US 3906451 A
Apparatus and method for erasing a selected gas discharge cell located between one of a plurality of X electrodes and one of a plurality of Y electrodes. Erasure is performed with non-coincident pulses and is particularly well suited for use with a sustaining pulse generator of the type which alternately applies sustaining pulses of a preselected amplitude and polarity to all the X electrodes and then to all the Y electrodes, since the sustaining pulse may be used as the conditioning pulse and with a slight modification as the erase pulse on the Y electrode.
Description (OCR text may contain errors)
United States Patent Strom Sept. 16, 1975  PLASMA PANEL ERASE APPARATUS 3,824,580 7/1974 Bringol 1, 340/324 M 3,840,779 l0 I974 Sch ho SIS 169 TV  Inventor: Richard Albert Strom, Richfield, firmer m Inn Primary Examiner-Donald J. Yusko  Assignee: Control Data Corporation, Attorney, Agent, or Firm-Edward L. Schwarz Minneapolis, Minn.
 Filed: Apr. 15, 1974  ABSTRACT ] Appl. No.1 461,044 Apparatus and method for erasing a selected gas discharge cell located between one of a plurality of X electrodes and one of a plurality of Y electrodes. Erag? a; sure is performed with non-coincident pulses and is l f 166 M particularly well suited for use with a sustaining pulse [5 l o arc I 315 generator of the type which alternately applies sustain ing pulses of a preselected amplitude and polarity to 56 R f ed all the X electrodes and then to all the Y electrodes, 1 e erences It since the sustaining pulse may be used as the condi- UNITED STATES PATENTS tioning pulse and with a slight modification as the 3,754.230 8/I973 Auger 340/324 M erase pulse on the Y electrode. 3,771,016 11/1973 Toba etal. 315/169 TV 3,81 1.124 5/1974 Kleen 315 169 TV X 13 Claims, 6 Drawing Flgures on '1 tug glue [29 A ems: was
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lso 1/8 l-ERASE seucr J SIGNAL aha-1116* susvnumo '71; 901.5: 11mm 127 \/26 /l4 PLASMA PANEL ERASE APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to the so-called plasma or gas discharge display panel, and in particular the erasing of those individual spots in the type of displays which have an inherent memory capability. In such inherent memory displays writing is the initiation of light emission in a gas volume located between two selected electrodes where light emission has not recently occurred. Writing is performed by imposing a voltage differential between the two electrodes having a peak greater than a threshold value, which threshold value is dependent on the characteristics of the display involved. Once emission occurs in a particular gas volume or cell, sustaining pulses having voltage peaks less than the writing threshold can be applied to one or the other, or both electrodes, which cause a previously written cell to emit light upon each application of sustaining pulses to the electrodes, and do not cause light emission from any cells which have not been specifically written. A more detailed description of writing and sustain techniques, as well as the physical characteristics of such displays, are contained in US. Pat. No. 3,573,542 (Mayer, et al.,) having an assignee common with this application, in US. Pat. No. 3,786,474 (Miller); and in US. Pat. No. 3,671,938 (Ngo). Reference to these patents will permit easier understanding of the instant invention. For general background in the gas discharge display art, Materials of High Vacuum Technology, Warner Espe, pub. Pergamon Press I968 is a valuable reference.
It is also necessary to be able to selectively extinguish or erase light emission from individual cells, in order to permit efficient use of the display.
2. Description of the Prior Art The Ngo each Mayer patents disclose the usual means of erasing a particular cell. The X electrode (on one side of the panel) and the Y electrode (on the other side of the panel) for that particular cell are ach simultaneously energized with a special half-select erase pulse. The erase pulses may have identical shapes, but are of opposite polarity as applied to the selected X electrode and Y electrode. Each is chosen such that by itself it is incapable of erasing any cell. The voltage caused across the cell between the two is, how ever, sufi'icient to cause the selected cell to be erased.
