Bilateral Selective Burst Erase System

Application Ser. No. 268,219 of T. W. Criscimagna and A. O. Piston for "Method and Apparatus For Gas Panel Display" and application Ser. No. 375,252, now U.S. Pat. No. 3,919,591 of T. W. Criscimagna for "Gas Panel With Improved Write-Erase and Sustain Circuits and Apparatus" assigned to the assignee of the present application, provide background information that may be helpful in understanding the general area of this invention.

This application relates to an improved erase system for multi-cell gas panel displays which provides a selective burst of bipolar signals to the particular cell in which erasing is desired.

Gas panels of the type to which this invention relates have two glass plates that are spaced apart by a seal to contain an ionizable medium. A set of horizontally extending insulated conductors are located on one glass plate and a set of vertically extending insulated conductors are located on the other plate. When a suitable voltage is applied between one horizontal conductor and one vertical conductor, ionization occurs at the crossover point of the two conductors and light is emitted. The crossover points are called cells, and a display pattern is formed by ionizing selected cells. The operation of initially ionizing a cell is called writing. Once a cell is written it is sustained by a continuously alternation potential called the sustainer. The operation of removing the wall charges from a previously written cell is called erasing. A cell is erased by applying a suitable voltage waveform to produce a controlled ionization so that the wall charge is reduced to or near to zero in the cell to discharge the cell. One object of this invention is to provide improved waveforms for erase operations.

As a result of the ionization that occurs during writing, positive and negative charges accumulate on opposite insulating walls of the cell. The voltage of this charge opposes the voltage applied between the vertical conductor and the horizontal conductor so that the sum of these voltages quickly falls below the voltage required for ionization and light is emitted from the cell for only a brief instant. The write voltage waveform is maintained for a sufficient interval after the light is extinguished for a substantial charge to be stored on the cell walls. After the write operation, periodic light output of the cell is sustained by an alternating polarity voltage that is called a sustain voltage. The sustain pulse following the write operation is opposite in polarity to the write pulse and thus is of the same polarity as the charge that was stored on the cell walls by the preceding write operation. Since the cell ionizes at a voltage that is the sum of the applied voltage and the voltage that is the sum of the applied voltage and the voltage of the stored charge, a previously written cell ionizes at an applied sustain voltage that is less than the write voltage. The sustain voltage is applied simultaneously to all cells and the previously written cells ionize and accumulate charge for the next sustain alternation but the previously erased cells with zero wall charge remain un-ionized.

A possible explanation for ionization in a gas panel will be helpful for understanding this invention. Independently of any voltage on the conductors of a cell, the cell medium ordinarily contains some free electrons and positive ions, and pilot lights may be located around the edge of the panel to establish a suitable level of ionization. The electrons and positive ions recombine and new ions are formed at an equalibrium rate. When a voltage is applied across the conductors of a cell, an electric field is formed in which the ions are accellerated so that ions collide more frequently with neutral atoms and thereby produce additional ions. At relatively low voltage levels an equalibrium condition may be reached where there is a high level of ionization but ions are lost by recombination as fast as they are created by collisions between atoms and ions. However, at some higher voltage level, ions are created faster than they are lost and these ions in turn produce additional ionization so that an avalanche of free charges occurs. Thus, both of the height and the width of the cell voltage waveform are important in establishing whether avalanche ionization will occur. As has already been explained, avalanche ionization is required for write, erase, and sustain operations.

It has been found that in attempting to do a selective erase operation, particularly in the case of erasing a cell which is surrounded by a number of lit cells to remain illuminated, the erase operation may be ineffective. It is believed that there are at present three basic techniques of erasing gas panel cells. One of these is the so-called narrow high amplitude erase which has the advantages that it erases better at Vs max. (sustain voltage maximum), it is insensitive to amplitude charges, it is selective and it is fast. It has the disadvantages that the high amplitude requires high voltage circuits for deselect and it is sensitive to width tolerances.

Another technique is the wide low amplitude erase which has the advantages that it is insensitive to width tolerances, low voltages may be used for deselect since it is a low amplitude erase, it is selective, and it is fast. It has the disadvantages that erasing at Vs max. may require critical amplitude adjustment and it is sensitive to amplitude tolerances.

