the principles of the invention relate to the operation of gas - filled glow discharge display devices . the invention is particularly useful with devices of the type shown by g . a . kupsky in an application entitled cold cathode display panel , filed on oct . 2 , 1968 . a typical display device 10 ( fig1 ) includes a plurality of discrete gas cells , each having an anode electrode and a cathode electrode . in one suitable arrangement , the cells are arrayed in rows and columns and are energizable , either singly or in groups and sequentially , to represent letters , numerals , messages , and the like . the gas cells may be located in individual envelopes separate from each other , or they may be part of a unitary panel , as shown , which includes a central plate 20 having apertures or cells 30 arrayed in rows and columns with parallel cathode electrode strips 40 positioned on the lower surface of the central plate . each cathode electrode strip is aligned with a line or row of cells 30 and includes a cathode button 42 in each cell 30 . the panel 10 also includes parallel apertured anode electrode strips 50 positioned on the upper surface of the central plate and each aligned with a line or column of cells . the apertures 60 in the anode electrode strips permit the glowing gas in the cells to be viewed , but apertured cathodes or transparent anode or cathode conductors could also be used . various other mechanical arrangements might be employed in anode constructions to permit cells to be viewed . the cathode electrodes are oriented at right angles to the anode electrodes , and one cell 30 is located at each intersection of an anode and a cathode . top and bottom glass plates 70 and 80 complete the panel 10 . the panel is shown as having three columns and three rows of cells 30 , although substantially any number of cells could be provided . in panels 10 with which the invention has been practiced , cells 30 had a diameter of 0 . 04 inch and a depth of 1 to 3 mm , and a density of sixteen cells per linear inch . the x and y electrodes were of stainless steel and were 0 . 042 inch wide and 5 mils thick . the gas content was as described below . in the description of the circuit operation below , the display panel 10 is shown schematically , and the row electrodes are called x lines , and the column electrodes are called y lines . this generalized representation is intended to indicate that , in any one of the various signal routines to be described below , the different types of pulses or signals can be applied in any desired polarity . this means that a first type of pulse can be applied between an x line as anode and y line as cathode , or vice - versa , and a second type of pulse can be applied in the same polarity as the first or in the opposite polarity . the orientation of the pulses might be determined in any case by the types of electrode material used , the inclusion of photoemissive materials in the cells , the inclusion of phosphor materials in the cells , etc . in the panel 10 , the cells 30 are filled with an ionizable gas atmosphere including about 99 . 5 % neon and about 0 . 5 % argon , to which is added a small quantity of mercury and a trace of krypton 85 . a suitable gas pressure is about 40 torr at room temperature . other gas mixtures and pressures can be selected by those skilled in the art to perform the required functions as described below . the gas cells 30 have a characteristic breakdown or firing voltage vf which , when applied to a cell for a suitable time period , causes breakdown and an ensuing glow discharge . the breakdown or firing voltage is determined generally by the physical dimensions of a cell , gas pressure , type of gas , electrode materials , etc ., as is well known in the art . a cell is turned on and emits light when it has been driven to breakdown and glow discharge . in practicing the invention , when it is desired to enter information into a cell and thus to turn a cell on and cause it to emit light , a suitable voltage , which may be equal to or greater than the firing voltage vf , is applied between its x line and its y line . this voltage vf is the characteristic d . c . firing voltage of the cell , and it may be applied : ( 1 ) by holding one line at a fixed voltage and applying the entire voltage to the other line , or ( 2 ) by applying a portion of the voltage to one line and a portion of opposite polarity to the other line . in the latter method , if the voltage portions are equal , and each is one - half of the total voltage , the method is known as half - select operation . unless otherwise indicated , half - select operation is normally intended in the description below for firing cells and for other operations to be described . the panel 10 may be coupled to a system ( fig2 ) which includes a data source 100 , such as a computer , which processes information through a series of circuit modules to a final display control module 104 , from which information is ultimately introduced