BRIEF DESCRIPTION OF THE INVENTION This invention comprises apparatus for interrupting a normal series of sustaining pulses applied to all of the electrodes, both X and Y, with first of all a conditioning pulse (which may be a sustaining pulse) which reverses the polarity of the residual wall charge of the cells commonly adjacent the X electrodes which supplies firing current to the cell to be erased, with respect to all the other cells in the display. The conditioning pulse should be of sufficient magnitude so as to duplicate the effect on the cells receiving a conditioning pulse of a normal sustaining pulse in that direction, and it is in fact preferred that it be a sustaining pulse. Then an erase pulse is applied to the Y electrode which supplies firing current to the cell to be erased, which is sufficient to remove the residual wall charge at the site of the selected cell, but affects all other cells adjacent that Y electrode very little because the erase pulse has the polarity of the most recent sustaining pulse applied to them. After the erase pulse has been completed, normal sustaining pulses can then resume and all cells previously emitting light, except for the erased cell will continue to emit. The erase pulses employed in the technique can be chosen to have maximum amplitude approximately equal to or less than that of the voltage of sustaining pulses. The preferred erase pulse is distinguished by its slow rise time relative the sustaining pulses. In order to more reliably accomplish the desired erasing, it is preferred that a few sustaining pulse cycles be applied to the panel between each separate erase cycle.
A possible variation allows the removal of the residual wall charge by partially overlapping the conditioning pulse on the X electrode with the erase pulse on the Y electrode, both pulses having normal sustaining pulse rise times and differing from normal sustain operation by their overlapping. Still another variation permits application of the conditioning pulses to all except the electrode adjacent the cell to be erased and then appli cation of the usual erase pulse to the Y electrode.
Accordingly, one object of this invention is to increase reliability in erasing of plasma cells.
A second object of this invention is to decrease the number of different pulse shapes required in the normal operation of a gas discharge panel.
Another object is to provide an erasing technique compatible with certain display operating systems.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. In is a combination block and circuit diagram of a gas discharge display panel and its operating system employing this invention.
FIG. lb and 1c are simple erase circuit diagrams.
FIG. 2 displays waveforms associated with this invention.
FIG. 3 discloses waveforms associated with a varia tion on this invention.
FIG. 4 is a modification to FIG. I necessary in implementing the variation on the invention associated with FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. la discloses apparatus incorporating this invention in a workable gas discharge display panel system in which individual gas volumes or cells may be selec tively written and erased. The diagram has been some what simplified by eliminating the writing circuitry and by making the display panel itself a matrix much smaller than that which normally would be employed in a commercial system. However, the principles displayed are applicable to a panel having any number of ionizable cells. Panel can be conveniently considered to comprise two identical parallel sheets formed of glass or other suitable dielectric, spaced a short distance apart, and closed around their periphery. Between them a gas-tight chamber is formed which is filled with any of the various well-known gas mixtures which can be locally ionized by the application of a voltage gradient across any desired volume thereof, and thereby made to produce visible light. X electrodes IOIu-10ld are placed adjacent the gas chamber in panel 100 on the near or front side of panel 100 as iewed in FIG. 1a. Y electrodes I02a-I02d are similarly placed adjacent panel 100 on its back or far side respect to the viewer of FIG. la. Both Y electrodes 1020-1021] and X electrodes 101u101d are each shown parallel to others of the same designation, but this need not be. Y electrodes 102u-102r! are shown generally perpendicular to X electrodes l01ul0ld, although this also need not be so. Both X electrodes 101u-101d and Y electrodes 102a-l02d are shown as wires, but it may be convenient to form them of conductive transparent films placed on the exterior of the dielectric sheets, conductive strips embedded within the dielectric sheets, or in any of the other various embodiments of the prior art. It should be understood that each X and Y electrode 101u-101d is in close and intimate contact with the dielectric sheets so as to create relatively high capacitance between it and the adjacent gas volume within panel 100.