Still another erase technique applies a burst of narrow sustain pulses which has the advantage that it is insensitive to width tolerances and erases at Vs max. very well. It has the disadvantages that it can not be applied selectively to only a desired cell or cells since it is on the sustain voltage and it is slow.

The subject improved erase technique has all of the advantages of the prior known erase techniques and none of the disadvantages. It is accordingly an object of this invention to provide a gas panel erase pulse technique that erases very well at Vs max., is insensitive to amplitude charges, is insensitive to width charges, is low amplitude and can be made selective with low voltage circuits for deselects, and is fast.

The erase waveform of this invention provides a wide low amplitude positive erase pulse for the selectric portion of the erase followed by a narrow high amplitude negative erase pulse for the unselected portion of the erase. A series of three of these bipolar pulse squares are provided, the series taking place in the normal time of a sustain pulse cycle. The wide low amplitude pulse permits low voltage circuitry to be used for the full select operation. The negative going narrow pulses of the erase series are generated by the normal sustain pulse circuitry and are simply narrowed versions of the erased sustain pulse. The negative going narrow pulses produce a better erase action of the Vs max. (V sustain maximum) and reduce the amplitude sensitivity of the wide selection erase pulse. A burst of these bipolar erase signals also decreases sensitivity to width and amplitude variation of the pulses and improves operation at Vs max.

Other objects, advantages, and features of the invention will be apparent from the following description of a detailed embodiment of the invention.

FIG. 1 shows the circuitry for producing the improved selective bipolar burst erase action at any desired cell of a gas panel display;

FIG. 2 shows waveforms that illustrate the operation of the circuit of FIG. 1;

FIG. 3 shows simplified waveform representations of various erase techniques;

FIG. 4 is a chart representation of Vs (sustain) plotted against Ve (erase) and further illustrates the improved erase action.

FIG. 3 waveform C shows in waveform representations the improved erase sequence of the subject application. In FIG. 3 waveform C the pulses designated 10 and 10a are the normal sustain pulse sequence applied to all cells in the usual manner to maintain previously illuminated (ignited) cells in the illuminated state until an erase operation is desired. The shaded low amplitude pulses 11a, 11b, and 11c, shown represent in part the improved erase pulse sequence applied to a selected cell to erase it. The erase pulses may for the particular panel used in this instance have a representable amplitude in the range of +20 to +80 volts and a representation duration of 1-3 micro-sec. Each pulse 11a, 11b, and 11c is followed by a negative polarity pulse alternation, 12a, 12b and 12c, respectively, which is actually a narrowed sustain pulse alternation. Three bipolar sequences of erase pulse such as 11a followed by negative alternation 12a are shown. This is representative only since the three pulse sequences work best for the particular gas panel utilized. Dependent upon panel characteristics, time series of 2 or more pulse sequences may be utilized. The erase pulses 11a etc. are shown shaded to indicate that they are applied only to the selected cell for erasing action. The narrowed sustain alternations are applied to all cells and are graphically represented as unshaded to illustrate this. In the particular panel utilized, the series of three erase bipolar alternations are applied during one normal sustain pulse alternation in the representative gas panel utilized or about 33 micro/sec. at 30 KHZ.

The A waveform as shown in FIG. 3 is a waveform representation of the commonly used prior art wide-low amplitude selective erase technique while the waveform B shown in FIG. 3 is a waveform representation of the prior art narrow-high amplitude selective erase technique. These prior art waveforms are shown as a frame of reference for the improved technique as shown by waveform (C). The detail circuit configuration for applying the improved erase pulse action to any selected cell of a gas panel display will now be described.

