US2645724A - Means for indicating condition of memory element - Google Patents

Description

July 14, 1953 J. A. RAJCHMAN ETAL 2,645,724

MEANS FOR INDICATING CONDITION OF MEMORY ELEMENT Filed June 24, 1950 PULSE Ml/L TIPI/El? 6 Erik Jemme Bg V Gttomeg Patented July 14, 1953 Stemme, Stockholm, Radio Corporation of America, a corporation of Delaware Sweden,

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MEANS Fon'INproAriNc coNnrrroN oF f MEMoRY ELEMENT Jan A. R21-jobmail, PrincetonpN. J., and '.Erik

assignorsiv to Appueatien rune 24, 1950, serial No. 170,082

s claims. (c1. 25o-214) Y This invention relates to an electronic storage device and in particular to an improved method of and means for indicating the condition of a memory element.

The invention herein described is an improvement in the one Vdescribed in Patent No. 2,442,985, issued June 8, 1948, for Method of and Means for Indicating Condition of Memory Element and Selectron to Jan A. Rajchman. In that patent, there is shown a system for reading the condition of an individual memory element in an electron discharge device having a storage target including a plurality of memory elements. Such an electron discharge device is described and claimed in Patent No. 2,494,670, issued January 17, 1950, to Jan A. Rajchrnan. The electron discharge device referred to has a storage target having a large number of memory elements. Means are provided, within the tube, forsele'cting a single one of these memory elements and 'then placing it in one or the other of two stable conditions indicative of a Condition to be remembered.

The principle by means of which the individual memory element is precisely and accurately selected is fully described in the aforementioned patent. In brief, the system utilizes one or more grid networks, each of which comprises a plurality of parallel spaced wires adapted to be individually biased with suitable control potential. The networks are positionedA so as to form a grid mesh. Electrons from a cathode source are directed toward the grid mesh, and by the suitable application of control potentials to adjacent pairs of Wires the electrons are permitted` to pass through selected windows dened by the intersecting grid wires and to impinge on a target electrode. The target electrode consists of dielectric secondary-emissive material, such as mica, and the small area which the electrons strike on passing through the grid window constitutes a memory element which may be conditioned to one or the other of two predetermined` stable conditions so as Vto retainer store information indicative of the condition to which it-wasl R. L. Snyder, issued Meren 15, 1949, ier Piurel Beam Cathode Ray Tubes.

As discussed more fully in Patent No. .2,442,985k

when a small area, or memory element,`of the secondary-electron'emissive dielectric surface is bombarded by electrons having suflicient energy to release more `secondary electrons than the number of bombarded electrons (that is, when the secondary emission ratio is greater than unity) and when there is a secondary-electron collector electrode adjacent the memory element, the surface of the dielectric will assume either the potential of the collector or that of the cathodes, as fully explained'in the above mentioned patent. This will then be a stable condition, since if the surface potential tends to rise above that of thel collector electrode thev secondary emission will be suppressed. Since' a greater number of negativeelectrons. remain on the di-` electric el'ement, it will have a lower potential.

On the other hand, if the dielectric element tends to goY below collector` potential, the secondary' any reason the dielectric potential tends to risel above cathode potential the surface will imme-l conditioned by applying a voltage to the signal plate and controlling the electron bombardment of the'selected element' so as to bring the element to collector or cathode potential, as the case lmay be. Thereafter the electron current will hold the potential of the element at the desired value. t

When the dielectric target is coated with liucrescent material, or alternatively, where the fluorescent material is coated directlyon a trans-v parent metallic signal plate and itself functions estrie dielectric, iight win be emitted from tii'ef element when it isbombarded by electrons of CollectorY sufficiently high velocity. Howeven'when the element is at cathode potential the electrons strike the element at such a low rate of speed andin such a smallquantity that no light is produced. Consequently, the presence or absence of light is indicative of the ycondition of the memory element.

As previously stated, the condition of each memory element may be retained indefinitely at its assigned value by opening all the Windows of the control grid'so as to cause electrons 'to impinge simultaneouslyv on all elements. Thisv may be called the standby condition. nthe operation of the device it is contemplated, therefore, that the tube will be maintained in a standby condition at all times except when a single window is opened for the purpose of applying information to or deriving information from` the selected memory element. A practical storage device Vmay have 4000 or more individual memory elements. collector potential it will be appreciated that a large portion or perhaps the entire surface will emit a light duringthe standby condition. It is a 'characteristic of most fluorescent materials that the light is not extinguished instantaneously when the electron current is cut oi, but dies down gradually in accordance with substantially exponential or hyperbolic curves. Compared to the amount of light produced by many elements, that emitted by a single selected element is extremely small; Y,

In Patent No. 2,442,985 there is described a system for using a light responsive indicator to indicate the condition cfa single memory element. This system providesfor desensitizing the light responsive indicator, after the general electron bombardment has been stopped for the period of time: required for the light, yproduced by all'th'c illuminated elements, to die down to a` value substantially less than that of one element. .The light responsive element would then be sensitized again to produce an output depending upon the condition of the element selected.