into one or more cells . for purposes of illustration of the invention , control module 104 feeding into the panel , provides a write signal made up of two signals w of equal and opposite polarity from sources 110 and 120 . the write pulses are used to turn cells on and thus to enter information therein . according to the invention , the control module 104 also provides a sustaining signal , made up of two sustaining voltage pulses s of equal and opposite polarity from sources 130 and 140 . the sustaining pulses are used to sustain the on state of a cell , without turning off cells on . the sustaining pulses and their operation are described in detail below . the sources 110 and 120 of write signals are coupled through current - limiting resistors 122 to selected x and y electrodes of display device 10 ; however , the sources of sustaining signals are coupled directly , that is , without current - limiting impedances , to all of these electrodes . diodes are normally provided , although they are not always illustrated , in all lines to separate write pulses from sustaining pulses and to prevent pulse crosstalk between lines . resistors are used in the write lines because the turn - on time of cells 30 during the application of write pulses may vary over a relatively wide range so that a cell which turns on early in the pulse period , might develop damaging currents if the current - limiting resistors were not provided . however , these resistors are only needed in one set of lines , either the x lines or the y lines , and , under some circumstances , for example where cells are quite uniform in characteristics , it may be possible to omit the resistors in the write circuit altogether and simply limit the duration of the write pulses to preclude a harmful current build - up in the cells . in the following description of the invention , it will be assumed that , initially , the panel 10 contains no information and none of the cells 30 is on and in glow discharge state . one method of writing information into a cell and sustaining the information in the cell is illustrated in fig3 and 4 and comprises applying sustaining pulses 3 from the sources 130 and 140 directly , either to selected x and y lines associated with cells to be turned on or to all x and y lines at the same time . the peak - to - peak amplitude of the sustaining pulses appearing across each cell is greater than the characteristic d . c . firing voltage vf of the cells and may conveniently be almost twice as large as vf ; however , the time duration of the sustaining pulses and the repetition rate is such that no significant ionization is produced in off cells , and these cells cannot turn on . in operating the panel 10 described above , sustaining pulses have had a peak - to - peak amplitude of 300 volts , a time duration of 1 to 11 / 2 microseconds , and a spacing between pulses of 50 microseconds . when it is desired to turn on a cell , such as cell 30 &# 39 ;, and thus enter information in the panel , write pulses w are applied to the x &# 39 ; and y &# 39 ; lines . the write pulses are applied during the time period between sustaining pulses s and , preferably , immediately after an applied sustaining voltage has terminated . the x and y write pulses together have a peak - to - peak amplitude which is larger , for example , something more than 1 to about 2 times larger , than vf . after the write pulses have terminated , the sustaining pulses continue and sustain cell 30 &# 39 ; on . in a modification of the method of the invention described above , as illustrated in fig5 and using the circuit of fig3 information is written into panel 10 by first applying write pulses from sources 110 and 120 to selected x &# 39 ; and y &# 39 ; lines , before any sustaining pulses have been applied , and then applying the sustaining pulses s to sustain the on state . the write operation and sustaining operation may also be carried out as illustrated in fig6 and 7 by applying sustaining pulses s from sources 130 and 140 to selected x &# 39 ; and y &# 39 ; lines or to all x and y lines , and then increasing the repetition rate ( fig7 ) or the amplitude ( fig8 ) or duration of the sustaining pulses on selected x and y lines for a sufficient time period to permit the selected cells , such as cell 30 &# 39 ; to turn on . the sustaining pulses s on the selected lines are then restored to their normal amplitude or duration or repetition rate to sustain the on state of the cell . it is to be noted that the write pulses or altered sustaining pulses employed to write in fig6 and 7 also appear on all cells associated with the selected x line and all cells associated with the selected y line other than the selected cell ; however , these other cells receive only half pulses or only one - half of the effect of the altered pulses and , accordingly , they do not turn on . only the selected cell or cells at the juncture or junctures of the selected x and y lines receive the full effect of the write pulses