Referring now to FIG. 1, there is shown a representative 6.times. 6 gas panel display 14 having six horizontal conductors 16a, 16b, 16c, 16d, 16e and 16f and six vertical conductors 17a, 17b, 17c, 17d, 17e and 17f. Transistors 19a, 19b, 19c, 19d, 19e and 19f and associated resistors 20a, 20b, 20c, 20 d, 20e and 20f respond, as will be explained, in accordance with selection signals applied at their related base terminal input 21 to connect the horizontal conductors 16a to 16f to either a lower horizontal bus line 22 or an upper horizontal bus line 23. Similarly, transistors 25a, 25b, 25c, 25d, 25e and 25f and associated resistors 26a, 26b, 26c, 26d, 26e and 26f respond to selection signals applied at their related base terminal input 28 to connect the vertical conductors 17a to 17f, to either a vertical lower bus 30 or a vertical upper bus 31. During a write or erase operation, a selected cell of the display receives the voltage of upper horizontal bus 23 with respect to the voltage vertical lower bus 30, an unselected cell receives the voltage of lower horizontal bus 22 and upper vertical bus 31, while half selected cells receive the voltages of both upper busses 23 and 31, or both lower busses 22 and 30.

Still referring to FIG. 1, a sustain circuit, generally indicated 32 is provided to apply the sustain waveform through a conductor 33 and associated secondary winding 34a and 34b of a transformer 34 to the horizontal upper bus 23 and horizontal lower bus 22. The normal sustain waveform applied to the upper and lower horizontal busses is as indicated by the pulse 10 in waveform (A) FIG. 2 and comprises a square waveswing from zero potential to a +Vs potential as indicated. The sustain waveform is applied periodically under the control of a timing circuit 36. The sustain circuit 32 is of the active pull up and active pull down type and comprises a transistor 37 having its collector connected to a potential source of +Vs. The emitter of transistor 37 and the collector of a second transistor 38 are commanded and linked to the previously mentioned conductor 33. The base of transistor 38 is connected to the timing circuit 36 through conductor 40, and the base and emitter of transistor 37 parallel the secondary of a transformer 41, one leg of the primary winding of which is connected to the timing circuit 36 through conductor 43 and the other leg to a +5 volt supply. The sustain circuit functions as follows. Assumming NPN transistor 37 is off, the timing circuit through conductor 43 pulls down the lower end of the primary of the transformer 41 relative to the +5 volt supply. As a result, the secondary winding of transformer 41, which is similarly poled as the primary, is pulled down to lower the potential of the emitter of transistor 37 below its base and render transistor 37 conductive. With transistor 37 conductive, previously mentioned conductor 33 is at the sustain potential of +Vs. The timing circuit thereafter raises conductor 43 to shut off transistor 37, and raises conductor 40 to render transistor 38 conductive. With transistor 37 off and transistor 38 on, conductor 33 is now at ground potential.

A similar sustain circuit generally indicated at 46 in FIG. 1 is provided to apply a sustain pulse 10 as indicated in waveform (C) of FIG. 2 through a conductor 47 and associated secondary windings 34c and 34d of the same transformer 34 mentioned previously to the vertical upper bus 31 and vertical lower bus 30. The normal sustain waveform applied to the busses 30 and 31 again is a wide square wave which swings from zero potential to +Vs. It should be noted that sustain circuit 32 is operated by the timing circuit 36 to generate spaced square wave sustain pulses 10 as indicated on the horizontal busses 22 and 23. It will also be noted, that during an erase operation, the sustain circuit 46 operates to generate on the vertical busses 30 and 31, a normal sustain pulse 10 followed by three spaced narrow sustain alternations 50a, 50b and 5c as indicated in waveforms C and D in FIG. 2. These narrowed sustain alternations generate in turn, part of the new erase sequence on a selected cell as will later be evident.

Referring again to FIG. 1, an erase control circuit generally designated 52 is provided which comprises a primary winding 34e of the previously mentioned transformer 34, one lead of the winding being linked through a suitable amplifier 53 and potentiometer 54 to a +12 volt source. The other lead of the primary winding is connected to ground through transistor 56, the base of which is controlled from timing circuit 36. When an erase operation is desired, the timing circuit 36 turns on transistor 56 causing the transistor end of primary winding 34e of transformer 34 to swing to ground. The magnitude of this swing is adjustable by potentiometer 54, so that there is induced in the secondary windings 34a, 34b, 34c and 34d, an approximate 25 volt positive going voltage swing. The windings 34a and 34b are poled so that +25 volts are additive on horizontal upper bus 23 relative to the potential of horizontal sustain conductor 33 and subtractive on horizontal lower bus 22 relative to the potential of conductor 33. Accordingly, horizontal upper bus 23 goes 25 volts above sustain line 33 potential while the horizontal lower bus 22 goes 25 volts below line 33 potential.