It is further assumed that application of a suitable voltage potential to a selected Y electrode 102:1-102d, say Y, electrode 102a, and a selected X electrode 101u-l01d, say X electrode 102e, will cause the gas volume or cell commonly adjacent both selected electrodes (designated by ref. num. 125 in FIG. 1a) to ionize and conduct current briefly, and in so doing, emit light. Current flow and light emission ceases when the inherent capacitance between the selected X electrode and selected Y electrode adjacent the cell becomes charged to a level sufficiently close to the difference between the voltages applied to the X electrode and Y electrode involved that insufficient voltage across the gas volume exists. To cause further conduction, the voltages on the X electrode and the Y electrode may be reversed before the residual charge in the inherent capacitance is discharged by leakage, allowing conduction in the opposite direction with a lower voltage than if no residual or wall charge is present. This is because the voltage caused by the wall charge in the inherent wall capacitance has polarity which tends to assist the firing of the cell by the voltage between the adjacent electrodes of polarity opposite to that of the most recent pulse. Therefore, each cell in panel 100 can be considered to be a single memory bit whose content is indicated by the decreased voltage difference needed between the X electrode and Y electrode adjacent that cell to cause light emission when wall charge caused by a recent cell firing is present. By the application of sustaining pulses of a certain polarity to all Y electrodes 10211-10211 alternately with similar pulses of the same polarity to X electrodes 10111-10111, the conductive/- nonconductive status of all cells may be maintained indefinitely. This is known as monopolar sustain operation. This memory characteristic, as well as means for writing individual cells are explained in greater detail in the previously mentioned Mayer and Ngo patents.
Sustaining is controlled in FIG. In by timing system 126 which supplies individual control pulses which cause other elements of the system to apply appropriate sustaining pulses on X electrodes 101u-l0ld and Y electrodes 10211-1020 to maintain cells of panel 100 in their current memory or light-emitting status. In the monopolar sustain method while a pulse is applied to all electrodes of a given type the other electrodes are grounded or otherwise connected to a reference voltage. Switches in FlGS. 10-14 are represented by blocks labeled SW, and may conveniently be of the type whose impedence between the current paths entering the block on opposite sides is essentially 0 whenever a suitable positive voltage is applied to the third. control path entering the block on a side between the other two (ill paths. Thus, switch 128 provides a very low impedence path between current paths 129 and 130 whenever suitable positive control voltage is present on control path 127. OR gates 110(1-1 10d provide the logical OR of the signal on path 112 and the respective paths 111 u-1 1111, on respective output paths 109a-109d. DC sustain voltage V, must have a magnitude selected for the particular physical dimensions and gas mixture present in panel 100. Selection of the sustain voltage magnitude can be done according to teachings of the prior art.
During normal sustain operation (when neither writing nor erasing occurs) timing system 126 supplies switch control signals which cause the production of waveforms 201 and 202 to the left of time T (HO. 2) on signal paths 112 and 127 respectively. It is assumed that the aforementioned wall charges exist adjacent written cells. When a sustain control pulse issues from sustain timing system 126, OR gates l10a-110d almost instantaneously produce an identical pulse on each control signal path 109a-109d, which closes switches 107ul07d, respectively. No pulse is applied to path 114 during this time, causing switch 113 to remain open. Voltage V, is applied through switches 107a-107d and diodes 124a-124d to X electrodes 1010-1011! as pulse 201a. A second pulse issued by timing system 126 on signal path 121 is coincident with pulse 201a and causes switch 117 to close and Y electrodes 102u-102d to be grounded or connected to an other reference voltage through diodes 103 (1-10311 respectively and switch 117 itself for the entire duration of pulse 201a. Pulse 201a has the appropriate rise time and duration to maintain or sustain the light-emitting status of the cells between the various X and Y electrodes. The voltage experienced by the cells of panel 100 due to this pulse, indicated as pulse 2011: there also. is shown by arbitrary selection in waveform 205 as positive, which means only that the positive direction of current flow is from X electrodes l0la-101d to Y electrodes 102u102d.
At the appropriate time after the trailing edge of pulse 201a, the signal closing switch 117 is removed from path 121 and a sustain control pulse is applied to signal path 127 which causes switch 128 to close and apply V, to Y electrodes 102a-102d as pulse 202a. At or before this time, timing system 126 issues a pulse on path 114 causing switch 113 to close for the duration of pulse 202a and connect X electrodes 1010-10111 to ground or other reference voltage. This causes a voltage to be applied between Y electrodes 102u-102d and X electrodes l01u-101d equal in magnitude but opposite in direction to that applied during pulse 201a. This causes cells between which have conducted during pulse 201a, to conduct again and emit light. Pulse 202a is shown in waveforms 202 and 204 as having positive polarity with respect to ground. In waveforms 205-208, pulse 20241 is shown as having negative polarity because its effect is to cause current to flow from Y electrodes 10211-10211 to X electrodes l01u-101d. The second X sustaining pulse 201]) is identical to pulse 201a. and occurs a preselected time after the trailing edge of Y sustaining pulse 20211. X and Y sustaining pulses continue to alternate in this fashion during normal sustaining operation. The interval between successive sustain pulses should be selected so as to produce the desired brilliance. as seen by the human eye. from each light emitting cell.