or modified sustain pulses and turn on . the theory of breakdown and generation of glow discharge in gases has been described extensively in the literature . according to the theory , the generation of glow discharge in a gas - filled cell requires initially the presence of one or more initiating electrons , and these electrons may be obtained in different ways , for example , by photoemission from an electrode or from an electron - emitting substance such as barium , or from random cosmic radiation , or the like . assuming that the required electron ( s ) are present , when a sufficiently large electric field , for example , that due to the above - described firing voltage or write signal , is applied to the electrodes of the gas cell , this field provides the energy for the complex process which produces breakdown and glow discharge . under the influence of the electric field , various energy transitions occur ; electrons collide with gas atoms , some of which are ionized to form electrons and gas ions , while others are raised to various excited states . in some gases , such as helium , neon , argon , and mercury , electrons of gas atoms can be raised to energy levels known as metastable levels or metastable excited states , and the atom itself is referred to as being in a metastable state . transitions from a metastable state to a lower energy level do not occur spontaneously but can occur , for example , by means of collisions with electrons or photons or with the wall of the enclosure . however , these collisions occur relatively rarely so that the life of a metastable state is relatively long . under ideal conditions , the life may be as long as 10 - 3 sec ., whereas the life of a normal excited state is of the order of 10 - 8 sec . when atoms of several gases are present , the excess energy of the metastable atom of one gas may be given up in exciting or ionizing another . this is known as the penning effect . in the discharge process in a gas mixture including neon , argon , and mercury , metastable atoms of neon may ionize argon atoms , metastable argon atoms may ionize mercury atoms , and all metastable atoms may collide with solids and cause secondary emission of electrons . the ionization process , including the liberation of electrons from the cathode and ionization of the gas atoms and the various actions and interactions with metastable atoms , is cumulative , and at some point in time , the energy changes are such that significant current flows , breakdown occurs , and glow discharge results . other physical phenomena , such as photo - electric effects , may also take part in the breakdown process . of course , the applied write or firing voltage must have sufficient amplitude and time duration to achieve the desired breakdown and glow discharge . it is to be noted that the presence of a gas or gases which can have metastable states is desirable in the present invention , since they are a source of electrons and ions ; however , this is not an absolute requirement . in operation of a panel according to the invention , after a cell has been turned on and the write pulses have been terminated , sustaining pulses are applied to sustain the on state of a cell . each sustaining pulse re - fires the cell and produces a burst of light at such a repetition rate that the glow discharge and light output appear to be continuous . referring again to the cell content after the write signal has been terminated , it is known that electrons leave the gas volume very quickly , and ions and metastable particles diffuse to the walls of the cells more slowly . the sustaining pulses are applied at a time when there are still sufficient electrons and ions present , or being generated by metastable state atoms , to sustain glow discharge , even though the sustaining pulses are of considerably shorter time duration than the write pulses . proper operation of the cells 30 by means of the sustaining pulses requires that sufficient current be generated or a suitable current level be reached to cause glow discharge at a favorable level of light output and power dissipation . in addition , the current level should be such that , if there is cathode sputtering , that this be minimal . conversely , the current must be kept below the level at which arcing or excessive power dissipation might occur . the current must also be kept low enough , depending on the closeness of cells to each other and their coupling aptitude , so that the ionization in one cell will not fire neighboring cells , or cause neighboring cells to be so unstable as to respond to half - select write pulses , ambient light , etc . the key to the efficacy of the present invention , that is the key to the sustaining of glow discharge by means of the sustaining pulses , resides in the &# 34 ; character &# 34 ; of these pulses . by character is meant the total quality of the parameters of the sustaining pulses , the parameters being the amplitude , duration , and repetition rate . these