US3778675A

US3778675A - Circuit for operating multiple position display tubes

Info

Publication number: US3778675A
Application number: US00324023A
Authority: US
Inventors: Harvey E Lloyd
Original assignee: Burroughs Corp
Current assignee: Unisys Corp
Priority date: 1973-01-16
Filing date: 1973-01-16
Publication date: 1973-12-11
Anticipated expiration: 1990-12-11

Abstract

The disclosed apparatus is adapted for operating multiple position display devices having several groups of display cathode segments or elements, with corresponding elements electrically interconnected, an anode associated with each group of segments, and an auxiliary electrode common to all of the groups of electrodes for electrically isolating the electrode groups and preventing spurious glow between them. The subject circuits include a common impedance or a voltage divider connected to both the auxiliary electrode and the cathode drivers to bias the auxiliary electrode proportional to a reference potential and to vary the common impedance inversely proportional to the number of cathode segments being energized, for inhibiting electrical discharge between the cathode segments and any other than the associated anode. Apparatus is also provided for biasing the OFF cathodes and the OFF anodes in a non-conductive state when not energized.

Description

United States Patent Harv [ Dec. 11, 1973 Primary ExaminerRudolph V. Rolinec Assistant Examiner-Marvin Nussbaum Attrney-George L. Kensinger et al.

[75] Inventor: Edgar Lloyd Harvey, Old Bridge,

N.J. [57] ABSTRACT [73] Asslgnee: ai L Corporatmn Detrolt The disclosed apparatus is adapted for operating mul- 1c tiple position display devices having several groups of [22] Filed: Jan. 16, 1973 display cathode segments or elements, with corresponding elements electrically interconnected, an [211 App! 324023 anode associated with each group of segments, and an Related US. Application Data auxiliary electrode common to all of the groups of [63] Continuation of 126,325 March 22 1971 electrodes for electrically isolating the electrode abandonei groups and preventing spurious glow between them. The subject circuits include a common impedance or [52] S. Cl 315/169 R, 315/334, 3l5/336, a voltage divider connected to both the auxiliary elec- 315/169 TV trode and the cathode drivers to bias the auxiliary 51 Cl 013 17 4 051 37 00 41 00 electrode proportional to a reference potential and to [58] Field of Search 315/167, 168, 169 R, y the common impedance inversely proportional to 315/169 TV, 334, 336; 313/1095 the number of cathode segments being energized, for inhibiting electrical discharge between the cathode [56] R f n Ci d segments and any other than the associated anode.

UNlTED STATES PATENTS Apparatus is also provided for biasing the OFF cathodes and the OFF anodes in a non-conductive. state 2,906,906 9/1959 McCauley et al 313/1095 when not ener ized 3,509,420 4/l970 Ogle 315/169 R g 21 Claims, 3 Drawing Figures ragga 210 I SEQUENCER l4 "24H #251 29 I 212% 0 H0O i :I A 255 295 T 250 I 290 id A I32 S m 245 2|5 e R L- 34 -35 39 I20 E 133 r W 23 7 V 7 V 230 /7-\ i I40 ,4! 150 5! I 9 21, 04 V U o I34 115 22 A A 25 x5 55 e imo PMENIEDnEc 1 1 I975 SHEET 2 0F 2 EDGAR L. HARVEY mwUZm30mm OOT ATTORNEY CIRCUIT FOR OPERATING MULTIPLE POSITION DISPLAY TUBES This patent application is a continuation of application Ser. No. 126,825, filed Mar. 22, 1971, now abandoned.

BACKGROUND OF THE INVENTION The invention relates to circuitry for operating multiple position gas discharge devices such as display panels having a plurality of interconnected cathode elements positioned in a row. More particularly, the invention relates to the operation of multiple position gas discharge devices having a plurality of segmented cathode electrodes for displaying characters side-by-side.

A multiple position display device which is available commercially is known as the PANAPLEX numeric panel display. This type of display panel includes a plurality of groups of display cathode elements in the form of elongated bars or segments. Each group of segments is arrayed in a figure 8 pattern or the like and the various segments, together with an anode, can be selectively energized to display a character at any desired cathode group or position by discharge in the gas about the electrodes. The electrical discharge results in cathode glow on the selectively energized segments which displays the desired character.

One such display panel includes an auxiliary electrode common to all the electrode groups which is biased intermediate the cathode and anode potentials for preventing spurious electrical discharges and the resultant spurious glow among the electrodes, as described and claimed in Harvey et al, S.N. 78,045, filed Oct. 5, 1970 now abandoned. The auxiliary electrode, however, receives a current from ionization in the device which tends to raise its potential in proportion to the number of cathode segments being energized and this reduces its effectiveness. At the same time, the initial bias on the auxiliary electrode cannot be reduced enough to offset completely this rise in potential when cathode segments are selected since the auxiliary electrode then tends to develop cathode glow itself.

Two recently developed circuits for operating such display panels are disclosed and claimed in G. E. Holz, et al, SN. 85,662, filed on Oct. 30, 1970 and in G. E. Holz, S.N. 87,058, filed on Nov. 5, 1970 now Pat. No. 3,694,693, issued Sept. 26, 1972. Those circuits, while suitable for operating many panels under certain conditions of operation, are limited in flexibility and introduce undesired costs or complexity to such panel displays.

SUMMARY OF THE INVENTION Accordingly, an object of this invention is to improve spurious glow suppression in multiple position display devices having interconnected cathode elements or segments.

Another object of the invention is to simplify and reduce the cost of such multiple position panel displays having auxiliary electrodes for spurious glow suppres sion.

In accordance with these objects, a circuit of the invention includes common impedance or voltage dividing means coupled between first and second reference terminals for the display device and having a junction to which is connected the auxiliary glow suppression electrode of a multiple position display panel, together with the output of each of the cathode drivers including second impedance means. The bias potential applied to the auxiliary electrode for suppressing spurious glow is thereby maintained proportional to the supply potential on the device. The effective biasing impedance is changed in inverse proportion to the number of cathode segments being energized since the cathode drivers, when activated, are in electrical parallel with the common impedance, draw current from the voltage dividing means, and reduce the potential on its junction. The circuit also includes means for biasing the OFF cathodes and the OFF anodes in a non-conductive state when not energized.

DESCRIPTION OF THE DRAWINGS Other objects and features of the subject invention will become clear from the following detailed description wherein:

FIG. I is an isometric view of a segmented-cathode display tube having individual anodes and an overlying auxiliary electrode;

FIG. 2 is an elevational view of a longitudinal section of the display tube or panel of FIG. I; and

FIG. 3 is an electrical schematic diagram of a preferred embodiment of the circuit of applicants invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The principles of the invention are particularly applicable to operation of the type of display device or panel described and claimed in the above-identified Harvey, et al. patent application. Some of the features of such a device as shown in FIGS. l and 2 for a display panel or tube 10 having three representative display positions. A plurality of groups of cathode segments 41-47, 51-57, and 61-67 are connected to corresponding conductors 2l.-27 which are spaced from each other on an insulating base plate 20 or within grooves in its surface as shown in FIG. 2. Ari

anode electrode

140, 150, 160, is provided for each group of segments. An insulating sheet or layer 30 shields cathode conductors 21-27 from the anodes, as shown in FIG. 2, to prevent them from glowing.

The anodes in FIG. 1 are each shown as a ring surrounding the associated grottps of cathodes, but they may be screen electrodes situated above the associated cathode groups, or they may take any other desired shape. In addition, the device includes an auxiliary screen electrode 230 disposed above the groups of cathodes and anodes and electrically associated with all of the electrode groups. Top and bottom

insulating plates

200 and 20, respectively, sealed together by a glass or ceramic frit 15 or the like, complete the panel or tube structure.

Briefly, in operation of the panel or tube shown in FIGS. 1 and 2, drive signals are applied to the cathodes via conductors 21-27 in accordance with the input information. Simultaneously, the anodes are energized in turn from left to right, or in any other desired order or sequence via

leads

34, 35, 36 in synchronism with the cathode drive signals, to cause the cathode segments in the selected groups to glow. A suitable bias potential intermediate the cathode and anode bias potentials is applied to lead 235 of screen electrode 230 for preventing spurious glow about the cathode segments and between adjacent display positions. The biased auxili ary electrode prevents extraneous discharges from occurring between an energized anode and the cathodes in adjacent groups which have operating potential applied to them.

A preferred embodiment of the invention is illustrated in FIG. 3 in which display device 19 is shown schematically. Display segments All, 51, 91 are connected to cathode conductor 21 which is driven from the collector of NPN transistor 121. Likewise, cathode segments 42-92, 13-93, 44-5 4 and 65-95 are connected to the collectors of

cathode drivers

122, 123, 124 and 125 by

conductors

22, 23, 2d and 25, respectively Cathode conductors 21-25 are also coupled to the cathode of Zener diode 118 at circuit junction by resistors 131-135, respectively. The anode of Zener diode 118 is connected to B reference terminal 120 and resistor 14 couples circuit junction 15 to E reference terminal 12.

A voltage

divider comprising resistors

13 and 19 is connected between anode bias terminal 12 and cathode bias terminal 121). The collectors of cathode driver transistors 121-125 are coupled to voltage divider junction 16 by resistors 111-115 connected in series with diodes 101-105, respectively. The cathodes are initially biased OFF by the voltage on circuit junction 15. The difference in potential between the OFF cathodes and the ON anodes must not exceed the ionization voltage in the panel.

The base electrodes of the cathode driver transistors 121-125 are connected to data source 100 for receiving positive-going information signals and they either limit cathode current in the device when operated or are connected to suitable current-limiting impedances. Data source 100 may include a computer and associated elements, a character generator or suitable decoders and encoders for providing information signals to the cathode drivers.

Anode leads 34-39 of anodes 140, 151 190 are connected to the outputs of anode drivers 240, 251), 290 and to the cathodes of

diodes

245, 255, 295, the anodes of which are connected to the cathode of Zener diode 218 at its junction 215 with resistor 212. The

input terminals

241, 251, 291 of the anode drivers are connected to a suitable sequencing circuit or device 210 for receiving anode control signals. The

anode drivers

240, 250, 290 are referenced to E potential terminal 12, as shown, and apply a suitable positive potential to each anode in turn under control of the sequencer.

The anode electrodes 1411-1911 are initially biased OFF through diodes 2&5, 255, 295 by the voltage across Zener diode 218, connected in series with resistor 212 between the voltage supply terminals 12 and 120. The potential difference between the OFF anodes and the ON cathodes is less than the ionization voltage for the device,

The display panel or tube 11 is operated in the multiplex mode. information signals are applied to cathode drivers 121-125 from data source 100 and, at the same time, operating potential is applied to the anodes in turn. As each anode is energized, the appropriate information signals are applied to the cathodes so that the desired information is displayed at each position.

Auxiliary electrode 230 is usually biased at a potential intermediate the anode and the cathode bias potentials from a voltage source having appreciable internal impedance or other impedance connected between them. During operation, auxiliary electrode 231) then conducts a leakage current from whichever anode Mid-1%) is energized, into the associated voltage source or impedance device. This raises the potential on the auxiliary electrode as more cathode segments are energized or as the cathode current is increased. This rise in the potential on the auxiliary electrode toward the anode potential reduces the effectiveness of the auxiliary electrode in suppressing spurious glow in the device.

in the present embodiment, electrode lead 235 of auxiliary electrode 2311 is connected to junction 16 of the common

voltage divider resistors

13 and 19, as are the collectors of cathode driver transistors 121-125, through resistors 111-115 and diodes 1111-1115. The voltage divider is, therefore, connected in common to the auxiliary electrode and to the output terminals of the cathode drivers. Since the emitters of cathode drivers 121-125 and resistor 19 of the voltage divider are all connected to E reference terminal 121), the resistors 111-115 in the driver collector circuits effectively appear in parallel to resistor 19 when the drivers are operated and reduce the effective biasing resistance for auxiliary electrode 2311 accordingly.

Since all of the cathodes are connected to circuit junction 16 of the voltage divider, current is conducted from voltage divider resistor 13 by the actuated ones of cathode drivers 121-125. These currents in resistor 13 tend to drop the potential on the auxiliary electrode in opposition to the potential rise caused by the abovementioned leakage current conducted by it from the actuated electrode. By proper selection of the values of

voltage divider resistors

13 and 19 and by-pass or coupling resistors 111-115, the opposing changes in potential on auxiliary electrode 230 may be made to counteract each other, without significantly affecting the glow inhibiting potential on the auxiliary electrode. The auxiliary electrode is thus held at or near the optimum potential for spurious glow suppression regardless of how many cathodes are energized or the particular pattern being displayed.

The incorporation of diodes 1111-1115 between the cathode driver transistors and the common voltage divider is made necessary by the direct biasing of the OFF cathodes to the E reference potential terminal 12 through resistors 1d and 131-135. Diodes 1111- prevent the OFF cathode bias potential on cathode conductors 121-125 from affecting the potential on junction 16 of the voltage divider and, consequently, the glow inhibiting potential applied to auxiliary electrode 231).

in operation of panel 1% to display a plurality of numerals, negative information signals are applied to one or more cathode conductors 21-27 from a suitable data source 1110, and a relatively positive potential may be applied to the first anode 1 m. The selected ones of cathode segments 41-47 which are thus energized, will glow and display a numeral at the first display position. A second group of information signals is applied to selected ones of cathode conductors 21-27 and the second anode 150, for example, is energized. The second numeral is displayed by the selected cathode segments 51-57 at the second display position. in the same way, the successive application of different information signals to selected cathode conductors and to the anodes, in turn, causes a character to be displayed at each character position. if this sequence is repeated through the display device from one end to the other at a sufficiently high rate, a stationary, but changeable, series of numerals can be displayed in the panel, one at each character position, if desired. As the cycling operation is carried out, the characters at the respective display positions change if the information signals on the cathode conductors change.

The above-described mode of operation utilizes sequential time-shared driving of

anode electrodes

140, 150, 160, etc. of the device. This sequentially energizes the successive display positions along the device. Another mode of operation involves time-shared driving of the cathode electrode segments. In this mode, the

anodes

140, 150, 160, etc. are selectively energized from a data source, for example, and cathode conductors 21-27 are energized successively for sequencing the display. The energization of

anode electrodes

140, 150, 160, etc. is changed as desired for varying the pattern of the display. The anodes and cathodes may be energized in any other desired order or sequence, also, depending upon the desired mode of display.

Although the preferred embodiment of the invention has been described in detail, it should be understood that the present disclosure has been by way of example only. Many modifications and variations of the invention are possible in light of the above teachings. it is, therefore, to be understood that the invention may be practiced otherwise than as specifically disclosed.

What is claimed is: 1. Apparatus for operating multiple-position display devices having a plurality of interconnected groups of cathode elements, an anode associated with each of the groups and an auxiliary electrode that is common to all of the groups and shields them from each other comprising a plurality of drivers having their output terminals coupled to corresponding ones of the interconnected cathode elements for energizing them selectively in response to a first set of input signals,

voltage dividing means coupled between a pair of reference terminals and having a divider junction,

means coupled to each of the anodes for energizing them selectively in response to a second set of input signals, and being biased in common with the voltage dividing means from one of said reference terminals,

the auxiliary electrode being coupled to the divider junction to be biased at a level that is directly proportional to the anode biasing potential for preventing spurious glow in the device irrespective of variations in the bias potential available for the device.

2. The apparatus for operating multiple-position display devices of claim 1 wherein the cathode drivers are also coupled to the divider junction so that the impedance of a portion of the divider will vary as an inverse function of the number of cathode elements that are being driven, to prevent spurious glow in the device irrespective of the display pattern.

3. The apparatus for operating multiple-position display devices of claim 2 wherein the output circuits of the cathode drivers are each coupled to the divider junction by impedance means and appear in electrical parallel with a portion of the divider when the cathode drivers are activated.

4. The apparatus defined in claim 3 wherein the voltage dividing means comprises a resistive voltage divider and the cathode driver output terminals are each coupled to the voltage divider junction by resistance means.

5. The apparatus for operating multiple-position display devices of claim 2 wherein the cathode drivers are each coupled in electrical parallel with a portion of the voltage dividing means.

6. The apparatus defined in claim 5 wherein the voltage dividing means comprises a resistive voltage divider and the cathode driver output terminals are each coupled to the voltage divider junction by resistance means.

7. The apparatus for operating multiple-position display devices of claim 1 wherein the means for energizing the anodes includes a driver for each of the anodes coupled for driving them responsive to the second set of input signals and in synchronism with the first set of input signals applied to the cathode drivers.

8. The apparatus defined in claim 7 further comprising a voltage reference device connected in series with an impedance means between a pair of reference terminals and the anodes each being coupled to the junction of them by reverse-biased assymetrically conductive means.

9. A character display circuit comprising an envelope containing an ionizable gas,

an anode and a plurality of cathodes within said envelope, each of said cathodes having the shape of a segment of a character to be displayed,

circuit means for establishing a glow potential betwee h said anode and different combinations of said cathodes to glow and thereby display different characters,

said circuit means including a plurality of cathode drivers, one connected to each of said cathodes, and means for selectively energizing said cathode drivers to render their connected cathodes glow ing,

an auxiliary electrode in contact with the ionizable gas and in operative relation with said anode and cathodes, the current through said auxiliary electrode being a function of the number of cathodes which are glowing, and bias circuit means for biasing said auxiliary electrode to a voltage intermediate the operating voltage of said anode and said cathodes, said bias circuit means including at least one impedance device electrically connected to said auxiliary electrode,

said cathode drivers being connected electrically across at least a portion of said impedance device and serving to change the level of the impedance connected to said auxiliary electrode as each cathode driver is energized.

lll. A character display circuit as in claim 9 wherein said current passesfrorn the anode through the ionizable gas to the auxiliary electrode, and at least a portion of said current flow passes through said impedance device and tends to increase the voltage on the auxiliary electrode as the number of glowing cathodes increases, and

wherein the level of the impedance connected to the auxiliary electrode is reduced as each additional cathode driver is energized, to compensate for the tendency of the auxiliary electrode to increase in voltage as the number of glowing cathodes increases.

11. A character display circuit as in claim 9 wherein the cathode drivers are transistors, and

wherein the means for selectively energizing said cathode drivers comprises circuit means connected to the transistors to render them more conductive in response to input signals, and

wherein the transistors are all connected across at least a portion of the impedance device to reduce the effective impedance connected to said auxiliary electrode for each transistor that is energized.

12. A character display circuit as in claim 9 further including a plurality of additional impedance devices, one connected in series with each of said current drivers and the series circuit is connected across at least a portion of the first-mentioned impedance device, and

wherein the current drivers are rendered conductive when energized, so that as each driver is energized its associated additional impedance device is effectively connected across the first-mentioned impedance device.

13. A character display circuit as in claim 9 wherein the impedance device comprises a voltage divider having a junction connected to the auxiliary electrode, a predetermined impedance connected from said junction to a first reference potential terminal, and a second predetermined impedance connected from said junction to a second reference potential terminal.

14. A character display circuit as in claim wherein each cathode driver is connected in series with an additional impedance driver, and each such series is connected from said junction to one of said reference potential terminals.

15. A circuit to compensate for voltage changes on an auxiliary electrode located in a gaseous atmosphere of a glow discharge indicator tube which has an anode and a plurality of cathodes in the gaseous atmosphere in operative relation to the auxiliary electrode, each of the cathodes being in the shape of a segment of a character to be displayed and being capable of exhibiting cathode glow when energized, comprising circuit means for energizing said anode and different combinations of said cathodes to display different characters,

said circuit means including a plurality of cathode drivers, each connected to one of said cathodes and serving to energize the cathode to cause it to glow response to an input signal, and

bias circuit means for biasing said auxiiiary electrode to a voltage intermediate the operating voltage of said anode and said cathodes,

said bias circuit means including at least one impedance device connected electrically to said auxiliary electrode, the current through the auxiliary electrode to said impedance device being a function of the number of cathodes which are glowing, and

said cathode drivers being connected electrically across at least a portion of said impedance device to modify the effective impedance connected to said auxiliary electrode as each of the cathode drivers responds to an input signal to cause its connected cathode to glow, so as to compensate at least in part for any voltage change which the increased current through the auxiliary electrode tends to produce as an increased number of cathodes glow.

16. A voltage compensation circuit as in claim wherein the impedance device comprises a voltage divider having a junction connected to the auxiliary electrode, a first predetermined impedance connected from said junction to a first reference potential terminal, and a second predetermined impedance connected from said junction to a second reference potential terminal.

17. A voltage compensation circuit as in claim 16 further including a plurality of resistors, one connected in series with each of said cathode drivers, and

wherein said cathode drivers are transistors, so that a plurality of resistor-transistor series circuits are connected from said junction to one of said reference potential terminals, and

wherein as each transistor is energized it causes its connected cathode to glow and its seriesconnected resistor to be effectively connected across one of said predetermined impedances.

18. A voltage compensation circuit as in claim 17 wherein said first and second predetermined impedances are first and second resistors.

19. A voltage compensation circuit as in claim 15 further including an anode voltage source terminal and circuit means for connecting said terminal to said anode,

a cathode voltage source terminal, with the cathode drivers being connected between the respective cathodes and said cathode voltage source terminal, and

wherein said impedance device comprises a voltage divider having a junction connected to said auxiliary electrode, a first predetermined impedance connected from said junction to said anode voltage source terminal, and a second predetermined impedance connected from said junction to said cathode voltage source terminal.

20. A character display circuit for displaying any one ofa plurality of characters in each of a plurality of sideby-side character positions, comprising a plurality of anodes, one in each of said character positions,

a piurality of groups of cathodes, each such group being associated with one of said character positions, each of the cathodes having the shape of a segment of one of the characters to be displayed,

each of said anodes and its associated cathodes being disposed in an envelope having an ionizahle gas atmosphere capable of sustaining cathode glow,

a plurality of cathode conductors each electrically connected to one of the cathodes in each character position,

a plurality of cathode drivers, each connected to one of said cathode conductors, for energizing said cathodes selectively in response to a first set of input signals,

circuit means for selectively energizing said anodes, in synchronism with the selective energization of said cathodes, in response to a second set of input signals, for displaying selected characters in each of said character positions,

an auxiliary electrode in the gaseous atmosphere associated with each character position, the current flow therethrough being a function of the number of cathodes which are glowing, and

bias circuit means for maintaining an electrical potential within a predetermined range on said auxiliary electrode, irrespective of the number of cathodes glowing,

cathode driver energizes one of said cathodes. 21. A character display circuit as in claim 20 wherein the auxiliary electrode contains a portion extending between the anode and cathodes of each character position and those of the next adjacent character position, to electrically isolate the electrodes of the respective character positions from each other.

Claims

1. Apparatus for operating multiple-position display devices having a plurality of interconnected groups of cathode elements, an anode associated with each of the groups and an auxiliary electrode that is common to all of the groups and shields them from each other comprising a plurality of drivers having their output terminals coupled to corresponding ones of the interconnected cathode elements for energizing them selectively in response to a first set of input signals, voltage dividing means coupled between a pair of reference terminals and having a divider junction, means coupled to each of the anodes for energizing them selectively in response to a second set of input signals, and being biased in common with the voltage dividing means from one of said reference terminals, the auxiliary electrode being coupled to the divider junction to be biased at a level that is directly proportional to the anode biasing potential for preventing spurious glow in the device irrespective of variations in the bias potential available for the device.

9. A character display circuit comprising an envelope containing an ionizable gas, an anode and a plurality of cathodes within said envelope, each of said cathodes having the shape of a segment of a character to be displayed, circuit means for establishing a glow potential between said anode and different combinations of said cathodes to glow and thereby display different characters, said circuit means including a plurality of cathode drivers, one connected to each of said cathodes, and means for selectively energizing said cathode drivers to renDer their connected cathodes glowing, an auxiliary electrode in contact with the ionizable gas and in operative relation with said anode and cathodes, the current through said auxiliary electrode being a function of the number of cathodes which are glowing, and bias circuit means for biasing said auxiliary electrode to a voltage intermediate the operating voltage of said anode and said cathodes, said bias circuit means including at least one impedance device electrically connected to said auxiliary electrode, said cathode drivers being connected electrically across at least a portion of said impedance device and serving to change the level of the impedance connected to said auxiliary electrode as each cathode driver is energized.

10. A character display circuit as in claim 9 wherein said current passes from the anode through the ionizable gas to the auxiliary electrode, and at least a portion of said current flow passes through said impedance device and tends to increase the voltage on the auxiliary electrode as the number of glowing cathodes increases, and wherein the level of the impedance connected to the auxiliary electrode is reduced as each additional cathode driver is energized, to compensate for the tendency of the auxiliary electrode to increase in voltage as the number of glowing cathodes increases.

11. A character display circuit as in claim 9 wherein the cathode drivers are transistors, and wherein the means for selectively energizing said cathode drivers comprises circuit means connected to the transistors to render them more conductive in response to input signals, and wherein the transistors are all connected across at least a portion of the impedance device to reduce the effective impedance connected to said auxiliary electrode for each transistor that is energized.

