Tracking Light Pen System

Abstract

Method and apparatus for tracking the movements of a light pen on a plasma display panel with a cursor for entering new information into the plasma panel and erasing other information therefrom. The invention comprises interrogating the cursor to determine the direction of motion, if any, of the light pen relative to the cursor, re-positioning the cursor under the light pen, and storing information over-written in the plasma panel by the cursor until the cursor has moved on, and then rewriting the stored information in its original location.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to a light pen system for use with plasma display panels, and in particular, to an improved method and apparatus for writing graphic information directly into a plasma display panel and editing other information therefrom.

Light pen arrangements have long been a popular means for manually inputing and editing graphic information directly on the cathode-ray tubes (CRTs) incorporated in some computer terminals. The CRT is well suited to the use of a light pen because the display is generally scanned in a serial manner, i.e., the positional light emitted by the CRT display occurs in a prescribed time sequence. Since the light emitting elements of a CRT have no memory other than the relatively short-lived persistence of the phosphor, the CRT display must be continually refreshed from a display buffer, typically once every 1/30th of a second. Thus, the provision of a cursor (i.e., a light point on the CRT indicating the position of the light pen) to track the light pen is relatively simple. This is generally accomplished by storing the cursor data in a buffer added onto the end of the display buffer.

With the development of computer terminals utilizing plasma display memory panels rather than CRTs wherein the plasma panel functions both as the display and as the display buffer or image file, however, new light pen arrangements have had to be developed.

As is well known in the art, the plasma panel is a two dimensional array of light emitting gas discharge elements that exhibit inherent memory. This type of plasma panel, as well as the operation of selectively addressing the plasma display panel to write information, and the sustaining operation, has been previously described in U.S. Pat. No. 3,559,190, issued Jan. 26, 1971, on an application of D. L. Bitzer, H. G. Slottow and R. H. Willson, entitled "Gaseous Display and Memory Apparatus", U.S. Ser. No. 613,693, filed Dec. 22, 1966, assigned to the same assignee has here.

One such light pen arrangement for plasma display panels is described in U.S. Pat. No. 3,651,509, issued Mar. 21, 1972, to D. Ngo. This prior art may be briefly summarized as follows.

The arrangement utilizes a light pen having a light tranducer which operates in conjunction with display scanning signals for selectively writing or erasing information in the display cells. The display scanning signals energize the display cells sequentially, each cell generating a momentary light pulse when energized. The momentary light pulses are of sufficient duration to be detected by the light pen transducer. The detected light pulses are employed to produce conventional write or erase signals, selectively, as the light pen is moved adjacent the display cells. Accordingly, information is written in a particular location on the display panel, i.e., the display cell is lit by placing the light pen adjacent the cell. During the next scanning cycle, the particular cell is momentarily energized by scanning signals applied thereto. The resulting light pulse is detected by the light pen, generating a write signal to turn the display cell ON. Thus, as an operator manipulates the light pen, each cell of the display that the light pen passes is lit to draw the desired image.

This arrangement, however, requires that complex new signals be introduced between the cycles of the sustaining signal. This cannot be done with presently commercially available plasma display panel and drive systems without completely changing the drive circuitry.

Moreover, this arrangement requires that the entire panel be scanned in moving the cursor a distance of one line, each scan requiring approximately five seconds. Accordingly, it would require five minutes to move one inch on a sixty line per inch panel.

SUMMARY OF THE INVENTION

In accordance with the present invention, method and apparatus are provided for tracking the movements of a light pen on a plasma display panel having an array of gaseous discharge cells with a cursor. In a specific embodiment of the invention apparatus including means for interrogating the cursor to determine the direction of motion, if any, of the light pen relative to the cursor, means for re-positioning the cursor under the light pen, means for storing information adjacent the leading edge of the cursor prior to re-positioning of the cursor, and means for writing the information previously displayed at the location on the plasma display panel vacated by the cursor adjacent the trailing edge of the cursor is provided. More particularly, the cursor comprises an arrangement of gas discharge cells in the ON state in the plasma display panel and the interrogation comprises sequentially discharging the cursor cells to generate light emissions, the combination of sequential light emissions monitored by the light pen determining the direction of motion if any of the light pen relative to the cursor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference now to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the several figures and in which:

FIG. 1 is a block diagram of an input/output computer terminal having a plasma display/memory panel for displaying graphic information and incorporating a light pen system in accordance with the present invention;

