ trated in FIG. 4, the computer program executed by the micropro¬ cessor 182 applies a low potential both to the diode control signal output terminal 180 and to the FET control signal output terminal 220. A low potential on the diode control signal output terminal 180 places an almost +5 V potential on the diode common line 108 thereby supplying an almost +5 V potential to the resistive layer 34 through the X-axis diodes 112. A low potential on the FET control signal output terminal 220 places a potential on the Y-axis control signal line 132 that is only slightly higher than ground potential, i.e. slightly above zero (0) volts, and a potential on the X-axis control signal line 152 that is only slightly less than the potential at the positive pole of the +12 V power supply. A low potential on the Y-axis control signal line 132 turns on the Y-axis FET switches 122 thereby supplying an almost +5 V potential to the resistive layer 34 through the source conduction electrodes 124. The almost +12 V potential applied to the gate control electrodes 154 turns off the X-axis FET switches 142. Thus, this combination of signals on the diode control signal output terminal 180 and the FET control signal output terminal 220 places an almost +5 V potential uniformly along the left-hand edge 62 and the top edge 52 of the resistive layer 34 through the X-axis diodes 112 and through the Y-axis FET switches 122. Since simultaneously applying low signals both to the diode control signal output terminal 180 and to the FET control signal output terminal 220 prevents current from flowing out ot the Y-axis diodes 102 and turns off the X-axis FET switches 142, the almost +5 V potential applied along the left-hand edge 62 and the top edge 52 is present uniformly everywhere across the resistive layer 34. Thus, if the computer program executed by the microprocessor 182 senses the presence of this almost +5 V potential on the stylus 38, the stylus 38 has contacted the resistive layer 34 as illustrated at the point 262 in FIG. 4. After the stylus 38 contacts the resistive layer 34, the computer program then begins a sequence of measurements that determine the coordinates of the location at which the point 44 contacts the tablet 32.

In measuring the Y-axis coordinate of the location at which the point 44 contacts the tablet 32 during a time interval T₀ illustrated in FIG. 4, the computer program changes the signal on the diode control signal output terminal 180 from a low potential to a high potential while retaining the signal on the FET control signal output terminal 220 at a low potential. Placing a high potential on the diode control signal output terminal 180 applies a potential to the diode common line 108 and to the cathode terminals 106 of the Y-axis diodes 102 that is only slightly above zero (0) volts. Because the Y-axis FET switches 122 have already been turned on by the low signal potential on the FET control signal output terminal 220, an electric current immediately begins flowing across the resistive layer 34 between the top edge 52 and the bottom edge 54 when the cathode terminals 106 of the Y-axis diodes 102 receive this low potential. As described previously, this electric current creates a gradient in electrical potential across the resistive layer 34 for determining the Y-axis coordinate at which the point 44 contacts the tablet 32. The shaded area for the voltage on the resistive layer 34 in FIG. 4 illustrates the presence of this voltage gradient.

After the computer program executed by the microprocessor 182 determines the Y-axis coordinates at which the point 44 contacts the tablet 32, it then applies signals to the diode control signal output terminal 180 and to the FET control signal output terminal 220 to permit determining the X-axis coordinates at which the point 44 contacts the tablet 32 during a time interval T__ illustrated in FIG. 4. To effect this determination, the computer program changes the signal applied to the diode control signal output terminal 180 from a high potential to a low potential, and the signal applied to the FET control signal output terminal 220 from a low potential to a high potential. The application these signals to the diode control signal output terminal 180 and to the FET control signal output terminal 220 cause an almost +5 V potential to be applied on the diode common line 108, an almost +12 V potential on the Y-axis control signal line 132, and an almost zero (0) volt potential on the X-axis control signal line 152. The presence of the almost +12 V potential on the Y-axis control signal line 132, +5 V potential on the diode common line 108, and the zero (0) volt potential on the X-axis control signal line 152 terminates the flow of electric current across the resistive layer 34 between the top edge 52 and the bottom edge 54, and immediately initiates a flow of electric current across the resistive layer 34 between the left-hand edge 62 and the right-hand edge 64. As described previously, this electric current creates a gradient in electri- cal potential across the resistive layer 34 for determining the X-axis coordinate at which the point 44 contacts the tablet 32. After determining the X-axis coordinate at which the point 44 contacts the tablet 32, the computer program returns to determine if the stylus 38 remains in contact with the tablet 32 before again repeating another determination of Y-axis and X-axis coordinates at which the point 44 contacts the tablet 32. If the stylus 38 remains in contact with the tablet 32, the computer program executed by the microprocessor 182 repetitively both senses the stylus 38 contacting the tablet 32 and determines the Y-axis and X-axis coordinates at which the point 44 contacts the tablet 32 two hundred times per second. However, before providing other computer programs such as Pen Windows that are being executed by the personal computer 24 with the coordinates of the location at which the point 44 contacts the tablet 32, the computer program executed by the microprocessor 182 repetitively determines those coordinates while cumulating between 5 and 10 successive measurements of Y-axis coordinates and of X-axis coordinates. After the computer program executed by the microprocessor 182 cumulates this coordinate data, it then averages that data before providing the averaged measurements to the other computer programs.

After an indeterminate interval of time, the point 44 of the stylus 38 ceases to contact the resistive layer 34. Breaking of contact between the point 44 and the resistive layer 34, indicated by the point 264 in FIG. 4, may occur during any of the three intervals T₀, T_x or T₂ in the operation of the digitizing system 20. Therefore, during such an interval identified as T_x in FIG. 4, it is possible that the signals present on diode control signal output terminal 180, FET control signal output terminal 220, diode common line 108, Y-axis control signal line 132, and X-axis control signal line 152 may be either in their high or low states as indicated by the hatching in FIG. 4. After the routine execution of the computer program by the microproces- sor 182 causes the digitizing system 20 to again enter the T₂ interval if it is not already in that interval, the T₂ interval persists until the point 44 of the stylus 38 once again contacts the resistive layer 34. Referring back to FIG. 3, depicted at the bottom of that FIG. is a circuit included in the interface card 22 that receives and processes the electrical signal from the stylus 38. As described previously, the stylus cable 42 conducts the signal present on the stylus 38 into the digitizer-display 28 at one of the sockets 94. Within the digitizer-display 28, the signal passes directly to the connector 98 and thence through the computer cable 26 to the interface card 22. At the interface card 22, the signal from the stylus 38 is applied to a junction at which terminals of two series connected, reverse-biased diodes 272 and 274 meet. The other terminal of the diode 272 connects to the +5 V potential present on the sensing current source line 126, while the other terminal of the diode 274 connects to ground potential. The diodes 272 and 274, which preferably are either MR816 or 1N4148 diodes, protect the circuit on the interface card 22 against static electricity discharges that may strike the stylus 38.

Also connected to the junction of the two diodes 272 and 274 is a first terminal of a 27 microhenry inductor 276. A second terminal of the inductor 276 connects to a first terminal of a 47 ohm resistor 278. A second terminal of the resistor 278 connects to a junction of a parallel connected 0.001 microfarad capacitor 282 and 1.0 M ohm resistor 284 with a non-inverting input 286 of an operational amplifier 288. The operational amplifier 288 is preferably a LMC662 operational amplifier. The other terminals of the parallel connected capacitor 282 and resistor 284 connect to ground potential. The inductor 276 rejects a wide spectrum of relatively high frequency noise that may be present in the signal received from the stylus 38. The series connected resistor 278 and capacitor 282 provide a passive low-pass filter. The resistor 284 provides a source of bias current for the non-inverting input 286 of the operational amplifier 288 when the stylus 38 is not contacting the tablet 32.

Connected between an output 292 of the operational amplifier 288 and ground potential are a series connected 2.0 K ohm resistor 294 and 100 K ohm resistor 296. The junction of the resistors 294 and 296 connects to an inverting input 298 of the operational amplifier 288. This feedback of the output signal from the operational amplifier 288 through the resistor 294 to the inverting input 298 limits the output signal from the operational amplifier 288 to only a slight gain in amplitude over the signal present at the non-inverting input 286. Also connected to the output 292 of the operational amplifier 288 is a first terminal of a 2 K ohm resistor 302. A second terminal of the resistor 302 connects to a first terminal of a 0.22 microfarad capacitor 304. A second terminal of the capacitor 304 connects to ground potential. The series connected resistor 302 and capacitor 304 provide a second passive low-pass filter.

A series connected 100 K ohm resistor 306 and 2 K ohm resistor 308 connect the junction of the resistor 302 and the capacitor 304 with a non-inverting input 312 of an operational amplifier 314. The operational amplifier 314 is preferably a LMC662 operational amplifier. A 180 picofarad capacitor 316 is connected between the non-inverting input 312 and ground potential. An output 318 of the operational amplifier 314 connects directly to its inverting input 322. A 470 picofarad capacitor 326 couples the signal from the output 318 to the junction of the resistor 306 with the resistor 308. The passive components 306, 308, 316 and 326 configure the operational amplifier 314 to operate as an active low-pass filter and control its voltage gain, upper cut-off frequency and flatness of its frequency response.

The signal present at the output 318 of the operational amplifier 314 is supplied directly to a stylus down input signal terminal 328 on the microprocessor 182. While the computer program executed by the microprocessor 182 causes a stylus contact sensing electrical potential to be present on the resistive layer 34, a high potential at the output 318 of the operational amplifier 314 which is supplied to the stylus down input signal terminal 328 informs the computer program that the stylus 38 has contacted the tablet 32. A 470 picofarad capacitor 332 couples the output 318 of the operational amplifier 314 to ground potential. A 620 ohm resistor 334 couples the output signal from the output 318 of the operational amplifier 314 to an analog signal input 336 of an analog-to-digital converter ("ADC") 338 located on the interface card 22 that is illustrated in FIG. 1. The capacitor 332 and the resistor 334 provide a final stage of filtering before the signal is applied to the analog signal input 336 of the ADC 338. The ADC 338 preferably a Model LTC1292 single chip 12-Bit ADC marketed by Linear Technology Corporation of Milpitas, Califor¬ nia.

Operating under the control of the computer program executed by the microprocessor 182, the ADC 338 converts the analog signal present at the output of the operational amplifier 314 into a digital value that is then transferred to the microprocessor 182. When the computer program executed by the microprocessor 182 causes an electric current to flow across the resistive layer 34 between the top edge 52 and the bottom edge 54 of the tablet 32, the digital value transferred to the microprocessor 182 repre¬ sents the Y-axis coordinate at which the point 44 contacts the tablet 32. When the computer program executed by the micropro¬ cessor 182 causes an electric current to flow across the resistive layer 34 between the left-hand edge 62 and the right-hand edge 64 of the tablet 32, the digital value trans¬ ferred to the microprocessor 182 represents the X-axis coordinate at which the point 44 contacts the tablet 32.

The interface card 22 illustrated in FIG. 1 includes a display control circuit 352 which permits a computer program executed by the personal computer 24 to simultaneously present images on the LCD 36 included in the digitizer-display 28 and on the computer display 48. The display control circuit 352 is preferably a conventional VGA display control of a type used extensively in personal computers that employ an ISA bus that is capable of presenting images both on VGA displays and on liquid crystal displays.

FIG. 5 is a perspective view illustrating a solid back wall 362 of the case 82. The back wall 362 includes a removable panel 364 that may be disengaged from the case 82. The inner surface of the removable panel 364 immediately adjacent to the LCD 36 is covered with a reflective, diffusing material to provide proper back-lighting for the LCD 36 with the removable panel 364 installed in the case 82. Disengaging the removable panel 364 from the back wall 362 extinguishes the back-lighting and provides a second aperture 366 through the case 82. Disengage¬ ment of the removable panel 364 permits transmission of light through the second aperture 366, the LCD 36, the tablet 32 and the first aperture 86 formed through the front wall 84 of the case 82. The removable panel 364 adapts the digitizer-display 28 for disposition on an overhead projector thus permitting enlargement of an image appearing on the LCD 36 and projecting that image onto a screen for viewing by a number of individuals.

Industrial Applicability

The juxtaposition of the LCD 36 with the tablet 32 in the digitizer-display 28 and the display control circuit 352 included in the interface card 22 permit a computer program executed by the personal computer 24, such as Pen Windows, to receive Y-axis and X-axis coordinates from the microprocessor 182 included in the interface card 22, and to then present on the LCD 36 an image of a path traced on the tablet 32 by the point 44 of the stylus 38 that is in registration with that path. Thus, the digitizing system 20 facilitates providing an operator using the digitizing system 20 immediately with a visual representation of the coordinate data received by the computer program executed by the personal computer 24.

It is readily apparent to those skilled in the art that reversing the top edge 52 and bottom edge 54 of the tablet 32 to which the Y-axis diodes 102 and the Y-axis FET switches 122 respectively connect is within the scope of the present inven¬ tion, and does not materially alter the operation of the digitizing system 20. Correspondingly, reversing the left-hand edge 62 and right-hand edge 64 of the tablet 32 to which the X-axis diodes 112 and the X-axis FET switches 142 respectively connect is also within the scope of the present invention, and also does not materially alter the operation of the digitizing system 20. Since the edges 52 and 54 to which the Y-axis diodes 102 and the Y-axis FET switches 122 connect is electrically independent of the edges 62 and 64 to which the X-axis diodes 112 and the stylus cable 42 connect, simultaneously reversing the connections of both sets is within the scope of the present invention, and also does not materially alter the operation of the digitizing system 20.

Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is purely illustrative and is not to be interpreted as limiting. Consequently, without departing from the spirit and scope of the invention, various alterations, modifications, and/or alternative applications of the invention will, no doubt, be suggested to those skilled in the art after having read the preceding disclosure. Accordingly, it is intended that the following claims be interpreted as encompassing all alterations, modifications, or alternative applications as fall within the true spirit and scope of the invention.

Claims

The ClaimsWhat is claimed is:

1. A method for electrically determining Y-axis and X-axis coordinates of a location at which a point of an electrically conductive stylus contacts an electrically conductive resistive layer present on a planar surface of a plate-shaped tablet, the tablet having a left-hand edge and a right-hand edge that are respectively disposed normal to the X-axis of said tablet, and also having a top edge and bottom edge that are respectively disposed normal to the Y-axis of said tablet, the method comprising the steps of repeatedly: applying a stylus contact sensing electrical potential to the resistive layer of the tablet; and if the stylus contact sensing electrical potential is present on the stylus, removing the stylus contact sensing electrical potential from the resistive layer, and then: providing a Y-axis unidirectional flow of electrical current across the resistive layer between the top edge and the bottom edge of the tablet thereby establishing a Y-axis sensing electrical potential across the resistive layer, and concurrently measuring an electrical potential present on the stylus to determine the Y-axis coordinate of the location at which the point of the stylus contacts the resistive layer; and providing a X-axis unidirectional flow of electrical current across the resistive layer between the left-hand edge and the right-hand edge of the tablet thereby estab¬ lishing a X-axis sensing electrical potential across the resistive layer, and concurrently measuring an electrical potential present on the stylus to determine the X-axis coordinate of the location at which the point of the stylus contacts the resistive layer.

2. The method of claim 1 further comprising the steps of: cumulating several measurements of the Y-axis coordinate and several measurements of the X-axis coordinate; averaging the several Y-axis coordinate measurements and averaging the several X-axis coordinate measurements to obtain Y-axis and X-axis coordinates of the location at which the point of the stylus contacts the resistive layer.

3. The method of claim 1 wherein said tablet and the electrically conductive resistive layer are both transparent, and a transparent, plate-shaped display screen is juxtaposed with a second planar surface of said tablet that is parallel to and on the opposite side of said tablet from that on which the electri¬ cally conductive resistive layer is present, the method further comprising the step of transmitting light through the display screen and through the tablet.

4. A digitizing system comprising: a plate-shaped tablet having a top edge and a bottom edge that are respectively disposed normal to a Y-axis of said tablet, and also having a left-hand edge and a right-hand edge that are respectively disposed normal to a X-axis of said tablet, said tablet also having a first planar surface that is surrounded by the edges and on which there is present an electrically conduc¬ tive resistive layer;

Y-axis current means for providing unidirectional flow of electrical current across the resistive layer between the top edge and the bottom edge thereby establishing a Y-axis sensing electrical potential;

X-axis current means for providing a unidirectional flow of electrical current across the resistive layer between the left-hand edge and the right-hand edge thereby establishing a X-axis sensing electrical potential; sensing current control means for alternatively activating either said Y-axis current means or said X-axis current means to alternatively and repetitively provide first one and then the other of their respective flows of electrical current across the resistive layer; an electrically conductive stylus having a point for contacting the resistive layer of said tablet to thereby receive either the Y-axis or the X-axis sensing electrical potential from the resistive layer; and sensing potential measuring means for receiving and measuring the electrical potential from said stylus while said stylus contacts the resistive layer to thereby determine the Y-axis coordinate of the location at which said stylus contacts the resistive layer while the Y-axis flow of electrical current passes across the resistive layer, or to determine the X-axis coordinate of the location at which said stylus contacts the resistive layer while the X-axis flow of electrical current passes across the resistive layer.

5. The digitizing system of claim 4 wherein said Y-axis current means includes a plurality of Y-axis diodes, a first terminal of all the Y-axis diodes being electrically connected in common to a first pole of a Y-axis electrical current source, a second terminal of each Y-axis diode being electrically connected to the resistive layer of said tablet at a location at the bottom edge of said tablet that is spaced apart from other locations along the bottom edge of said tablet at which the second terminals of other Y-axis diodes connect to the resistive layer, all of the Y-axis diodes being oriented for conducting electricity in the same direction; said Y-axis current means also including a plurality of Y-axis switches, a first conduction electrode of all the Y-axis switches being connected in common to a second pole of the Y-axis electrical current source, a second conduction electrode of each Y-axis switch being electri¬ cally connected to the resistive layer of said tablet at a location at the top edge of said tablet that is spaced apart from other locations along the top edge of said tablet at which the second conduction electrodes of other Y-axis switches connect to the resistive layer, all of the Y-axis switches being oriented for conducting electricity in a direction that corresponds with the direction of current flow through the Y-axis diodes.

6. The digitizing system of claim 5 wherein said X-axis current means includes a plurality of X-axis diodes, a first terminal of all the X-axis diodes being electrically connected in common to a first pole of a X-axis electrical current source, a second terminal of each X-axis diode being electrically connected to the resistive layer of said tablet at a location at the bottom edge of said tablet that is spaced apart from other locations along the bottom edge of said tablet at which the second terminals of other X-axis diodes connect to the resistive layer, all of the X-axis diodes being oriented for conducting electricity in the same direction; said X-axis current means also including a plurality of X-axis switches, a first conduction electrode of all the X-axis switches being connected in common to a second pole of the X-axis electrical current source, a second conduction electrode of each X-axis switch being electri¬ cally connected to the resistive layer of said tablet at a location at the top edge of said tablet that is spaced apart from other locations along the top edge of said tablet at which the second conduction electrodes of other X-axis switches connect to the resistive layer, all of the X-axis switches being oriented for conducting electricity in a direction that corresponds with the direction of current flow through the X-axis diodes.

7. The digitizing system of claim 6 wherein the X-axis diodes are oriented for conducting electricity with respect to the resistive layer in an opposite direction to the orientation of the Y-axis diodes.

8. The digitizing system of claim 7 wherein the first terminals of said Y-axis diodes that are connected together in common are also electrically connected directly to the first terminals of said X-axis diodes that are connected together in common.

9. The digitizing system of claim 6 further comprising stylus contact sensing means that includes: means for suspending operation of said sensing current control means thereby inactivating both said Y-axis current means and said X-axis current means; means for applying a stylus contact sensing electrical potential to the resistive layer of said tablet while said Y-axis current means and said X-axis current means are inactivated; and means for removing the stylus contact sensing electrical potential from the resistive layer and for restoring operation of said sensing current control means if the stylus contact sensing electrical potential becomes present on the stylus.

10. The digitizing system of claim 6 wherein said tablet and the electrically conductive resistive layer are both transparent; and said digitizing system further comprises a plate-shaped display screen juxtaposed with a second planar surface of said tablet that is parallel to and on the opposite side of said tablet from that on which the electrically conduc- tive resistive layer is present.

11. The digitizing system of claim 10 further comprising display control means for presenting on said display screen an image of a path traced by the point of said stylus on the resistive layer of said tablet, the image appearing on said display screen following and in registration with the path traced by the point of the stylus on the resistive layer.

12. The digitizing system of claim 10 wherein said display screen is also transparent; and said digitizing system further comprises a case that surrounds said display screen and all edges of said tablet, said case having a solid back wall which encloses a surface of said display screen furthest from said tablet and also having a front wall immediately adjacent to the resistive layer of said tablet, the front wall being pierced by a first aperture having edges that surround a central region of the resistive layer of the tablet, the solid back wall of said case having formed therein a removable panel whose disengagement from the back wall provides a second aperture that permits transmis- sion of light through the second aperture, through said display screen, through said tablet, and through the first aperture.

13. The digitizing system of claim 4 wherein said X-axis current means includes a plurality of X-axis diodes, a first terminal of all the X-axis diodes being electrically connected in common to a first pole of a X-axis electrical current source, a second terminal of each X-axis diode being electrically connected to the resistive layer of said tablet at a location at the bottom edge of said tablet that is spaced apart from other locations along the bottom edge of said tablet at which the second terminals of other X-axis diodes connect to the resistive layer, all of the X-axis diodes being oriented for conducting electricity in the same direction; said X-axis current means also including a plurality of X-axis switches, a first conduction electrode of all the X-axis switches being connected in common to a second pole of the X-axis electrical current source, a second conduction electrode of each X-axis switch being electri¬ cally connected to the resistive layer of said tablet at a location at the top edge of said tablet that is spaced apart from other locations along the top edge of said tablet at which the second conduction electrodes of other X-axis switches connect to the resistive layer, all of the X-axis switches being oriented for conducting electricity in a direction that corresponds with the direction of current flow through the X-axis diodes.

14. The digitizing system of claim 4 further comprising stylus contact sensing means that includes: means for suspending operation of said sensing current control means thereby inactivating both said Y-axis current means and said X-axis current means; means for applying a stylus contact sensing electrical potential to the resistive layer of said tablet while said Y-axis current means and said X-axis current means are inactivated; and means for removing the stylus contact sensing electrical potential from the resistive layer and for restoring operation of said sensing current control means if the stylus contact sensing electrical potential becomes present on the stylus.

15. The digitizing system of claim 4 wherein said tablet and the electrically conductive resistive layer are both transparent; and said digitizing system further comprises a plate-shaped display screen juxtaposed with a second planar surface of said tablet that is parallel to and on the opposite side of said tablet from that on which the electrically conduc- tive resistive layer is present.

16. The digitizing system of claim 15 further comprising display control means for presenting on said display screen an image of a path traced by the point of said stylus on the resistive layer of said tablet, the image appearing on said display screen following and in registration with the path traced by the point of the stylus on the resistive layer.

17. The digitizing system of claim 17 wherein said display screen is also transparent; and said digitizing system further comprises a case that surrounds said display screen and all edges of said tablet, said case having a solid back wall which encloses a surface of said display screen furthest from said tablet and also having a front wall immediately adjacent to the resistive layer of said tablet, the front wall being pierced by a first aperture having edges that surround a central region of the resistive layer of the tablet, the solid back wall of said case having formed therein a removable panel whose disengagement from the back wall provides a second aperture that permits trans is- sion of light through the second aperture, through said display screen, through said tablet, and through the first aperture.