The transformer secondary winding 34c and 34d associated with the vertical upper bus 31 and vertical lower bus 30 are similarly poled and respond in the same manner to drive the vertical upper bus 31, 25 volts above the potential of vertical sustain line 47 and drive the lower vertical bus 25 volts below the potential of vertical sustain line 47. After a time period which may be in the range of 2-4 microseconds, the timing circuit 36 turns transistor 56 off to drop the voltage across primary winding 34e of transformer 34 to zero and accordingly causes a similar swing of the bus lines 22, 23, 30 and 31. The timing circuit 36 controls the transformer 34 primary in such a manner so that three spaced square waves 60a, 60b and 60c are generated in the horizontal upper bus 23 as illustrated in waveform A of FIG. 2, while three similar spaced negative square waves 61a, 61b and 61c are generated on the horizontal lower bus 22. The magnitude of these pulses is 1/2 the normal erase pulse magnitude for the gas panel.

The above described circuits provide the required structure for applying the improved erase waveform action to a selected cell of an array and a representative operation will now be described. Referring to FIG. 1, assume that the cell position 64, at the intersection of horizontal line 16c, and vertical line 17c was previously written into. Also assume that adjacent surrounding cells were also written into. After the write operation, sustain waveforms are applied in the usual manner to maintain each written cell in its illuminated state.

Assume now that it is desired to erase only cell 64, in the mid area of the plurality of illuminated cells. Conventional addressing circuitry (not shown) will accordingly turn associated transistor 19c OFF, and transistor 25c ON. With transistor 19c OFF, the upper horizontal bus 23 is linked through the resistor 20c associated with OFF transistor 19c to the horizontal line 16c of the display, the desired cell 64 being on this line. Similarly, the vertical lower bus 30 is linked through ON transistor 25c to the vertical line 17c of the display in which our desired cell 64 resides. Since this is an erase sequence, the previous described erase control circuit 52 and horizontal sustain circuit 32 are activated under timing circuit 36 control so that there is generated on the horizontal upper bus 23 the usual square wave sustain signal 10 followed by three spaced 1/2 select pulses of magnitude V erase/2 as represented by the waveform A of FIG. 2. This waveform is of course applied to the desired horizontal line 16c of the display which was selected as noted above. Similarly, the erase control circuit 52 and vertical sustain circuit 46 are actuated under timing circuit control so that there is generated on the vertical lower bus as shown in waveform D of FIG. 2, the usual square wave sustain signal 10, followed by a negative going 1/2 select voltage pulse 65 of Ve/2, followed by a shortened width sustain signal 50a, the half selects and shortened sustain cycle being repeated twice more before the generation of the normal sustain pulse 10. This waveform D of FIG. 2 is of course applied to the desired vertical line 17c of the display which was selected as noted above. With the application of the erase sequence waveforms of waveform A and D to the respective horizontal line 16c and vertical line 17c of the display, which of course intersects at the desired cell 64 which we wish to erase, the selected cell sees a resultant wave change as represented by the waveform A-D of FIG. 2 which is the algebraic resultant of the horizontal upper bus waveform A and the lower vertical bus waveform D. Examining the waveform A-D we note that it involves a normal width positive going sustain pulse 10, followed by a normal width negative going sustain pulse 10a, followed by a positive erase pulse 66a, which is twice the magnitude or Ve of the two half select erase pulses 60a (Ve/2) and 65a (Ve/2) of which it is comprised, followed by a narrowed negative going sustain pulse 67a. The full magnitude erase and narrowed sustain pulse sequence is repeated two more times on the selected cell to complete the erase action.