The X erase select signal on line 118 digitally specifies which of the X electrodes l01a-101d passes adjacent the cell to be erased. Similarly, the Y erase select signal on path 120 specifies the Y electrode which passes adjacent the cell to be erased. In this way any desired cell to be erased may be easily and accurately designated. For the remainder of the discussion, assume that cell 125 which is adjacent X electrode 101( and Y, electrode 1020 is specified by the select signals on paths 118 and 120.
FIG. 2, previously mentioned in conjunction with normal sustain operation, discloses a number of waveforms comprising pulses and resulting cell voltages before, during and after a typical erasing operation. It is most convenient to explain the invention in terms of the times and types of pulse applied, rather than in terms of the actual circuitry producing them, since circuits capable of producing them at the specified times are well known. The pulse train applied by X electrodes other than the X electrode 1010 to panel 100 can be conveniently denoted by V waveform 201 (unse lected X electrode voltage) and that applied by Y electrodes other than Y, electrode 102a can be referred to as V,,,,. V and V are the voltages applied to the X and Y electrodes selected by X and Y erase signals respectively on paths 118 and 120 to halt light emission from the selected cell, cell 125 for explanatory purposes. Since the sustaining pulse in waveforms 203 and 204 are identical to those in waveforms 201 and 202, they have been given identical reference numerals.
Placing an erase synchronizing signal on path 150 at time T initiates an erase operation. Timing system 126 interrupts issuance of sustaining pulses on lines 127 and 112 after the next trailing edge of a Y sustaining pulse, say pulse 202b. The X erase select signal on path 118 causes X-line conditioning select system 115 to produce a control signal on the appropriate conditioning control signal path Illa-111d, in this case path 111(- for electrode X,,, a preselected time after the occurance of the erase synchronizing signal. OR gate 110cproduces an essentially similar pulse on path 109(, which causes switch 107(- to close, applying voltage V to X, electrode 1010 via diode 1241'. This voltage is represented as conditioning pulse 203a in FIG. 2. It is preferable that the conditioning pulse control signal on path 111C be such that conditioning pulse 2030 is similar or identical to a normal sustaining pulse 201a or 201b, differing only in that conditioning pulse 203a is applied to and affects only X, electrode 1010, rather than all X electrodes 10111-10111 Prior to the leading edge of pulse 203a, timing system 126 issues the appropriate control signal on signal path 121 to cause switch 117 to ground all Y electrodes 102u-102d. Therefore, conditioning pulse 203a affects X electrode 101a only, exactly the way sustaining pulse 20111 affected it. As previously mentioned, sustain timing system 126 interrupts sustaining signals after a sustaining signal to Y electrodes 102u102d has issued, and before the next sustaining pulse for X electrodes l0lul0ld has issued. The effect of conditioning pulse 2030 is to reverse the polarity of the residual wall charge in the dielectric spaced between selected X electrode 101:- and the portions of Y electrodes 1020-1020 adjacent it, including the dielectric adjacent cell 125. Upon completion of conditioning pulse 203a, sustain timing system 126 removes the grounding signal to switch 117.
The next step of the erasing of selected cells 125 involves placing erase pulse 204a on Y, electrode 102a, the electrode specified by the Y erase select signal on signal path 120. At the time the control signal is removed from switch 117 or shortly thereafter, a similar control signal is applied to path 114 for a preselected time by timing system 126 causing switch 113 to close and ground all X electrodes 101a-l0ld. At that time, or shortly thereafter, Y-line erase select system 1 16 energizes for a preselected time that one of the four erase control signal paths 106-106d specified by y erase select signal, in this case path 106a, causing switch 1050 to close. OR gate 131 passes this switch closure signal through to signal path 119 to provide a control signal to erase pulse generator 122. In response to the control signal on path 119, erase pulse generator emits erase pulse 204a on path 123 which is applied to the power input terminal of switches l05a-105d. Since only switch a is closed, erase pulse 2040 is applied to only Y, electrode 102a. Pulse 204a is chosen to have a shape and duration suitable to remove the residual electric charge in the walls of panel 100 adjacent cell 125, to prevent later conduction by cell 125 upon the application of normal sustaining pulses 2011' and 2020.
Referring to waveform 208, it can be seen that cell 125 is subjected to a positive conditioning pulse 203a and the following negative erase pulse 2040. By comparing waveform 208 with waveforms 205-207 it can be seen that between time T,., and T only cell 125 has the pulses of waveform 208 applied to it. During this time, no voltage pulses are applied to those cells not adjacent Y, electrode 102a or X, electrode 1010, as displayed by waveform 205. Cells adjacent X electrode 1011', excepting cell 125, are subjected during this time to the voltages of waveform 207, which essentially comprises conditioning pulse 203a only. Cells adjacent Y, electrode 102a excepting cell 125, are subjected to the voltages of waveform 206, which comprises erase pulse 2040 during the interval between times T,. and T,,,. In analyzing the eflect of these pulses in cells other than cell 125, one must realize that a pulse which is of the same polarity and approximately the magnitude of a sustaining pulse immediately preceding it does not affect the wall charge in the dielectric adjacent the electrodes to which the pulse is applied, and hence does not affect the memory condition of the cells involved. Pulse 2030 in waveform 207 merely acts as a normal sustaining pulse whose function would normally be performed by pulse 201:, and hence following it with pulse 20k does not affect the condition of cells adjacent X, electrode 101v. Pulse 2040 in waveform 206 duplicates the effect of sustaining pulse 202b, and hence does not af fect the residual electric charge in the walls adjacent Y, electrode 102a. except at cell 125.
The net result of the application of conditioning pulse 20311 to X;, electrode 101(' and erase pulse 20411 to Y, electrode 1020, is to remove the wall charge adjacent cell 125, and to leave the wall charge adjacent all other cells essentially unchanged. Thus, cell 125 has been selectively erased without the necessity of even temporarily erasing other cells.
The actual characteristics of erase pulse 20411 to most effectively cause the removal of the residual electric charge adjacent cell 125 is almost completely dependent on physical characteristics of panel 100 and the gas mixture within its gas chamber. As a general guide line, rise time on pulse 204a should be at least 10 times and preferrably 50 to lUU times slower than that of a normal sustaining pulse 201a. etc. Maximum amplitude of pulse 2040 is preferrably between 50 percent and 100 percent that of pulse 201a. Pulse 20411 is shown to have approximately an exponential decay shape. such as may be produced by a standard R-C circuit. This is not essential, and a ramp or multi-step pulse can be substituted. However, the exponential decay shape is preferred because of ease of generation.
Turning next to FIG. 1b, a simple circuit for produc ing an erase pulse 2040 having an exponentially decreasing rate of change is displayed. Switch 140 receives the output of OR gate 131 and closes while the output pulse is on path 119, thereby applying Dc erase voltage V, to the terminal of resistor 141 connected to switch 140. Capacitor 142 is assumed to be discharged and will initially absorb most of the current flowing through resistor 141 from voltage source V... As capacitor 142 charges to a higher voltage, voltage on path 123 increases according to the exponential decay curve. Resistor 143 is provided to discharge capacitor 142 after the signal on line 106a ceases, and may be chosen to discharge capacitor 142 over an interval several times that of the duration of pulse 204a.
Waveforms 209-211 disclose different timing and shape for erase pulse which may be easily incorporated into the apparatus of FIG. 1a. Whereas erase pulse 204a removed the residual wall charge over a relatively long period of time, with relatively small average current during the existence of pulse 204a. it is also possible to remove the wall charge with a relatively short pulse which induces a relatively large current flow through the gas volume to be erased during its existence. In general. current flow through a gas volume increases markedly with shortened rise time of the voltage pulse. Accordingly, Xline conditioning select sys tem 115 may be designed to produce conditioning pulse 209a rather than pulse 203a. Pulse 209a may be of the same amplitude of pulse 203a, but have a somewhat longcr duration, as shown. Responsive to the output of OR gate 131 on path 119, erase pulse generator 122 issues erase pulse 210a on path 123. Pulses 209a and 2101: must be timed with respect to each other to cause the trailing edge of pulse 2091: to occur during pulse 210a. During the time common to both pulses 209 a and 210a, neither Y electrodes 102u102d nor X electrodes 1011140111 are grounded. However, Y electrodes l02a102d are grounded over the duration of pulse 209a preceding the leading edge of erase pulse 210a. After the occurance of the trailing edge of pulse 209a, X electrodes [Ola-101d must be grounded. Both grounding operations can be controlled by the timing system 126 with reference to the synchronizing signal on path 150.