12. A character display circuit as in claim 9 further including a plurality of additional impedance devices, one connected in series with each of said current drivers and the series circuit is connected across at least a portion of the first-mentioned impedance device, and wherein the current drivers are rendered conductive when energized, so that as each driver is energized its associated additional impedance device is effectively connected across the first-mentioned impedance device.

14. A character display circuit as in claim 5 wherein each cathode driver is connected in series with an additional impedance driver, and each such series is connected from said junction to one of said reference potential terminals.

15. A circuit to compensate for voltage changes on an auxiliary electrode located in a gaseous atmosphere of a glow discharge indicator tube which has an anode and a plurality of cathodes in the gaseous atmosphere in operative relation to the auxiliary electrode, each of the cathodes being in the shape of a segment of a character to be displayed and being capable of exhibiting cathode glow when energized, comprising circuit means for energizing said anode and different combinations of said cathodes to display different characters, said circuit means including a plurality of cathode drivers, each connected to one of said cathodes and serving to energize the cathode to cause it to glow in response to an input signal, and bias circuit means for biasing said auxiliary electrode to a voltage intermediate the operating voltage of said anode and said cathodes, said bias circuit means including at least one impedance device connected electrically to said auxiliary electrode, the current through the auxiliary electrode to said impedance device being a function of the number of cathodes which are glowing, and said cathode drivers being connected electrically across at least a portion of said impedance device to modify the effective impedance connected to said auxiliary electrode as each of the cathode drivers responds to an input signal to cause its connected cathode to glow, so as to compensate at least in part for any voltage change which the increased current through the auxiliary electrode tends to produce as an increased number of cathodes glow.

16. A voltage compensation circuit as in claim 15 wherein the impedance device comprises a voltage divider having a junction connected to the auxiliary electrode, a first predetermined impedance connected from said junction to a first reference potential terminal, and a second predetermined impedance connected from said junction to a second reference potential terminal.

17. A voltage compensation circuit as in claim 16 further including a plurality of resistors, one connected in series with each of said cathode drivers, and wherein said cathode drivers are transistors, so that a plurality of resistor-transistor series circuits are connected from said junction to one of said reference potential terminals, and wherein as each transistor is energized it causes its connected cathode to glow and its series-connected resistor to be effectively connected across one of said predetermined impedances.

19. A voltage compensation circuit as in claim 15 further including an anode voltage source terminal and circuit means for connecting said terminal to said anode, a cathode voltage source terminal, with the cathode drivers being connected between the respective cathodes and said cathode voltage source terminal, and wherein said impedance device comprises a voltage divider having a junction connected to said auxiliary electrode, a first predetermined impedance connected from said junction to said anode voltage source terminal, and a second predetermined impedance connected from said junction to said cathode voltage source terminal.

20. A character display circuit for displaying any one of a plurality of characters in each of a plurality of side-by-side character positions, comprising a plurality of anodes, one in each of said character positions, a plurality of groups of cathodes, each such group being associated with one of said character positions, each of the cathodes having the shape of a segment of one of the characters to be displayed, each of said anodes and its associated cathodes being disposed in an envelope having an ionizable gas atmosphere capable of sustaining cathode glow, a plurality of cathode conductors each electrically connected to one of the cathodes in each character position, a plurality of cathode drivers, each connected to one of said cathode conductors, for energizing said cathodes selectively in response to a first set of input signals, circuit means for selectively energizing said anodes, in synchronism with the selective energization of said cathodes, in response to a second set of input signals, for displaying selected characters in each of said character positions, an auxiliary electrode in the gaseous atmosphere associated with each character position, the current flow therethrough being a function of the number of cathodes which are glowing, and bias circuit means for maintaining an electrical potential within a predetermined range on said auxiliary electrode, irrespective of the number of cathodes glowing, said bias circuit means including at least one impedance device in series with said auxiliary electrode, a plurality of resistors, each of which is connected to one of said cathode drivers, and circuit means connecting each said cathode driver and connected resistor across at least a portion of said impedance device, so as to reduce the effective impedance coNnected to said auxiliary electrode as each such cathode driver energizes one of said cathodes.

21. A character display circuit as in claim 20 wherein the auxiliary electrode contains a portion extending between the anode and cathodes of each character position and those of the next adjacent character position, to electrically isolate the electrodes of the respective character positions from each other.