parameters are tailored to achieve the desired operation and to satisfy the required conditions set forth above . the required character of the sustaining pulses is affected by many factors such as cell size and volume , type of gas , gas pressure , etc . ; however , the required sustaining pulses and their use for any particular display panel or circuit can be readily determined by those skilled in the art with minimal experimentation . to achieve proper operation of glow discharge devices and to satisfy the foregoing requirements imposed on the sustaining pulses , the prior art has always considered it necessary to use relatively complex and expensive circuit arrangements including current - limiting resistors or other impedance elements in the leads to either the anode or cathode electrodes in each cell . according to the present invention , the desired operation is achieved by the sustaining pulses alone and without current - limiting resistors or other relatively complex circuit arrangements . as described above , the sustaining pulses can be applied to a panel before a write signal is applied , in which case , cells are unaffected , or they may be applied after a writte signal has been applied and a cell has been turned on , in which case , the cell ( s ) which is on is sustained on , and cells which are off are not affected . the method of the invention permits relatively wide variations in the parameters or character of the sustaining pulses , for which the desired sustaining operation can be achieved . however , outside the range of these permissible variations , on cells might turn off spontaneously , or off cells might turn on spontaneously . the method of the invention also tolerates variations in the electrical characteristics , particularly firing voltage , from cell to cell in the display panel . since the write and sustain operating signals or pulses are so much larger in amplitude than vf , even if vf varies from cell to cell , the operation takes place at a level which is beyond that at which it could be affected by these variations in vf . variations which might occur in the circuit parameters are generally of no significance for the same reason . an auxiliary mechanism known as sweepout may also be used in the present invention . the theory of sweepout is as follows . as described above , ions and electrons are continually being generated by the action of metastable atoms during the time between successive sustaining pulses . also , they are generated by the sustaining pulses themselves while these pulses are present . in addition , it is possible for spurious ionization to occur due to photoelectric effects , field emission , coupling betwen cells , etc ., so that , under some circumstances , off cells can turn on spuriously . in order to minimize this sensitivity of off cells to spurious ionization effects , the invention utilizes an electric field known as a sweepout field to remove ions from cells at a rate greater than the normal diffusion rate . the sweepout field does not affect uncharged particles , such as the metastable atoms , but it does attract the charged particles to one of the cell electrodes , depending upon the polarity applied , and thus removes them . the use of a sweepout field to remove ions renders the operation of sustaining glow discharge by the sustaining pulses dependent primarily on the generation of new ions and electrons , especially those generated by the metastable atoms by the penning effect or by the electron emission - inducing action of the metastable atoms at the surfaces of the electrodes . when a sweepout field is employed in operating a panel , a d . c . sweepout voltage may be used , but sweepout pulses are preferred , and such pulses are paired as partners with the sustaining pulses in the operating signal routine . when used in a normal sustaining operation , sweepout pulses are of relatively low amplitude , well below the firing voltage . typical pulses are 10 volts in amplitude and 5 usec . in duration . the effectiveness of sweepout pulses can be controlled by adjusting their amplitude , time duration , and timing with respect to the other signals used in operating a cell . to maximize their effectiveness , they should be applied immediately before the sustaining pulses , as shown schematically in fig9 . however , because the sweepout field removes ions and electrons , in order to achieve the desired sustaining effect of the sustaining pulses , as described above , an appropriate increase in one or more of the parameters of the sustaining pulses must be made . sweepout signals may be used not only for minimizing the sensitivity of off cells to spurious turn on , but they may be used in conjuction with sustaining signals to write information into a panel , as follows . referring to fig1 , in this method of writing information , sustaining pulses s and sweepout pulses sw are applied to all x lines and all y lines , with the proper time spacing between them and with the required signal isolation provided . in this operating routine , the sustaining pulses are set at about the maximum amplitude , duration and repetition rate permissible without having cells spontaneously turn on , and without having the writing operation , to be described , turn on non - selected cells . then to write or turn on a cell 30 &# 39 ;, the sweepout signals are removed from the x &# 39 ; and y &# 39 ; lines which intersect at cell 30 &# 39 ; and the sustaining pulses now applied across cell 30 &# 39 ; have such character as to cause cell 30 &# 39 ; to turn on . the other cells intersected by the x &# 39 ; and y &# 39 ; lines separately receive a signal routine which includes the full value of sustaining pulses but only half of the full value of the sweepout pulses , since sweepout pulses are present on only one of the lines associated with these other cells . this signal routine is of such character that these other cells intersected by the x &# 39 ; and y &# 39 ; lines cannot turn on . finally , after writing is completed , or the write period is terminated , the sustaining pulses are restored to such character that they , alone or in conjunction with sweepout pulses , can sustain glow at the desired level and power consumption in the cells just turned on . as set forth above under the heading write and sustain operations , and as illustrated in fig4 a display panel can be operated by a signal routine which includes applying sustaining pulses and then applying write pulses to turn on a particular cell ( s ). in this type of routine , the write pulses should be applied as close to the end of a sustaining pulse as possible , and , where sweepout pulses are used , as far from the next sweepout and sustaining pulse as possible . now that the sweepout mechanism has been explained , the reason for this preferred spacing of the write pulses becomes clear . since the combination of sustaining pulse and sweepout pulse is adjusted to provide the desired sustaining operation , if the write pulses are applied too close to the start of a sustaining pulse , the half - write pulses on the non - selected cells might have the effect of a large sweepout pulse and might cause cells which are on to turn off . sweepout signals can also be used in conjunction with sustaining pulses to erase information from a panel , that is , to turn on cell ( s ) off . referring again to fig1 , sustaining pulses are applied to all x lines and y lines to sustain on a cell ( s ) which has been turned on , and they are adjusted to be near the minimum amplitude or time duration or repetition rate necessary to sustain the on state . to erase a cell ( s ), sweepout signals sw are applied to the selected x &# 39 ; and y &# 39 ; lines which intersect at the cell ( s ) to be erased . if the signal routine already includes sweepout pulses , then the amplitude or time duration of the sweepout signals on the selected x &# 39 ; and y &# 39 ; lines is increased , or the sweepout pulses are shifted closer to the beginning of the associated sustaining pulses which follow them in time . as above , the other cells on the x &# 39 ; and y &# 39 ; line , which may be on and are not to be erased , receive only half of the applied erase sweepout signals so that , assuming that the sustaining pulses are of the proper character , these unselected cells are maintained on during this erase operation . according to still another method of erasing on cells , alternate sustaining pulses are omitted on selected x &# 39 ; and y &# 39 ; lines , and , after several pulses are omitted , a cell which had been on will turn off . other cells on the selected x &# 39 ; and y &# 39 ; lines which had been on are not affected by this routine since they receive alternately half - value and full - value sustaining pulses . as a further alternative , the sustaining pulses on the x &# 39 ; lines and those on the y &# 39 ; lines may be alternately omitted so that the cells selected to be erased will have only half - value sustaining pulses for a time , while the non - select cells have alternate half - value and full - value sustaining pulses . another method of erasing an on cell ( s ) is illustrated in fig1 and 12 and utilizes the fact , illustrated in the curve of fig1 , that a sweepout voltage of about 0 . 8 vf applied across a cell by means of short duration pulses can act as an erase voltage and can turn an on cell off . in the description of this method , it is assumed that it is desired to turn off cell 30 &# 39 ; which is at the intersection of lines x &# 39 ; and y &# 39 ;, and it is further assumed that sustaining pulses are being applied continually to sustain cell 30 &# 39 ; in the on state as described above . to turn off cell 30 &# 39 ;, immediately before a sustaining pulse would normally be applied , a pulse approximately equal to the firing voltage of the cell and of very short duration is applied to the line x &# 39 ; and a pulse of the same polarity but of approximately 20 % of the amplitude of the first pulse is applied to line y &# 39 ;. in addition , pulses of about 0 . 