As can be seen in waveform 211, during the period of time of overlap for pulses 209a and 2101:, the voltage across cell 125 is the difference between the amplitude of pulses 209a and 210a. The rural duration of erase pulse 210a should be such that pulse 21 lb is just long enough and has a sufficiently steep leading edge to cause the residual wall charge adjacent gas volume 125 to be removed. As discussed in the explanation involving erase pulse 204a, the prior art teaches the procedure for determining the amount of residual wall charge adjacent cell 125 which must be removed to prevent subsequent conduction by cell 215 during sus raining pulses.
FIG. 10 discloses a simple erase pulse generator 122 comprising one-shot 144 and switch 145, for generating pulse 2100. In response to an output signal from OR gate 131. one-shot 144 places a closure signal on path 146 causing switch 145 to close while one-shot 144 is set. The width of one-shot 144 output pulses should be chosen equal to the duration of erase pulse 210a. Since the duration of pulse 2111) must be selected with much greater precision than pulse 204a, one-shot 144 is provided to accurately control the width of the pulse 21 lb. It is preferable that erase voltage V of pulse 2llb be selected to be approximately percent that of 21 normal sustain pulse. Selecting this amplitude decreases the precision required in regulating the duration of pulse 21 lb.
Turning next to FIG. 3, therein are shown waveforms 301308 which illustrate electrode-to-ground voltages and voltages across individual cells in the same manner that FIG. 2 illustrates such voltages, and use notation consistent therewith. The X electrode drive circuitry of FIG. 1, to implement this variation, must be modified as shown in FIG. 4, wherein all circuit elements of P16. 1 are retained. except for switch 113, which is replaced by switches 1130-41311, and signal paths 1 14., replaced by similar paths 1140-1 14d. Timing system 126 has been replaced by timing system 126', whose operation is slightly different therefrom, as will be explained below. Since grounding of the X electrode is selective, X erase select signal on path 118 must be applied to timing system 126' as well. Erase pulse generator 122 must also be capable of generating pulse 3040, which is of polarity opposite to that of pulse 204a, but may be gen erated similarly Operation of the combined apparatus of FIGS. 1a
* and 4 to perform this variation is quite similar to that described in conjunction with FIG. 2. Operation during normal sustaining is identical, except that X electrode grounding is done by timing system 126 causing all switches 113a-l13d to close. When sustaining pulses are interrupted, the interruption must occur following an X electrode sustaining pulse, such as pulse 201!) in waveform 301. The principle followed in this variation is to place similar or identical conditioning pulses on all of the X electrodes Ha-101d. Accordingly, conditioning pulse 201a is applied simultaneously to all the X electrodes not selected by the X erase select signal on path 118, and conditioning pulse 3020 is applied simultaneously with pulse 3010 to the Y electrodes. It is convenient that these pulses be identical to sustaining pulses 201a. 2021:, etc. and if so, can be caused as a normal sustaining pulse by timing system 126'. If the example of the erasing of cell 125 is still followed, these conditioning pulses will be applied to X,, X and X electrodes 101a, lOlh. and 101d and all the Y electrodes 102a-l02d. The electrode specified by the signal on path 118 is grounded by timing system 126' during these conditioning pulses, i.e. X electrode 101(' is grounded for this example. The voltages which these pulses apply to cells which are adjacent neither selected X electrode l01c nor selected Y electrode 102a. is shown in waveform 305 as causing in effect no voltage to appear across these cells. However, since no conditioning pulse occurs on selected X electrode 1010. which is instead grounded, pulse 302a does appear across all cells adjacent X; electrode 1016. as shown by waveforms 307 and 308. Therefore, the conditioning pulses cause all these cells to have a negative residual wall charge. Following conditioning pulses 302a, erase pulse 304a is applied to the selected Y electrode 102a. Erase pulse generator, 122 generates erase pulse 304a and Y-line erase select system 116, by enabling switch 105a, gates the erase pulse to Y electrode 102a. Erase pulse 3040 has no effect on cells common to unselected X electrodes 101a, lOlb, and 101d because it is, as shown in waveform 306, an erase pulse of the same polarity as the latest sustaining pulse (20111) applied to them.
Waveform 308 displays the cell voltage across cell 125 during the erase interval. Only this cell is subjected to a negative polarity pulse 302a similar to a sustaining pulse and immediately thereafter a positive erase pulse 304a. Pulse 3040 must be selected to cause the residual wall charge adjacent cell 125 to be removed as was the wall charge by the pulses shown in FIG. 2. Such pulse generation is well known and easily understood by those skilled in the art, so no problem in practicing this alternative variation should occur. Following the end of the erase interval, normal sustain operation begins with sustaining pulse 2010.
Because of the asymmetry of the voltages across cells which are not erased during any of the above-described erase operations, normal residual wall charges do not exist adjacent unerased cells. it is necessary to apply at least one X sustaining pulse 201( and one Y sustaining pulse 202C to panel 100 to restore the wall charges to approximately their normal condition, before further erasing can reliably occur, as is shown in FIGS. 2 and 3. Preferably, 4-6 pulses are applied between individual cell erase operations, to provide maximum reliability. The number of pulses necessary to completely restore normal wall charges is dependent on the physical characteristics of panel 100 and the gas mixture within [t is also possible to erase several cells along a specified Y electrode, by applying conditioning pulses to each electrode adjacent the cell on the selected Y electrode which is to be erased. Many other variations on the basic concepts herein disclosed are possible, by slight modifications of the teachings of this invention.
What I claim as my invention is:
1. In a gas discharge display matrix of the type having a chamber containing ionizable gas and formed of a dielectric, and having a plurality of spaced-apart first electrodes on a first side of the chamber and a plurality of spaced-apart second electrodes on a second side of the chamber and nonparallel to the first electrodes, said electrodes located so as to interpose an ionizable gas volume and a portion of the dielectric between each first electrode and each second electrode; and having a pulse generator for sustaining light emission from all gas volumes having a wall charge voltage exceeding a predetermined value by applying sustaining pulses of predetermined voltage to at least one of the pluralities of first and second electrodes causing voltage potential on the first electrodes to alternately rise above and fall below the potential on the second electrodes; wherein the invention comprises improved ap paratus for extinguishing light emission from the selected gas volume between a selected first and a selected second electrode specified by first and second erase select signals, respectively, comprising:
a. means for interrupting for a preselected time application of sustaining pulses after substantial completion of a selected sustaining pulse to the first electrodes, responsive to an erase select signal;
b. first switch means for connecting at least the selected first electrode to a reference voltage responsive to a first switch closure signal;
c. second switch means for connecting at least the selected second electrode to a reference voltage responsive to a second switch closure signal;
d. means for applying a first switch closure signal to the first switch means responsive to an erase select signal;
e. means responsive to the second erase select signal for applying to the selected second electrode a conditioning pulse which overlaps the period of con nection of the selected first electrode to the reference voltage and which produces conduction causing reversal in polarity of the wall charge in the dielectric adjacent at least the selected gas volume; means responsive to an erase select signal for applying a second switch closure signal following the conditioning pulse, to the second switch means; and means responsive to the first erase select signal for applying to the selected first electrode an erase pulse whose duration is greater than that of a sustain pulse and whose voltage for a substantial part of its duration is substantially less than peak sustain pulse voltage, said erase pulse overlapping the per iod of connection of the second electrode to the reference voltage and causing conduction reducing below that necessary to permit sustaining of light emission, the wall charge in the dielectric adjacent the selected gas volume.
2. The apparatus of claim 1, wherein at least one of the switch means connects all of its associated electrodes to the reference voltage responsive to its associ ated closure signal.
3. The apparatus of claim 1, wherein the first switch means connects all the first electrodes to the reference voltage responsive to the associated closure signal, and wherein the sustaining pulse interrupting means and the Conditioning pulse applying means in combination comprise means responsive to the second erase select signal for interrupting application of the next sustaining pulse to all but the selected second electrode following substantial completion of the application of the selected sustaining pulse to the first electrodes.