Inorder for this system to function properly however, great care must betaken torascertain that the light from one element is sufficiently strong to be distinguishable from stray light and the light responsive device dark current. The time required for reading the element condition depends upon the phosphor decay time. This den If a substantial number of these are at cay time is a variable and no positive indication is available to indicate when the light, from the elements which have been switched ofi, has decayed to at least the value of the light from the selected element which has been left on. A

It is an object of the present invention to provide, in a devicel having a plurality of elements capable of being conditioned to either one of two conditions, and which emit light in one of the conditions, an improved method of and means for determining the condition of a selected one of the elements.

It is a further object of the present invention to provide, in a devicel having a plurality of elements capable of being conditioned to either one of two conditions, and which emit light in one of the conditions, a more accurate method of and means for determining the condition of a selected one of the elements.

It is still a further object of the present invention to provide, in a device having a plurality of elements capable of being conditioned to either one of two conditions, and which emit light in one of the conditions, a more rapid. method-,of

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and means for determining the condition of a selected one of the elements.

These and other objects are achieved in the present invention by providing a reading system utilizing a pulse reading technique. In most phosphors, the rise time of illumination is much shorter than the decay time. Such a phosphor isemployed in the electron discharge device the condition of an element of which is desired to be read. Y The electron bombardment of all the elements in the electron discharge tube is terminated. After a period of time not necessarily the period required for the light from all the elements to decay to a value less than that of a single element the electron bombardment to a selected element is turned on. If the element is at collector potential a light pulse with a steep front is superimposed upon the decaying light fromv the other elements. The resulting signal generated by an associated photoelectric multiplier isiamplied and differentiated 'in a circuit connected to the output of the multiplier. Since this detecting circuit isan alternating current coupled device, any direct current components from the stray light and the dark current of the multiplier are eliminated. The differentiated pulse is applied to a gating tube which is pulsed to be responsive only during the time of occurrence of such pulse.

The novel features of the invention as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description, when read in connection with the accompanying drawings, in

. which,

Figure l is a graph showing curves of decaying phosphor illumination, as Well as a superimposed increase in such illumination versus time.

Figure 2 is a schematic diagram of an embodiment of the invention, and

Figure 3 is a representation of some of the Waveshapes required and obtained-in the operation of the embodiment of the invention shown.

Referring to Figure 1, the light from a numl, ber of illuminated elements, having an exponentially rising and decaying phosphor, decays along a curve Td within a time tr. Time tr is assumed to be the minimum Waiting time after which reading may start. When a single element, at the end of time tr, is bombarded with electrons, its illumination rises sharply along the curve Tr. Let the total number of light-producing elements in a memory tube be N. Let the maximum light from each element be qs. The condition for reliable indication, namely that the light from the decaying elements should be equal or less than the light from the selected element, is represented by the equation t,=Td loge N-l-Ta loger d Since, as may be seen from the curves in Figure 1, TT T@1, the actual reading time is shorter for the pulse reading system, since the logarithm of a quantity smaller than one is negative. Therefore, the quantity Y is a negative quantity.

Referring to Figure 2, an electron storage device is shown schematically. This tube is preferably an electron discharge device of the type described in the aforementioned Patent No. 2,442,985 by Rajchman. For convenience, the tube is shown schematically and for the purpose of illustrating the present invention it is assumed that a single rectangular target electrode is employed, although the tube may be made in accordance with all the structural modications suggested in the patent. The tube contains a cathode 1, an 'accelerating grid 9, a vertical grid network Il, only one Wire of which is shown, a horizontal grid network I3, a collecting electrode l5 and a target electrode l1. The target electrode may comprise a thin dielectric mica sheet on the inner surface of which is deposited a thin layer of fluorescent material. 'Ihis material is selected to have a characteristic such that when bombarded by electrons it will become illuminated at a much greater rate than its rate of decrease of illumination upon termination of the electron bombardment. On the outersurface of the target is a thin, transparent layer of conductive metal to which the lead I9 is connected.

For the sake of simplicity, the tube is assumed to have only four horizontal and four vertical grid wires, although it is to be understood that these may be multiplied to provide the desired number of memory elements in accordance with the aforementioned earlier application. Furthermore, the tube may be constructed either as a potential barrier or deiiection device and the construction and arrangement of the auxiliary electrodes may be modified accordingly. In order to illustrate the processes of grid control in its simplest form, aplurality of manually operated switches have been shown for connecting the individual wires of the control grid networks to suitable pulse sources of potentialsfor opening and closing the selected windows. In the case illustrated the required pulse pol tentials are -10 v. and +100 v. which are produced by each of the pulse sources represented by rectangles. This assumes that the tube is of the deflection type. If the potential barrier type of construction is employed, it will be understood that the pulse potentials will be O and 100 v.

By means of two conductors 23 and 25, the pulse potentials are applied to the respective contact points of the four vertical selecting switches 21 and the four horizontal selecting switches 29. In the'normal or standby'condition both pulse sources apply an opening potential to both conductors. At this time, selection of any one of the I6 memory elements may be made for reading by operating the vertical and horizontal switches, which are connected to the grid wires defining a window opposite a selected element, to be connected to the selected grid pulse source 30. The remaining vertical and horizontal switches are operated to the nonselected grid pulse source 32. A key 36 and battery 34 are connected to both selected and non-selected grid pulse sourcesand the dynode pulse source 38. rIfhese are used forl simultane-` the desired one offa plurality of memory ele-Y ments including those describedv in a copending application of J. A. Rajchman,` Serial No; '702,775 led October 11, 1946, now Patent No.'

The accelerating and collecting electrodes are connected to suitable sources of positive potential provided by a .battery 33. The signal plate, which is a part ofthe target electrode, is connected by conductor i9 to the positive terminal of a battery 35 through a switch 31, the negative terminal of the battery beingconnected tov ground. The lead`l9 is also connected to ground through a resistor 39 and a capacitor 4|. .The purpose of switch 31 isto apply aY pulse to the signal plate to set the potential of the selected memory element at ther desired value. The

purpose of the capacitor and resistor is to prevent the voltage pulse from reducing in amplitude too rapidly when switch 31 is opened.

Associated with the target electrode is a photomultiplier tube 45. -A lens d3 serves to focus anylight from the target electrode on to the cathode 53 'of the photo-multiplier tube 5. successively greater positive potentials are applied to the nine dynodes by means of a battery 131 and a voltage divider 49 in a-conventional manner. In order to prevent the photo-multiplier tube from being overloaded during the quiescent period of the memory tube when every positively charged element is illuminated, the iirst dynode v5l is connected to the'sam'epo;V tential as thecathode 53 and the gainrof the multiplier tube is reduced to a low value. During the reading period a positive pulse is .applied to this dynode 5| from the multiplier pulse source 38 'which sensitizes vthe .tube Vfor the period of its use. l

The output of the photo-multipliertube is connected to'an ampliier 55 which inverts and amplies the output to it from the photo-multiplier tube Q5. The output of the amplifier stage is conconstants of the resistor 51 and condenser 53 of the differentiating circuit are selected in a mariner Well-known to the art to provide discrimina-` tion between a fast-rising and a slow-decliningsignal. The differentiating circuit output is applied to'a normally closed electron gate. This may consist of an electron discharge tube Gi having at least a `control grid 63 `and a screen grid 65 and being normally biased below cut-off. The control grid G3 isconnected to the differentiating circuit Vwhile the screen grid 35 is connected to a Vsource of positive pulses or al gate pulse source G1. This supplies a positive pulse at the proper time to allow the tube to provide. an output in accordance with the signal applied to its control grid. Whether or not there isfan output depends upon the condition of the memory element which has been selected.

After the switches have been 'set so that av desired memory element has beenrselected, the key' 3S is momentarily closed to apply a pulse simuln y screen grid 65.

, 7.y bombardment of all the memory elements and extinguish their illumination. Referring now to Figure 3, from the non-selected grid pulse source 32, a pulse having a long duration is applied to all the selecting grid wires except the ones defining a window opposite the desired element. Simultaneously a pulse is obtained from the selected grid pulse source 3G which has a shorter duration. This is equal to the time required for the rate of decay of the light from all the elements to be lessthan the rate of rise of the light due to illuminating the selected element. The time is less than the time required for the light from all the illuminated elements todecay below the light from one element. The multiplier pulse.

source 38 supplies a positive pulse to the iirst dynode 5l of the photo-multiplier tube to overcome the bias from the cathode 53 and to restore the gain of the tube 4| to its normal value. The duration of this pulse to the ,photo-multiplier tube may be made as long as the non-selected grid pulse. At the application of this gate pulse the photo-multiplier tube output rises responsive to the light from all the memory elements. As this light slowly decays the tube output slowly decays until a sharp rise in its output occurs again when the selected memory element is illuminated. l

The photo-multiplier output is amplified and inverted by the first amplifier stage 55 and then applied to the diierentiating circuit. The differentiated photo-multiplier tube output first shows a large amplitude diierentiated signal rapidly falling to zero in view of the time constants selected for the circuit. Then a smaller amplitude diierentiated signal occurs at the end of time tr when the selected element is bombarded with electrons. This second signal occurs only provided the condition of the element is such as to allow it to be illuminated when bombarded by electrons.

The gate tube 65 is biased off'until at the end of time tr when a positive pulse is applied to its Thus the only pulse which the gate tube amplies is the one which occurs when and if, the memory elementis illuminated. The gate pulse source 6l may be keyed by the termination of the biasing off :pulse from the selected grid pulse source 3%. The gate pulse is terminated before the multiplier and non-selected grid pulses to avoid transient indications. All the pulse sources represented by rectangles may be iiip flop or slideback types of unistable state circuits which are well known to the art and which revert to their single stable state after a time determined by the time constants selected. The pulse sources may also be trigger circuits which are made responsive to trigger pulses from a central electronic pulse source.

It will b'e appreciated that in View of the differentiating circuit and gating stage andthe method of operation of the memory tube, no false readings of elements occur.l The reading system is responsive only tothe sharp rise time of the selected element which is in a condition to be illuminated. No output occurs if the selected element is not in such condition. The non-selected grid pulse and multiplier gate pulse may be terminated at any time after the differentiated signal caused by illumination of the selected element occurs.

Although the description 0f the method and means for indicating the conditionv of a memory element has been described in connection with a memory tube wherein the element serves as both 8- the indicator and the memory element, the method and means described herein are also applicable to tubes wherein the indicating and memory functions are performed by separate targets.

From the foregoing description it may be seen that the present invention provides an improved, more accurate and more rapid method and means for determining the condition of a selected element in a device having a plurality of elements capable of being conditioned to either one of two conditions, and which emit light in one of the conditions. Although a single embodiment has been shown and described, it should be apparent that manychanges may be made in the particular embodiment herein'disclosed and that many other embodiments are possible, all within the spirit and scope of the present invention. AIt is therefore desired that the foregoing description shall be taken as illustrative and not as limiting'.

What is claimed is:

l. The combination with a device having a plurality oi conditionable elements adapted to be bombarded with electrons which emit light under bombardment when in one condition but not when in another condition, said device also having selecting means to control the iow of electrons to bombard said conditionable elements, and said device also having leads' connecting from said selecting means external-to said device selection of certain yones of which permit selection of a conditional element'to be bombarded, of means to apply a first pulse of a given duration to all but the ones of said leads which select a desired element to be bombarded to extinguish all elements but said desired one,'means to apply aV second pulse simultaneously with saidv first pulse to the remaining ones of said leads to selectively control the bombardment ofv allor a selected one of said elements by electrons, and said device having a plurality of leads connecting said means Yexternal to said device to permit external biasing of said selective control. means,

of switch means to select vthe leads connected to the part of said selective control means controlling the bombardment of the one of said-elements whose condition it is desired to determine, means to apply a rlrst bias for a given duration to all but'said selected leads to prevent bombardmentby electrons of all but said one element, means to apply a second bias to said selected leads simultaneously with said rst bias to prevent bombardment by electrons of said one element, said sec-Y ond bias being applied for a shorter duration than said ilrst bias, said shorter duration being de-j termined by the time required for'v the rate of to said light responsive means responsive only to a rapid increase in electrical output of said light responsive means.

3. The combination with a device having a plurality of conditionable elements adapted to be bombarded with electrons which emit light under bombardment when in one condition but not when in another condition, said device also having means to selectively control the bombardment of all or a selected one of said elements by electrons and said device having a plurality of leads connecting said means external to said devlce to permit external biasing of said selective control means, of switch means to select the leads connected to t-he part of said selective control means controlling the bombardment of the one of said elements whose condition it is desired to determine, means to apply a rst bias for a given duration to all but said selected leads to prevent bombardment by electrons of all but said one element, means to apply a second bias to said selected leads simultaneously with said rst bias to prevent bombardment by electrons of said one element, said second bias being applied for a shorter duration than said iirst bias, said shorter duration being determined by the time required for the rate of decay of the total light from all said elements to be less than the rate of rise of the light from one element, light responsive means associated with said device, means to maintain said light responsive means insensitive until the application of said first bias, a differentiating network coupled to said light responsive means output, a normally closed electron gate to which said differentiating network is coupled and means to open said normally closed electron gate at the termination of the application of said second bias to provide an output dependent upon the condition of said selected element.

Y JAN A. RAJCHMAN.

ERIK STEMME.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,442,985 Rajchman June 8, 1948 2,503,165 Meyer Apr. 4, 1950

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