1 vf are preferably applied to the other x lins , but this is not essential . the net voltage which appears at cell 30 &# 39 ; is about 0 . 8 vf , and this acts as a strong sweepout and turns off cell 30 &# 39 ;. the net voltages appearing across all other cells are shown in fig1 , and these cells which carry vf or 0 . 1 vf , or even 0 . 2 vf , are not significantly disturbed . having in mind the various factors and functions already described , it may be well to consider a typical cycle of operation of the described panel . referring to fig1 and assuming that binary logic information &# 34 ; 101 &# 34 ; is to be written into and displayed in the lefthand y column of gas cells 30a , 30d , 30g , by rendering the respective cells from top to bottom on - off - on , the following methods of operation may be employed . sustaining pulses of the proper polarity are applied to all of the x lines and all of the y lines , and then write pulses w are applied to lines x1 , x3 , and y1 , preferably close to the end of a sustaining pulse as described above . this causes cells 30a and 30g to fire and provide visible glow , and this glow is sustained by the continuing sustaining pulses . if desired , the sustaining pulses need be applied only to lines x1 , x3 , and y1 . sweepout fields may be used in connection with this method , or in connection with any of the following methods . as already discussed , these fields may be produced by a d . c . voltage applied to all of the cells , or only to the selected cells , but are preferably produced by pulses applied to all or selected ones of the cells just prior to the start of each sustaining pulse . in this method of operation , write pulses are first applied to lines x1 , x3 , and y1 to turn on cells 30a and 30g , and then sustaining pulses s are applied either to lines x1 , x3 , and y1 , or to all of the lines . in this method , sustaining pulses are applied either to lines x1 , x3 , and y1 , or to all of the lines , and then either : 1 . the repetition rate of the sustaining pulses is increased on lines x1 , x3 , and y1 until cells 30a and 30g turn on , and then the repetition rate is returned to the normal sustaining rate , or 2 . the amplitude of the sustaining pulses is increased on lines x1 , x3 , and y1 until cells 30a and 30g turn on , then the amplitude of the sustaining pulses is returned to the normal sustaining level , or 3 . the time duration of the sustaining pulses on lines x1 , x3 , and y1 is increased until cells 30a and 30g turn on , and and the duration is returned to the normal duration for the sustaining operation . a fourth method involves the application of sweepout pulses and sustaining pulses to all of the x and y lines . the sweepout pulses are preferably timed so that one occurs just prior to the start of each sustaining pulse , and the sustaining pulses preferably have an amplitude , duration and repetition rate just below that which would cause any of the cells to be turned on spontaneously . to write , the sweepout pulses are selectively removed from lines x1 , x3 and y1 . thereafter , the sustaining pulses may be reset to their normal level and the sweepout pulses either dropped or retained . this method could employ a d . c . sweepout voltage , rather than sweepout pulses , with the voltage being discontinued or offset by an oppositely directed voltage , on the selected lines x1 , x3 and y1 . however , the use of sweepout pulses is preferred . any of the erase techniques already discussed may be used in connection with the writing and sustaining techniques of methods i to iv . for example , the stored display of binary number 101 , in the lefthand column of the panel in fig1 , can be erased by ( 1 ) temporarily omitting the sustaining pulses from all of the x and y lines or from all of the x or y lines alone , which erases the entire panel , or ( 2 ) temporarily omitting alternate sustaining pulses from lines x1 , x3 and y1 , or ( 3 ) temporarily omitting sustaining pulses from the relevant x lines ( x1 and x3 ) and line y1 , alternately , ( 4 ) temporarily changing the character of the sustaining pulses , on a panel - wide basis or selective basis , in any way which would render these pulses insufficient to sustain the glowing cells , i . e ., by reducing the amplitude or duration or repetition rate of these pulses , or some combination of these parameters . also , erase can be accomplished by ( 5 ) the application of sweepout signals on an overall or selective basis , preferably in the form of pulses applied just prior to the successive sustaining pulses , or ( 6 ) where the applied pulse pattern already includes a sweepout signal , by an overall or selective increase in the sweepout signal , or a shift of the sweepout pulses closer to the beginning of the sustaining pulses which follow them in time , or ( 7 ) the application of sweepout voltages in keeping with the curve of fig1 , as already discussed .