4. The apparatus of claim 1, wherein the erase pulse applying means comprises a pulse generator producing a pulse having a relatively slow rise time in comparison to the sustaining pulses, and whose maximum amplitude is between approximately 50 percent and percent that of the sustaining pulses.
5. The apparatus of claim 1, wherein the conditioning pulse applying means comprises a conditioning pulse generator applying a conditioning pulse having a predetermined width and the erase pulse applying means issues an erase pulse overlapping and ceasing after the conditioning pulse by predetermined times.
6. The apparatus of claim I, further including erase sequencing means for spacing consecutive erase operations by a predetermined number of sustaining pulses.
7. The apparatus of claim 6, wherein the predetermined number of sustaining pulses is at least 2.
8. The apparatus of claim 1 adapted to operate with apparatus of the type in which the sustaining pulse generator comprises a pulse generator alternately applying sustaining pulses of a predetermined polarity to the first electrodes and to the second electrodes, and grounding each plurality of electrodes while the other receives a sustaining pulse, wherein the erase pulse generating means comprises an R-C network generating an erase pulse having an exponentially decreasing rate of change.
9. The apparatus of claim 8 wherein the maximum amplitude of the pulses supplied by the erase pulse generating means is between approximately 50 percent and I percent of the maximum amplitude of the voltage applied to a gas volume by a sustaining pulse.
10. In a gas discharge display matrix of the type having a chamber containing ionizable gas and formed of a dielectric having a plurality of spaced-apart first electrodes on a first side of the chamber and a plurality of spaced-apart second electrodes on a second side of the chamber nonparallel to the first electrodes, said electrodes located so as to interpose an ionizable gas volume and a portion of the dielectric between each first electrode and each second electrode; and having a pulse generator for sustaining light emission from all gas volumes having a wall charge voltage exceeding a predetermined value by applying sustaining pulses of predetermined voltage to at least one of the pluralities of first and second electrodes causing voltage potential on the first electrodes to alternately rise above and fall below the potential on the second electrodes; wherein the invention comprises improved apparatus for extinguishing light emission from the selected gas volume between a selected first and a selected second electrode specified by first and second erase select signals. respectively. comprising:
a. means for interrupting for a preselected time application of sustaining pulses after substantial completion ofa selected sustaining pulse, responsive to an erase select signal;
b. switch means for connecting to a reference volt age, responsive to a switch closure signal. each second electrode specified thereby;
c. means responsive to the second erase select signal for applying a switch closure signal to the switch means specifying the selected second electrode;
d. means responsive to the second erase select signal for applying simultaneously to all the first and second electrodes excepting the selected second electrode a conditioning pulse which overlaps the period of connection of the selected second electrode to the reference voltage and which is sufficient to cause reversal in polarity of the residual electric charge in the dielectric adjacent the selected second electrode; and
e. means responsive to the first erase select signal for applying to the selected first electrode an erase pulse following the conditioning pulse and overlapping the period of connection of the selected second electrode to the reference voltage, and which is sufficient to reduce below that necessary to permit sustaining of light emission, the residual electric charge in the dielectric adjacent the gas volume between the two selected electrodes.
11. The apparatus of claim 10, wherein the switch means connects all the second electrodes to the reference voltage at least during the erase pulse, and wherein the sustaining pulse interrupting means and the conditioning pulse applying means in combination comprise means responsive to the second erase select signal for applying the next sustaining pulse to all the first and second electrodes, excepting the selected second electrode, following substantial completion of the application of the selected sustaining pulse to the first electrodes.
12. The apparatus of claim 11. wherein the erase pulse applying means comprises a pulse generator producing a pulse having a relatively slow rise time in comparison to the sustaining pulses, and whose maximum amplitude is between approximately 50 percent and percent that of the sustaining pulses.
13. The apparatus of claim 10 adapted to operate with apparatus of the type in which the sustaining pulse generator comprises a pulse generator alternately applying sustaining pulses of a predetermined polarity to the first electrodes and to the second electrodes, and grounding each plurality of electrodes while the other receives a sustaining pulse. wherein the erase pulse generating means comprises an R-C network generating an erase pulse having an exponentially decreasing rate of change.