FIG. 1A is a partial view of the plasma display panel in FIG. 1 illustrating one particular cursor arrangement which can be utilized by the light pen system of the present invention;

FIG. 2 illustrates various wave forms associated with interrogation of the plasma panel by the light pen system of the present invention;

FIGS. 3, 4, 5, 6, 7, 8 and 9 illustrate the operations performed in tracking the light pen with a cursor for a particular example, to wit: a downward movement of light pen; and

FIG. 10 is a combined schematic and block diagram illustrating in greater detail the input/output computer terminal incorporating the light pen system shown in FIG. 1.

DETAILED DESCRIPTION

As previously indicated above, the complete description of a plasma panel is set forth in U.S. Pat. No. 3,559,190, and reference may be made thereto. However, for purposes of describing the present invention, a brief summary of some basic principles is herein provided.

Physically, the plasma display/memory panel is an array, generally rectangular, of gas discharge cells that are separated from associated row (x) and column (y) electrodes by dielectric sheets. The intersection of any two electrodes can be selectively addressed by suitable addressing means coupled to the electrodes.

Initially, the selective application of suitable drive signals to those cells discharges the gaseous medium within the respective cells so that a cell wall charge is formed therein. The formed wall charge, however, opposes the drive signal thereby rapidly extinguishing the discharge.

Thereafter, except when the information is changed, every cell of the array is periodically excited, i.e., discharged, by a sustaining signal such as that illustrated in FIG. 2. In the most commonly used mode of excitation, the sustaining voltage waveform, by itself, is not of sufficient magnitude to initiate a discharge in any of the elements. If, however, previous electrical activity in a cell has left the cell dielectric end walls charged to an adequate level, the resultant wall voltage can augment the applied sustaining voltage and cause a discharge in the cell. During this discharge, electrons and ions flow to the walls in response to the applied field, and light is emitted. These charges depress the internal field and thereby extinguish the discharge. In the next half cycle, the charge collected on the end walls will again augment the applied voltage and cause a discharge to take place in the opposite direction. Thus, the alternating or sustaining voltage, which by itself is not able to initiate a discharge, will sustain a stable sequence of discharges, repetitively, in an element if appropriate wall voltage conditions are established by each previous discharge.

The state of a cell which sustains a stable sequence of discharges, i.e., one discharge each one-half cycle of the applied voltage, is also characterized by light output and is usually defined as the "one" or ON state. The other state, which exhibits no discharge activity, and has no light output, is called the "zero" or OFF state. State changes in a plasma display element are accomplished by establishing the wall charge condition of the desired state. Although there are now well known techniques for accomplishing state changes, the primary technique utilizes coincidentally applied voltage perturbations (control pulses) as described for instance in a published article of R. L. Johnson, D. L. Bitzer and H. G. Slottow, "The Device Characteristics of the Plasma Display Element", I.E.E.E. Transactions On Electron Devices, Vol. ED-18, No. 9, pp. 642-649, September, 1971.

Referring now to FIG. 1, there is shown in block diagram form an input/output computer terminal having a plasma display/memory panel for displaying graphic information and incorporating a light pen system in accordance with the present invention. The light pen system may be utilized to manually input, i.e., "write", graphic information into the plasma panel for visual display and, conversely, to edit or erase information from the panel.

It is to be understood that while the present invention is described in connection with the plasma display panel, the invention is useful with any addressable matrix of light emitting elements exhibiting inherent memory.

In the present embodiment, the input/output computer terminal 11 is coupled to a remote computer 13 over voice grade telephone circuits 15. The terminal 11 includes a terminal processor 17 which, responsive to instructions and data received from the computer 13, drives the plasma display panel 19 to graphically display information thereon. In particular, the terminal processor 17 also includes a control signal generator developing the drive signals for setting up wall charges in the appropriate cells, i.e., writing information into the plasma planel 19, and a sustaining signal generator which supplies the required alternating sustaining signals to the panel 19 for repetitively discharging the ON cells in a sustaining sequence.

In FIG. 1, the light pen system illustrated there includes a light pen 21 which the terminal user can move across the surface of the plasma display panel 19 to input new information into the panel or erase other information therefrom. In the embodiment shown in FIG. 1, the light pen 21 comprises a bundle of optical fibers, typically eight cells in diameter, for coupling light emitted from the portion of panel 19 covered by the end of pen 21 to the remainder of the light pen system circuitry.

As the light pen 21 is moved across the panel surface, a visible cursor such as the cross-shaped (+) arrangement of lit gas discharge cells illustrated in FIG. 1A tracks the pen 21. The tracking of light pen 21 by the cursor is implemented by sequentially interrogating the four cells A, B, C, and D at the vertices of the cursor to determine the direction of motion, if any, of the pen 21 across the plasma panel 19.

More particularly, the interrogation technique practiced by the light pen system of the present invention utilizes the existing sustaining and address circuitry of the terminal processor 17 to provide the requisite interrogation signals in a manner well known in the art. That is, an interrogation signal, i.e., an erase pulse, sufficient to discharge a cell in the ON state but not sufficient to discharge a cell in the OFF state is applied to the selected vertice cell during the period between the normally supplied sustaining signals. Accordingly, the vertice cell, which is in the ON state, will be discharged by the interrogation signal and emit light while none of the other cells in the panel 19 are discharged. Subsequently, a restoration pulse, i.e., a write pulse, can be applied to the interrogated cell during the next interval between sustaining pulses to "rewrite"a wall charge into the cell. A typical sustain voltage waveform, the light output for both states of an element and the control (write and erase) pulses which are superimposed on the sustaining waveform are illustrated in FIG. 2. In turn, cells B, C, and D are interrogated in a similar manner.

The method of interrogation to determine the direction the cursor must move to track the light pen as it moves on the plasma display panel may be more readily understood by reference now to FIG. 3 which illustrates a portion of the plasma display panel 19 with the letters g and E written thereupon. The cursor is shown near the center of the display.

Operationally, the four cells A, B, C, and D at the vertices of the cursor are sequentially interrogated. By sequentially interrogating the vertice cells, an area photodetector 23 coupled to the light pen 21 is able to distinguish between the interrogations of the four cells. Accordingly, during the interrogation, cell A is initially erased. If the light emission resulting from this erasure is captured by the light pen 21, a binary 1 is shifted into the most significant bit position of a five-bit S register 25 (FIGS. 1 and 3) coupled to the output of the photodetector 23, and cell A is then rewritten on the panel. If, on the other hand, the light emission is not monitored by the pen 21, a binary O is shifted into the S register 25. Cell B, C and D are then similarly interrogated until all four cells have been sequentially processed. After the interrogation of each vertice cell, the corresponding 1 or 0 is registered in the most significant bit (MSB) position of the S Register 25 and the previous interrogation results are shifted toward the least significant bit (LSB) position. Consequently, after the interrogation has been completed, the bit of information corresponding to the interrogation of cell D is located in the MSB position and the bit corresponding to cell A is located in the second to the least most significant bit position. Since only four interrogations are undertaken, the LSB bit position is always 0.

Accordingly, if for example the cursor is aligned with the light pen, identified generally by circle 21 in FIG. 3, the light emissions during each of the four interrogations will be coupled through the optical fibers of the light pen 21 to the photodetector 23. That is, if the light pen 21 is stationary so that it can observe all four interrogations, the S register 25 will be loaded with the binary word 11110.

Whenever one or more of the light emissions from the four interrogations is not observed, however, the light pen system reacts to reposition the cursor beneath the pen 21. If, for example, the light pen 21 is moved down one row on the plasma panel 19, as illustrated in FIG. 4, cell A is no longer in the area monitored by pen 21, and accordingly, interrogation of cell A is not observed by the pen 21. Consequently, the five bit S register 25 is set to 11100 which is interpreted as a downward movement of the light pen 21.

Accordingly, it should be understood that the various combinations of vertice cells which are "observed" by the light pen 21 during the sequential interrogation of the vertice cell is indicative of the direction of motion of the light pen 21.

Since the plasma display panel 19 functions both as the display and the display buffer or image file, the information originally displayed on the panel at a particular location is lost when it is "over-written" by the cursor unless it is temporarily stored elsewhere. Consequently, the original information contained in the five by five array of cells (identified generally by dashed line box 27 in FIG. 4) subsequently overwritten by the cursor, as it moves on the panel 19, is stored in the 5 .times. 5 register 29 illustrated in FIGS. 1 and 4.

As long as the interrogation of the cursor's vertices indicates that the light pen 21 is stationary, the contents of the 5 .times. 5 register 29 remain unchanged. But when cursor must be repositioned, as is the case with the example shown in FIG. 4, the contents of the 5 .times. 5 register 29 must be changed to reflect the information which will be overwritten as the cursor moves downward.

This is accomplished by interrogating the five cells in the row adjacent to the leading edge of the 5- by -5 array 27 incorporating the cursor. First, the S register 25 is cleared. Then, responsive to the controller 31, the cells, identified as 1 through 5 in FIG. 4, are interrogated by sequentially erasing each of the five cells, starting with cell 1 and continuing through cell 5 (FIG. 4), and shifting a binary 1 into the S register 25 if the interrogated cell is ON or conversely, a 0 if the cell is OFF. FIG. 5 illustrates the results of the leading edge interrogation in the example under consideration. There, it may be seen that the S register 25 is loaded with the binary word 10000 after this operation is complete.

This information, in turn, is shifted into the bottom row of the 5 .times. 5 register 29 by means of a parallel transfer of the data from the S register 25. Each row of data previously stored in the 5 .times. 5 register 29 is likewise shifted up one row. Thus, after the transfer, the 5 .times. 5 register 29 contains the image data corresponding to the 5 by 5 cell area presently monitored by the light pen 21 after it has moved down one row.

When the contents of the S register 25 is shifted into the bottom row of the 5 .times. 5 register 29 and the other rows already stored therein are shifted up, the top row is transferred out of the 5 .times. 5 register 29 and loaded back into the S register 25. FIG. 6 illustrates the contents of the 5 .times. 5 register 29 and the S register 25 after the leading edge data has been shifted into the 5 .times. 5 register 29 and the trailing edge data has been transferred to the S register 25.

Next, the contents of the S register 25 are coupled to the light pen display processor 33 and written into the appropriate cells, identified as 1 through 5 in FIG. 7, through an adder circuit 35, at the trailing edge of the cursor so that the original information is displayed on the panel 17 after the cursor has passed. Operationally, this is accomplished by writing the most significant bit stored in the S register 25 into cell 5 (FIG. 7) at the trailing edge of the cursor. The contents of the S register 25 are then shifted one position to the left, and again the most significant bit is written, this time in cell 4. This procedure continues until the information is completely re-written as shown in FIG. 8, erasing cell A.

The cursor is then moved down one row by erasing the horizontal bar and rewriting it in the next row down, and subsequently, a cell is written at the bottom of the vertical bar comprising the cursor. Accordingly, the cursor is re-written, i.e., moved, so that it once again lies beneath the light pen 21, thus appearing to follow the pen 21. FIG. 9 illustrates the status of the plasma panel display upon the completion of the one downward movement.

While in the present example a simple one line downward movement has been illustrated, any movement of the light pen can be interpreted as a combination of moves in either the X or the Y direction. Thus, the procedure which was followed for single move in the downward direction is functionally identical to that employed for a move in any of the other three directions, or in a diagonal direction.

The gas discharge cells in the plasma panel 19 corresponding to the path of cell X as the cursor travels over the panel 19 may be maintained in a lit state. This leaves a track corresponding to the movements of the light pen 21 on the plasma panel 19, and accordingly, the user can write information into the panel 19. Alternatively, to edit information from the panel 19, the information temporarily stored in the 5 .times. 5 register 29 is not re-written into the panel 19 after the cursor has been relocated.

The novel apparatus comprising a light pen system in accordance with the present invention is shown in greater detail in FIG. 10.

More particularly, signals for controlling the operation of the various registers, including incrementing and decrementing, loading, and shifting, are generated by the controller 51 upon receipt of an instruction from the sequencing and processing unit, identified generally at 53. The sequencing and processing unit 53 includes a 256 .times. 8-bit re-programmable read-only memory (ROM) 55 which stores the instructions for directing the processing activities of the controller 51.

The controller 51, in turn, decodes the upper four bits of the 8-bit instruction from the sequencing and processing unit 53 into 16 possible instructions while the lower four bits modify these instructions. For example, when the upper four bits decode into an instruction to increment a particular register, the lower bits may indicate the size of the increment, or when an instruction indicates that the 5 .times. 5 register 29 is to be shifted, then the lower four bits indicate in which of the four possible directions the stored data is to be shifted. Once these instructions are decoded, the appropriate circuits respond automatically to the control signals generated by the controller 51.

The major function of the sequencing and processing unit 53 then is to direct the various instructions stored in the ROM 55 to the controller 51 in the correct sequential manner. Accordingly, all processing operations are synchronous with a master clock. In particular, the system operates on a major cycle consisting of four minor clock pulses. That is, the "decode" clock pulse which is applied to the decoder 57 decodes the instruction presently at the output of ROM 55 so that it can subsequently be coupled to the controller 51. A "set-up" pulse is then applied to the controller 51 which, in turn, generates a control signal setting up the inputs to the various registers. A subsequent "load" pulse coupled to the controller 51 causes it to load or shift the various registers. Finally, an "increment" pulse increments the 8-bit loadable counter comprising the ROM address register 59 which always contains the address of the instruction to be performed.

Normally, the ROM address register 59 is incremented once each major cycle in order to move sequentially through the program. However, there are a number of jump instructions in the ROM 55 which, when decoded, change the contents of the ROM address register 59 to a new memory location. For example, one of these jump instructions loads the ROM address register 59 with the contents of the S register 25. This instruction is executed immediately after interrogating the four vertices of the cursor for the determination of the direction of movement. Since at this point, the S register 25 holds a 4-bit code indicating the particular direction of light pen motion, this code can be loaded into the lower bits of the ROM address register 59 to cause a branch to the particular area of the ROM 55 containing the set of instructions required for this move.

A center point (C) register 61 comprising two 9-bit registers, C.sub.x and C.sub.y, containing the x and y coordinates of the cursor center point (i.e., cell X) address, respectively, is controlled by the controller 51. The C.sub.x and C.sub.y registers can be independently incremented or decremented responsive to the controller 51 to reflect the movements of the cursor center point. For example, when the light pen 21 is moved down one line, the C.sub.y register is decremented by one count while the C.sub.x register is left unchanged at the completion of the cycle.

As the cursor's movements across the plasma panel 19 are registered in the C register 61, the cells in the plasma panel corresponding to the coordinates in the C.sub.x and C.sub.y sub-registers may be lit so that the cursor's path across the panel is recorded. In this manner, information can be written into the plasma panel 19 by the terminal user.

The operating point (0) register 63 also comprises a pair of 9-bit registers, O.sub.x and O.sub.y, and is controlled by controller 51. The O.sub.x and O.sub.y registers contain the address in the plasma display panel 19 at which the WRITE or ERASE operations are to be performed. The O registers 63 are loaded from the corresponding C registers 61 with the center point coordinates and can be incremented or decremented to control, via the ADDRESS means 65, the location on the panel 19, relative to the center point of the cursor, where the WRITE or ERASE operation is performed.

Operationally, the interrogation data from the photodetector 23 is coupled to the input of the S register 25, a 5-bit register capable of serially shifting data therein to input and output the data in a serial manner. Moreover, the S register 25 can simultaneously input or output five bits of data in parallel, e.g., when transferring the "leading edge" data to the 5 .times. 5 register 29.

During the interrogation of the cursor's vertices, i.e., cells A, B, C, and D, the S register 25 is operative in the serial mode to receive the results of the sequential interrogations. The resultant binary word, indicative of the direction in which the light has been moved, then coupled to the sequencing and processing unit 53, i.e., ROM address register 59.

If motion is detected, the operating point (0) register 63, responsive to the controller 51, is counted to the row of cells at the leading edge of the cursor (e.g., cell 1 in FIG. 4). The five cells adjacent the leading edge are then sequentially interrogated and the resultant data is serially loaded into the S register 25. This data is subsequently outputed from the S register 25 and transferred in parallel into the corresponding row in the 5 .times. 5 register 29.

The 5 .times. 5 register 29 comprises an array of twenty-five storage devices arranged in five rows of five devices each for storing the data originally displayed in the area occupied by the cursor.

Upon receipt of a shift comman, multiplexors associated with each device in the 5 .times. 5 register 29 and each of the five storage devices comprising the S register 25 are toggled to input the data from the S register 25 in the 5 .times. 5 register 29 and the contents of the top row of 5 .times. 5 register 29 are input into the corresponding devices in the S register 25 (FIG. 5).

Thus, as a result of the shift command, the leading edge data contained in the S register 25 is shifted into the appropriate row in the 5 .times. 5 register 29 (e.g., if the light pen is moved horizontally to the right, the leading edge data is shifted into the right side column of the 5 .times. 5 register 29), the contents of the 5 .times. 5 register 29 is shifted by one row, and the trailing edge data is shifted into the S register 25. Accordingly, the S register 25 now contains the trailing edge data and the 5 .times. 5 register 29 has stored the information which will be overwritten by the cursor when it moves to follow the light pen 21.

The data in the S register 25 is subsequently coupled to control means 67 and is written into the row of cells at the trailing edge of the cursor as it is relocated. In particular, the operating point (0) register 63 is loaded with the address of cell 5 in the plasma panel 19. Thus, the address in the plasma panel 19 at which data is written is contained in the O register 63. Initially, the bit located in the most significant bit (NSB) position is re-written into cell 5. The O register 63 is then incremented (or decremented) and the data in the S register 25 is shifted toward the NSB position output so that it can be re-written into cell 4. The process is repeated until all of the data in the S register 25 is re-written into its original location on the plasma panel 19.

Finally, the cursor B relocated below the light pen 21, and the center point (C) register 61 is incremented (or decremented) to coincide with the new address of the cursor's center point.

While the writing operation of the light pen system has just been described, it should be apparent that information can also be edited from the panel by not re-writing the information temporarily stored in the 5 .times. 5 register 29 into the plasma panel 19.

Accordingly, a tracking light pen system for use with an input/output graphics terminal has been shown. The tracking light pen system just described exhibits tracking speeds in excess of 16 inches per seconds. The light pen system allow the user to move a visible cursor around the plasma display panel for purposes of editing and creating graphical and textual material.

In many graphic display terminal applications, a light pen is often used to provide the user with a means of manually selecting one item from many which are displayed on the screen. The extension of the tracking light pen system of the present invention to include a pointing light pen capability can be accomplished by using a modified control algorithm and adding a data buffer large enough to store the display addresses that correspond to the user's choices.

To provide the pointing capability, a pointing algorithm is added to the tracking algorithm.

First, the C register 61 and the O register 63 are loaded with the first address in the pointing address buffer. The cell at this address is written and erased and the photodetector output is sampled during the erasure time to determine whether or not the light pen is positioned over the addressed cell. If light is detected during the erasure, the address contained in the C register 61 is returned to the computer thereby indicating that the light pen is over the cell location. If, however, no light is detected during the erasure, the C and O registers 61 and 63, respectively, are loaded with the next address contained in the address buffer and the previous operation is repeated. This operation is cyclically continued through the address buffer.

In order to provide for a large number of positional choices in a pointing light pen system, a hardware address buffer is added to the tracking system. The buffer consists of a dual 512-bit shift register which will hold up to 50 display panel addresses and is loaded from the existing terminal interface. This hardware unit has the ability to keep track of the shift register address both during loading and during the process of being read into the C register 61.

While a particular embodiment of the present invention has been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention in its broader aspects. Accordingly, the aim in the appended claims is to cover all such changes and modifications as may fall within the true spirit and scope of the invention.

Claims

What is claimed is:

1. A method for tracking the movements of a light pen on a plasma display panel with a cursor, the plasma panel comprising an array of gaseous discharge cells for storing and visually displaying the information, the cursor comprising a sub-array of the gaseous discharge cells in the plasma display panel including an arrangement of the gaseous discharge cells which are aligned with the orthogonal axes of the cursor sub-array and located on opposite sides of the cursor, the method comprising:

sequentially interrogating the cursor cells;

monitoring the plasma display panel with the light pen for the sequential cursor cell interrogations to determine the direction in which the light pen is displaced relative to the cursor, the combination of sequential cursor cell interrogations monitored by the light pen being indicative of the direction in which the light pen is displaced relative to the cursor; and

electronically repositioning the cursor in the plasma display panel to be under the light pen and coincident therewith responsive to the determination of the direction of the displacement of the light pen from the cursor.

2. A method as claimed in claim 1 including selectively lighting at least one of the gaseous discharge cells in the plasma display panel coincident with the cursor after each time the cursor is electronically repositioned to track the light pen and thereby plot successive movements of the light pen to selectively enter information into the plasma display panel for storage and display.

3. A method as claimed in claim 1 including storing the information contained in the gaseous discharge cells located immediately adjacent the cursor and in the direction in which the light pen is displaced relative to the cursor before electronically repositioning the cursor, the cursor overwriting the information previously contained in the adjacent gaseous discharge cells to edit the information from the plasma display panel when the cursor is electronically repositioned under the light pen, and selectively writing the stored information into the gaseous discharge cells comprising the location in which the stored information was previously contained prior to being overwritten by the cursor after the cursor has moved from the repositioned location to thereby selectively re-enter the edited information into the plasma display panel.

4. A method as claimed in claim 3 including selectively lighting at least one of the gaseous discharge cells in the plasma display panel coincident with the cursor after each time the cursor is electronically repositioned to track the light pen and thereby plot successive movements of the light pen to selectively enter information into the plasma display panel for storage and display.

5. A method for tracking the movements of a light pen on a plasma display panel with an electronically-generated cursor, the plasma display panel comprising an array of gaseous discharge cells being selectively switchable between an OFF state and an ON state to store information and visually display the information via light emissions from the gaseous discharge cells in the ON state when the gaseous discharge cells are periodically discharged, the cursor comprising a sub-array of the gaseous discharge cells in the plasma display panel including an arrangement of the gaseous discharge cells in the ON state which are aligned with the orthogonal axes of the cursor sub-array and located on opposite sides of the cursor, the method comprising:

sequentially discharging the ON cursor cells to generate corresponding sequential light emissions therefrom;

monitoring the plasma display panel with the light pen for the sequential light emissions from the ON cursor cells to determine the direction in which the light pen is displaced relative to the cursor, the combination of sequential light emissions monitored by the light pen being indicative of the direction in which the light pen is displaced relative to the cursor; and

electronically repositioning the cursor sub-array in the plasma display panel to be under the light pen and coincident therewith responsive to the determination of the direction of displacement of the light pen from the cursor.

6. A method as claimed in claim 5 including selectively switching at least one of the gaseous discharge cells in the portion of the plasma display panel coincident with the cursor sub-array to the ON state to emit light when the gaseous discharge cells are periodically discharged, at least one of the cells being switched to the ON state after each time the cursor is electronically repositioned to track the light pen and thereby plot successive movements of the light pen to selectively enter information into the plasma display panel for storage and display.

7. A method as claimed in claim 5 including storing the information corresponding to the state of the gaseous discharge cells located immediately adjacent the cursor sub-array and in the direction in which the light pen is displaced relative to the cursor before electronically repositioning the cursor, the the cursor sub-array overwriting the information previously contained in the adjacent gaseous discharge cells by altering the state of the gaseous discharge cells to conform to the state of the gaseous discharge cells comprising the portion of the cursor sub-array overwriting the adjacent gaseous discharge cells and thereby editing the information from the plasma display panel when the cursor is electronically repositioned under the light pen, and selectively writing the stored information into the gaseous discharge cells comprising the location in which the stored information was previously contained prior to being overwritten by the cursor sub-array after the cursor Sub-array has moved from the repositioned location by altering the state of the adjacent gaseous discharge cells in accordance with the stored information to correspond to the state of the adjacent cells prior to the adjacent cells having been overwritten by the cursor to thereby selectively re-enter the edited information into the plasma display panel.

8. A method as claimed in claim 7 including selectively switching at least one of the gaseous discharge cells in the portion of the plasma display panel coincident with the cursor sub-array to the ON state to emit light when the gaseous discharge cells are periodically discharged, at least one of the cells being switched to the ON state after each time the cursor is electronically repositioned to track the light pen and thereby plot successive movements of the light pen to selectively enter information into the plasma display panel for storage and display.

9. Apparatus for tracking the movements of a light pen on a plasma display panel with a cursor, the plasma panel comprising an array of gaseous discharge cells for storing and visually displaying the information, the cursor comprising a sub-array of the gaseous discharge cells in the plasma display panel including an arrangement of the gaseous discharge cells which are aligned with the orthogonal axes of the cursor sub-array and located on opposite sides of the cursor, the apparatus comprising:

means for sequentially interrogating the cursor cells;

means including a light pen for monitoring the plasma display panel for the sequential cursor cell interrogations to determine the direction in which the light pen is displaced relative to the cursor, the combination of sequential cursor cell interrogations monitored by the light pen being indicative of the direction in which the light pen is displaced relative to the cursor; and

means for electronically repositioning the cursor in the plasma display panel to be under the light pen and coincident therewith responsive to the determination of the direction of the displacement of the light pen from the cursor.

10. Apparatus in accordance with claim 9 including means for selectively lighting at least one of the gaseous discharge cells in the plasma display panel coincident with the cursor after each time the cursor electronically repositioned to track the light pen and thereby plot successive movements of the light pen to selectively enter information into the plasma display panel for storage and display.

11. Apparatus in accordance with claim 9 including means for storing the information contained in the gaseous discharge cells located immediately adjacent the cursor and in the direction in which the light pen is displaced relative to the cursor before electronically repositioning the cursor, the cursor overwriting the information previously contained in the adjacent gaseous discharge cells to edit the information from the plasma display panel when the cursor is electronically repositioned under the light pen, and means for selectively writing the stored information into the gaseous discharge cells comprising the location in which the stored information was previously contained prior to being overwritten by the cursor after the cursor has moved from the repositioned location to thereby selectively re-enter the edited information into the plasma display panel.

12. Apparatus for tracking the movements of a light pen on a plasma display panel with a cursor as claimed in claim 11 including means for selectively lighting at least one of the gaseous discharge cells in the plasma display panel coincident with the cursor after each time the cursor is electronically repositioned to track the light pen and thereby plot successive movements of the light pen to selectively enter information into the plasma display panel for storage and display.

13. Apparatus for tracking the movements of a light pen on a plasma display panel with an electronically-generated cursor, the plasma display panel comprising an array of gaseous discharge cells being selectively switchable between an OFF state and an ON state to store information and visually display the information via light emissions from the gaseous discharge cells in the ON state when the gaseous discharge cells are periodically discharged, the cursor comprising a sub-array of the gaseous discharge cells in the plasma display panel including an arrangement of the gaseous discharge cells in the ON state which are aligned with the orthogonal axes of the cursor sub-array and located on opposite sides of the cursor, the apparatus comprising:

means for sequentially discharging the ON cursor cells to generate corresponding sequential light emissions therefrom;

means including a light pen for monitoring the plasma display panel for the sequential light emissions from the ON cursor cells to determine the direction in which the light pen is displaced relative to the cursor, the combination of sequential light emissions monitored by the light pen being indicative of the direction in which the light pen is displaced relative to the cursor; and

means for electronically repositioning the cursor sub-array in the plasma display panel to be under the light pen and coincident therewith reponsive to the determination of the direction of displacement of the light pen from the cursor.

14. Apparatus in accordance with claim 13 including means for selectively switching at least one of the gaseous discharge cells in the portion of the plasma display panel coincident with the cursor sub-array to the ON state to emit light when the gaseous discharge cells are periodically discharged, at least one of the cells being switched to the ON state after each time the cursor is electronically repositioned to track the light pen and thereby plot successive movements of the light pen to selectively enter information into the plasma display panel for storage and display.

15. Apparatus in accordance with claim 13 including means for storing the information corresponding to the state of the gaseous discharge cells located immediately adjacent the cursor sub-array and in the direction in which the light pen is displaced relative to the cursor before electronically repositioning the cursor, the cursor sub-array overwriting the information previously contained in the adjacent gaseous discharge cells by altering the state of the gaseous discharge cells to conform to the state of the gaseous discharge cells comprising the portion of the cursor sub-array overwriting the adjacent gaseous discharge cells and thereby editing the information from the plasma display panel when the cursor is electronically repositioned under the light pen, and means for selectively writing the stored information into the gaseous discharge cells comprising the location in which the stored information was previously contained prior to being overwritten by the cursor sub-array after the scursor sub-array has moved from the repositioned location by altering the state of the adjacent gaseous discharge cells in accordance with the stored information to correspond to the state of the adjacent cells prior to the adjacent cells having been overwritten by the cursor to thereby selectively re-enter the edited information into the plasma display panel.

16. Apparatus in accordance with claim 15 inclusing means for selectively switching at least one of the gaseous discharge cells in the portion of the plasma display panel coincident with the cursor sub-array to the ON state to emit light when the gaseous discharge cells are periodically discharged, at least one of the cells being switched to the ON state after each time the cursor is electronically repositioned to track the light pen and thereby plot successive movements of the light pen to selectively enter information into the plasma display panel for storage and display.