The action of the selection process to effect the erase function on a desired cell has the following action on the other cells on that same horizontal line 16c. The horizontal upper bus waveform A is of course seen by all cells on the line 16c, however, the related selection transistor 25 associated with all the remaining vertical lines other than line 17c remains OFF. Accordingly these OFF transistors cause the vertical upper bus 31 to be connected to all the associated vertical lines of the gas panel. The vertical upper bus has a waveform pattern as illustrated by the waveform C of FIG. 2 which includes a normal sustain pulse 10, followed by repetitive pairs of a deselect pulse 68 and a shortened duration sustain pulse 50a in the same direction as the deselect pulse 68. The resultant charge pattern seen by the non-selected cells on the horizontal line 16c is accordingly the algebraic addition of waveform A and C or waveform A-C as indicated in FIG. 2 for half selected cells. It will be noted in this waveform that the cells see a series of narrowed negative sustain pulses 67a but does not see the associated required erase pulse swing 66a so that no erase action takes place on these cells. The cells other than our desired cell 64 on horizontal display line 16c may accordingly be considered as only half selected and unaffected by the erase action.

Similarly, the action of the selection process to effect the erase or the desired cell 64 causes the following action on other cells on the same vertical line 17c of the display panel. The vertical lower bus waveform 30 is of course seen by all cells on vertical line 17c, however the selector transistor 19 associated with all the remaining horizontal lines other than line 16c remains ON. Accordingly, then ON transistors 19 cause the lower horizontal bus 22 to be connected to all remaining associated horizontal lines of the panel. The lower horizontal bus 22 has a waveform pattern thereon as illustrated by waveform B of FIG. 2 which includes a normal positive sustain pulse 10 followed by three spaced negative going deselect pulses 61a, 61b and 61c resulting from the action of transformer 34. The resultant waveform seen by the non-selected cells on the vertical line 17c is accordingly the algebraic addition of waveforms B and D or waveform B-D (identical to A-D). Here again no erase action takes place on these remaining cells in the selected vertical line 17c and they may be considered to be only half selected and unaffected by the erase action.

The action of the selection and subsequent erase action on the selected cell 64 has the following action on cells that reside on neither the selected horizontal line 16c or the vertical line 17c of the panel. These lines by reason of their related selection transistor 19 being ON have their related horizontal line 16 on the display subjected to the waveform B of the horizontal lower bus 22 and by reason of their related transistor 25 being OFF have their related vertical line 17 of the display subjected to the waveform C of the vertical upper bus 31. Here again the waveform at the intersections of these latter vertical and horizontal lines of the display are the resultant algebraic addition of waveforms B and C or waveform B-C. These cells are referred to as unselected cells and it is evident from waveform B-C that the cells are subjected after the usual sustain pulse operation, to a negative going low amplitude pulse 68 followed by the negative going shortened narrow sustain 67a. This latter pulse pair both being in the same direction as normal sustain 10a are of the wrong polarity to erase. There are three pairs of these pulses as illustrated in waveform B-C of FIG. 2. The unselected cells remain unaffected by the erase action.

It is thus evident that we have a circuit system where only the selected cell sees burst voltage disturbances that are of the right bipolar form and magnitude to effect erase, the other cells see disturbances of the wrong polarity to erase and therefore remain unaffected. The operation of selecting only a single cell has been described, however multiple cells on a line can also be selected or erased by appropriate conditioning of more than one transistor 19 and only one transistor 25 or vice versa.

Referring now to FIG. 4 which illustrates in general plots of erase voltage amplitude (Ve) versus sustain voltage (Vs) along the horizontal axis and two limiting voltage points for sustain voltage are indicated. Vs minimum is that point, below which cells previously written will not sustain. Vs maximum is that point above which cells not written will turn ON spontaneously without a write pulse. Obviously these two limits define the sustain voltage range of operation for a gas panel. The solid line plot illustrates the Ve maximum and minimum that will produce a successful erase operation for any given sustain voltage with only one erase pulse. It should be noted that there is no voltage Ve that will successfully erase the panel cells in question at Vs maximum. These cells in question are usually cells that are surrounded by lit cells. Consequently using the single pulse erase does not allow for maximum sustain voltage range inherent in the panel.

The dotted line plot illustrates the Ve maximum and minimum that will produce a successful erase operation for any given sustain voltage with a burst of erase pulses as described in this invention. In this plot the cells in question can be erased at Vs maximum by a burst of erase pulses thereby allowing full use of the panel's inherernt sustain voltage range.

While the invention has been shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention.