US3437869A - Display apparatus - Google Patents

Description

April 8, 1969 Rio. coB ET AL 3,437,869

DISPLAY APPARATUS Filed Nov. 1, 1965 Sheet ofS FIGW TRANSLATOR TRANSLATOR FIG.5

INVENTORS RICHARD OICOBB RAYMOND A. SKOV ATTORNEY April 8, 1969 Filed Nov. 1. 1965 DISPLAY APPARATUS Sheet 3 ors FIG. 3

l I 1 l x 50 1 4c 50 6C 1c) 3,437,869 Patented Apr. 8, 1969 York Filed Nov. 1, 1965, Ser. No. 505,795 Int. C1. 1101 29/72 11.5. Cl. 315-18 8 Claims ABSTRACT OF THE DISCLOSURE A display wherein X, Y input data in one number set is translated into another set X(A), Y(A); X(B), Y(B); or X(C), Y(C) having more orders and having symmetry around a central position defined by one of the additional orders. By a plurality of such translations sharing the same central position, characters of different sizes A, B, C can be drawn, each symmetrical about the central position. The character generator deflection is held at that relative position during non-character mode vector operation of the display, so that characters, when later drawn on the vectors, will appear in centered relation thereon.

This invention relates to data processing systems, and more particularly to a display system and data translating means forming a component of the same.

Display apparatus has been found useful in input-output devices in data processing or handling systems. For example, in a cathode ray tube scanner or display apparatus, the beam of the tube may be deflectable to digital addresses on the tube screen for executing vectors which (when unblanked) yield traces defining a desired configuration. This configuration may be an arrangement of lines constituting a drawing, or it may be a character (alpha, numeric, or arbitrary symbol), or it may be a time-interleaved and positionally overlayed combination of the two.

The exact position of the character may be of some importance. For example, in an input drawing-scanner apparatus, a line follower symbol may be projected onto a drawing in such a manner that it is, desirably, centered on a predicted position on the line. In a display of a family of curves, each may have symbols thereon (e.g., identifying the same) which, desirably, are centered on the corresponding curve. In a display having a special symbol which is to be sensed by a manipulatable photocell device (such as a light gun or a light pen) or wherein such a symbol is merely indicative of some operator input (such as in a joy stick or voltage pen type of device), the special symbol is, desirably, centered on a known cordinate.

Moreover, in each case there are occasions when it Would be desirable to change the size of the character without changing its center. In this way the character may be larger or smaller but still have predetermined, perfect centering on a predetermined position on the display or scanning field.

A usual means for generating characters in a display (or scanner) device is to provide incremental deflection for the electron beam whereby the beam is made to follow a prescribed path in terms of deviations from some position defined by the main deflection system. In digital systems the available increments are definite units in a dimensional field which may have no addressable center. For example, in a three-bit (i.e., three order) binary system the addressable positions (in decimal) for each of X and Y in the dimensional field are 0, 1, 2, 3, 4, 5, 6 and 7. The center would be X=3.5, Y=3.5, which is not addressable.

A difficulty therefore arises in attempting to draw characters which are perfectly centered on a display address. If the character deflection system is kept at its zero X, zero Y condition during the drawing of the display, superposed characters will be completely off-centered with respect to a line which has been drawn by the display since the lower left hand corner of the character will be the point of origin by which the display is drawn. If some other character input is utilized during the drawing of the display, characters will inherently be ofl center because of the above noted fact that in a binary system the center of a deflection system such as a character deflection system is inherently off center. Moreover, this off centered relationship becomes aggravated as the character is expanded.

The prior art contains examples of means for relocating and expanding portions of a display, but these prior art techniques do not completely satisfy the need to which the present invention is addressed. For example, US. Patent No. 3,011,164 shows a display apparatus wherein binary data in a given set are given changed weights, with certain orders being complemented to give the eflect of two way expansion about one of the orders. In another prior art example, a disclosure authored by J. W. Carlson entitled, Display Centering and Expansion Apparatus, published in the IBM Technical Disclosure Bulletin, vol. 5, No. 11, pages 89-91 (April 1963), there is shown a data manipulating system wherein a new origin is selected by subtraction of a certain value and then the display is expanded by order weight alteration (shifting). Of course, expansion is also possible by simple analog amplification.

In each of the above prior art examples, the data is expanded about some addressable point to some other point in the same scale system, be it digital or analog. Given a binary input which is inherently non-centered, such a display is centerable only by use of a bias, which bias must then be changed with each level of expansion. Moreover, the above cited prior art does not deal with changes in mode of operation from character to non-character mode in order to form superimposed images.

In accordance with the present invention, means are provided wherein a set of input data is translated into another set having more orders and having symmetry around a position, hereinafter called the Home Position, a position defined by one of the additional orders. Moreover, means are provided affording a plurality of such translations having diflerent scale factors and sharing the same home position. The result is that the invention enables the generation of a character which is perfectly centered on a home position and therefore is perfectly centered on a display element which has been drawn while the character control is in that home position. Moreover, the invention provides unchanging symmetry about that center with change in size of the character drawn thereabout. Finally, these advantages are carried out by simple, accurate, digital manipulation.

Accordingly, it is a primary object of the invention to provide, in a data processing apparatus, improved means for executing controlled deflections of an electron beam or similar display or scanner instrumentality.

It is another object of the invention to provide apparatus as aforesaid means defining an operative deflection field or grid for an electron beam for operation of the apparatus in a character generating mode, with the grid having a true center rest position providing an operative address at which the character deflection system is held during other deflections of the electron beam in non-character mode, so as to yield in character mode characters which are perfectly centered on a display drawn in noncharacter mode.

Still another object of the invention is to provide, in an apparatus as aforesaid, a data translation system wherein a set of digital input information is translated into another set having more orders for enabling symmetry and symmetrical expansion.

Yet another object of the invention is to provide the above features by use of simple, accurate, digital circuitry.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIG. 1 is a diagram of a data processing system including one preferred embodiment of the invention.

FIG. 2 is a schematic diagram of a translator circuit representative of a part of FIG. 1.

FIG. 3 is a diagram showing related deflection grids which show the operative ranges of deflection of an electron beam resulting from the operation of translator circuitry of FIGS. 1 and 2.

FIG. 4 is illustrative of a display such as may be generated by operation of the apparatus of FIG. 1; and

FIG. 5 is a detail showing steps in generation of display indicia of the general kind seen in FIG. 4, as related to an operative grid system of FIG. 3.

Referring more particularly to FIG. 1, the invention may be embodied in a data processing apparatus comprising a central processor unit or computer having an output apparatus including a cathode ray tube 12. In the illustrated embodiment, the cathode ray tube 12 is pro vided with main XY deflection yoke having windings 14, 16, and a secondary or character yoke having additional X and Y windings 18, 20, hereinafter sometimes referred to as the AX and AY deflection coils. The deflection coil arrangements may be of the kind wherein each deflection coil has two halves in which the currents flow in relative opposition so that when there are equal currents there is no deflection. Such a winding arrangement may have three connections for each coil, represented in the draw ing by the line groups 22, 24, 26 and 28. It will be understood that this connection arrangement is representative only; for example, in some systems the common connec tion to the two halves of each deflection winding is actually two wires for enabling use of certain additional bal ancing or surge drive networks or the like.

The several line groups are driven by digital to analog converter and driver circuits 30, 32, 34, 36, the main deflection driver circuits 30, 32 being under the control of the central processor unit 10 via input cables 38, 40, and the character deflection driver circuits 34, 36 being under the indirect control of the central processor unit .10 via, in the illustrated embodiment, a character generator 42 together with digital translator circuits 44, 46 of the invention.

The input signals to the translator circuits 44, 46 on cables 48, 50 could be provided directly from the central processor unit, but the character generator 42 is shown because such an apparatus is advantageously included in display systems for removing some of the burden of steps in character generation from the central processor. Accordingly, the central processor may provide digital commands on output cable 52 which identify the character to be generated, and the character generator 42 may contain circuitry which thereupon issues series of digital signals on its output cables 48, 50 for execution of particular strokes of the electron beam of the cathode ray tube 12, by operation of the character yokes 18, thereof, so as to trace the desired character on the face of the cathode ray tube. Ordinarily, such deflections are minor in extent compared to the face 54 of the tube and account only for the configuration of the character. The placement of the character on the face is effected by operation of the main deflection yokes 14, 16 and the control circuitry leading thereto.

As will be dealt with more fully hereinafter, the apparatus may have both character and non-character modes of operation, and during a non-character mode of operation, the dynamic operation of the electron beam may be solely under the control of the main deflection yoke 14, 16, with the character yoke 18, 20 held in a steady state condition. Conversely, in character mode, the character yoke may have dynamic deflection control while the main yoke is at a steady state. In either case, there are occasions when it is desirable to blank the operation of the electron beam so as to enable adjustments of the position thereof without efiecting a trace. Such a blanking control is indicated as S6 and is shown to be operative in response to a logical OR circuit 58 under selective control of the central processor unit 10 or the character generator unit 42 via lines 60, 62, as may be required during operation of the apparatus. It will be understood that this blanking control 56 may be operative in any conventional manner, such as by operation of the control grid of the cathode ray tube (not shown), and that the cathode ray tube may be fitted with other conventional parts and connections such as anode and focusing connections and the like, also not shown.

In the illustrated apparatus the signal translation circuitries 44, 46 are operative to serve three main functions. First, they provide signal outputs on cables 64, 66 which are operative to provide a Home Position of which is at a precisely known location within and preferably near the center of the deflection capability of the AX and AY deflection windings 18, 20. As will become apparent hereinafter, it is at this Home Position that the character deflection system is held during the non-character mode of operation of the device. Secondly, the translator circuits 44, 46 provide ranges or sets of outputs symmetrically disposed about the aforesaid Home Position for enabling the generation of characters which will have a true center on that Home Position and therefore can be truly centered on a desired position in the display generated during non-character mode. Thirdly, the translator circuits 44, 46 provide for generating a plurality of such sets of output signals having different scale factors that the aforesaid character can be made to have different sizes. Accordingly, inputs to the translation circuits include a character mode-non-character mode command line 63 and character size information lines 70, 72, in addition to the character form information On cables 48, 50.

Since the translator circuits 44, 46 of the system of FIG. I may be identical, only one of them, the X signal translator circuit 44, is shown in detail in the drawings. This circuit is shown in FIG. 2 and may comprise a size input register for storing input signals received from the central processor unit 10 via lines 70, 72. In the illustrated apparatus, provision is made for three sizes of characters hereinafter referred to as the A size, the B size, and the C size. In a binary system, two orders or bits of information are needed to define the three possibilities and therefore the register 80 has two inputs 70, 72 and two outputs 82, 84. The levels on lines 82, 84 are decoded in a conventional manner in a circuit 86 to yield a unique output on one of lines 88, 90, 92 corresponding respectively to the three sizes of which the apparatus is capable. Cable 48 contains lines corresponding to three binary orders identified in the drawing by the decimal notation 1, 2, 4, as shown. The three orders enable the communication of values of 000 through 111 in binary notation, or 0-7 in decimal notation. Although there are only three binary orders of inputs in the cable 48, there are six binary orders of outputs in the cable 64, as indicated by the symbols D1 through D32 representing the weights of driver current in the corresponding AX D/A converter 34 which are activated by those lines.

The translator circuit 44 includes a matrix by which the three bit input to six bit output conversion is made in accordance with the size information provided on the size indicator lines 88, 90, 92. This matrix is activated only during character mode of operation of the devices. Therefore the character-non-character line 68, which is assumed to be up or have a significant signal during noncharacter mode in the present apparatus, is connected through an inverter 94 to the conditioning inputs of AND gate circuits 96, 98, 100 to admit the signals from the decoder 86 to the translator matrix only during character mode operation. Thus, during size A character mode operation, matrix AND circuits 102, 104, 106, 108 are conditioned. During size B character mode operation matrix, AND circuits 110, 112, 1.14 are conditioned. During size C character mode operation only AND circuits 116, 118 are conditioned.

The outputs of this matrix are then ORed via circuits 120, 122, 124, 126 to the respective binary order lines of cable 64, in the manner shown. However, the highest order output line in the cable 64 is not energized from the aforedescribed AND circuit matrix. Rather, it is energized through an OR circuit 128 directly from the high order, 4 weight input line .130 of the cable 48. This input is utilized also through an inverter 132 which yields afnot 4 output on line 134 to energize one input of AND circuits 102, 104, 110, as shown. The D32 or high order output line in cable 64 is also energizable through an input 136 which is up or on during non-character mode operation. As will be seen more fully hereinafter, this results in the provision of deflection to the Home Position during non-character mode operation of the translator circuit.

FIG. 3 in conjunction with Table I and II is illustrative of the operation of the translator circuit of FIG. 2. The X (C) and Y(C) axis are marked, as referenced for example at 140, with small intervals corresponding to the smallest deflection executable by the AX or AY deflection coil system by and upon a lowest order increment of the six bit signal on cable 64 or on cable 66, respectively. In FIG. 2, this increment results by and upon the change of 1 to 0 or 0 to 1 in the value of the signal on the lowest order output line D1 of the circuit. The coordinate values 0C through 7C along the X(C) and Y(C) axes correspond, in size C, to the 0 through 7 values at the input of the X translator circuit 44 (FIGS. 1 and 2) and its companion Y translator 46 (FIG. 1), respectively. Since there are three sizes available, these input values 0-7, 0-7 are seen three times on different scales in FIG. 3 to form three grids, the A grid, the B grid and the C grid corresponding to size A, size B and size C. To avoid crowding in the drawing, only the Y axis values are shown in size A.

The center position 142 common to each'of the grids is the Home Position, which is a position that is non addressable, that is, corresponds to a value of 26:35, Y=3.5 in each of the grids. This value cannot be represented numerically by the three bit signal input to the matrix of FIG. 2. However, the Home Position is utilized during non-character mode of the device, and is definable by the six bit outputs of the translator circuits 44, 46, as a value in each of X and Y of 100000 in binary or weight D32 in decimal.

Table I shows the decimal equivalents of the translator inputs, as used in FIG. 3 and Table II. Table II summarizes the operation of the translator 44 or 46. As will be seen in Table II, 0A through 7A, 0B through 7B, and 0C through 7C comprise sets bearing a 1, 2, 4 size relationship which have been fitted to the binary capability of the output of the circuit of FIG. 2 in such manner that each is symmetrically disposed about the Home Position.

TABLE I Input Decimal Binary (48, 50) Equivalent (X, Y)

111 HOME (CM) TABLE II Input Input Output Output Size Code Size Code A B O A B 0 CM-HOME (3.5) 100000 000000 In Table II, the six bit binary coded output has order positions, reading left to right, of thirty-two weight, sixteen weight, eight weight, four weight, two weight, and one weight. Binary signals on lines D32 through D1 in cables 64, 66 are communicative of these values to the corresponding converter 34, 36. The converters 34, 36 may be of the kind having a binarily weighted current driver for each order, selectively connected to one half of the corresponding yoke winding 18, 20 or the other. Thus, for example, a signal of D32 alone in each of AX and AY would yield thirty-two units of current in one half of each of windings 18, 20 opposed by thirty-one units in the other halves, resulting in a net deflection of plus one unit in each of AX and AY or Home Position.

FIG. 4 shows the result of the Home Position-character grid orientations of FIG. 3 and Table 11. Let it be assumed that the data processing apparatus of FIG. 1 is to be used to portray on the face of the cathode ray tube 12 a display comprising a family of curves 150, 152 on a coordinate axis system 154, 156, and that the curve is to have displayed on it one or more representations of a symbol such as a square 158, and the curve 152 is to have a diflFerent symbol such as shown at 162, 164. Such symbols may be utilized to identify the curves in the family of curves or to show the positions of experimental data from which the curves were drawn. In either case but particularly in the latter case it is highly desirable that the symbols be centered on the curve, that is, that they be drawn symmetrically about a home position falling on the curve.

Moreover, it is further highly desirable that if these symbols are changed in size that they not change in position; namely, that they maintain the same Home Position. This is illustrated in the case of a circular symbol 166 such as is sometimes used as a symbol indicative of a light gun or light pen operative position. In the operation of such a device it is sometimes desired to have the symbol be larger or smaller while maintaining the same data-significant position. This is enabled by drawing the symbol 166 in larger versions such as indicated at 168 or 170 about the same Home Position in accordance with the operation of the invention.

Although the curves 150, 152 in FIG. 4 appear to be smooth lines, one means of producing such curves, frequently used in digital apparatus, is to deflect the electron beam by operation of the main yoke coils 14, 16 through a succession of vectors. This is done by operating the digital to analog converters 30, 32 in accordance with a succession of values fed to them from the central processor unit 10 through the connecting cables '38, 40. Since the main yoke coils 14, 16 may be operative to deflect the electron beam throughout the entire extent of the face 54 of the cathode ray tube, the main deflection signals on cables 38, 40 are desirably of high resolution, such as the ten bit signals indicated in the drawing.

It will be understood, however, that no matter what the main deflection X, Y may be, there is added to this deflection the AX, AY resulting from operation of the character yoke deflection coils 1'8, 20. Since the deflect-ion imparted by these coils is held at the Home Position of the character yoke during non-character mode, the character yoke imparts no distortion to the noncharacter made generated display. However, in the illustrated embodiment of the invention it does impart a small constant deflection to the electron beam (a small bend in the electron beam) of the cathode ray tube. This results from the fact that the inputs to the AX and AY digital to analog converters 34, 36 which would correspond to no deflection by the character yoke would be not 100000, 100000 but one half a unit of the lowest order in each of X and Y less than that, a value which would be unobtainable in a binary system but is very closely approached in the six bit system illustrated. This slight deflection could be biased out but it is ordinarily so unimportant that this is not required. It produces no distortion in the non-character mode display (e.g., 150, 152, 154, 156) because it is present in the Home P- sition during the generation of those elements.

FIGURE 5 is illustrative of the operation of the main and character yokes of the illustrated apparatus during change from non-character mode to character mode and then back to non-character mode. Let it be assumed that the apparatus is in non-character mode and has just finished execution of a trace 180. Since the character yoke coil 18, are always energized with a D32 input corresponding to Home Position when the apparatus is in non-character mode, the center of the invisible character grid (the Home Position) will be at the end of the vector 180. Let it be assumed that the next command in the program of the central processor unit 10 is to draw a circle around the end of the vector 180. The central processor units sends an eight bit command to the character generator 42 via cable 52 defining the shape of the character to be executed, namely, a circle. The

CPU 10 also sends on line 70, 72 the size code for the circle to be executed, and shifts the level on line 68 to its down or off condition thereby signalling the change from non-character mode (OM) to character mode (CM). Let it further be assumed that the character generator 42 terminates each character with an output on cables 48, 50 of 000,000 corresponding to the lower left hand corner of the character grid (no matter what its size).

Referring to FIG. 2, there will therefore be a zero significant or down signal on all three lines of cable 48, and when the signal on line 68 goes down, OR circuit 128 will become deconditioned and AND circuits 9'6, 98, 100 will become conditioned. If the size command on line 70, 7-2 dictates size A, outputs D1 and D8 will be up. If size B isspecified, outputs D1 and D16 will be up. If size C is prescribed, output D4 alone will be up. Accordingly, no matter what size character grid is prescribed the position of the electron beam will be deflected to the lower left hand corner of the prescribed character grid, or character grid address 0,0 in decimal. This will result in the blanked adjustment of the electron beam position indicated by FIG. 5 by the dotted vector from position 315, 3.5 to 0,0. The blanking of the electron beam during this time could be 8 under the control of the character generator 42 via line 62 or the control of the CPU via line 60, it being assumed in this discussion that it is under the control of the CPU via lines 60 as a part of the programmed change from non-character mode to character mode.

The first step of the operation of the character generator 42 is to change its output from 0, 0 decimal equivalent to 0, 2 decimal equivalent while maintaining the cathode ray tube blanked by operation of an output on line 62. This then executes a blanked repositioning as shown by the dotted vector from 0,0 to 0,2 in FIG. 5. The next step in the operation of the character generator is to change its output from 0,2 to 0,5 while unblanking the electron beams by operation of line 62 (the signal on line 60 having been previously dropped when the character generator was put into operation) so as to execute the visible trace from 0.2 to 0.5 in FIG. 5. Subsequent steps of operation of the character generator feed new vector endpoint values of 2,7; 5,7; 7,5; 7,2; 5,0; 2,0 and 0,2 in succession to execute the corresponding visible traces as seen in FIG. 5. This results in a generally circular figure having the Home Position of 3.5, 3.5 at its exact center.

It will be seen that a three bit binary code (0-7 decimal) provides sufficient flexibility to enable the generation of reasonably smoothly shaped characters; of course if greater flexibility is desired more bits could be used at the cost of more elaborate circuitry. It will be understood that, with the circuitry shown, the larger circles 168 and 170 in FIG. 4 would show as octagonal approximations of circles although this would not be so obvious in the small circle 166. However, in every case the center of the figure would be exactly in Home Position and therefore exactly on the trace 1'50.

Returning to FIG. 5, and following the previously stated assumption that the character generator is one which returns to 0,0 decimal output (000,000 binary) at the end of each character, the final operation of the character generator would be to execute a blanked trace from 0,2 to 0,0 as seen by the dotted vector in FIG. 5 between those positions.

If the CPU 10 was programmed to return to noncharacter mode so as to continue on with a trace 182 by operation of the main deflection system, the signal on line 68 will be brought back up, resulting in the deconditioning of AND circuits 96, 98 and 100 in translator 44 as well as the corresponding circuits in translator 46, so as to return the operation of the output of the translators to D32 in each case, corresponding to the Home Position. If the CPU 10 blanks the electron beam by operation of an appropriate signal on line 60, this beam adjustment to the center of the character grid will not be seen, and this blanked readjustment is indicated by the dotted trace between 0,0 and 3.5, 3.5 in FIG. 5. With the character yoke 18, 20 again adjusted for Home Position, operation of the electron beam can be unblanked and the main yokes 14, 16 operated to preceed to draw a trace along path 182. The aforedescribed operation will have resulted in drawing the octagonal circle shown in FIG. 5 exactly centered on the continuous line 180, 182.

As explained above, the small deflection introduced by the character yoke and Home Position introduces no distortion in a visual display. When the invention is used to produce a line following symbol for analyzing some other graphic input such as a drawing, that graphic input can be maintained in such position as to compensate for that deflection.

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

What is claimed:

1. In a digital data display system having a display delineating device and non-character and character modes of operation of the same,

deflection means for controlling the movement of said delineating device, said deflection means including a data translating means to convert input data thereto of one number set into a second number set,

said second set being characterized by symmetry around a predetermined value in said second set not present in said one set,

and means for holding said translating means at said predetermined value during non-character mode of operation of said system,

whereby displays can be generated in said non-char-- acter mode in position for registry with the centers of characters generated through operation of said translating means.

2. Apparatus in accordance with claim 1 wherein,

said translating means comprises means to selectively convert said input data into said second set and an additional set having a different extent about said predetermined value,

whereby a change in size of a character delineatable by said device is effected.

3. In a digital data display system having a cathode ray tube and graphic and character modes of operation of the same,

deflection means for controlling the movement of an electrom beam,

said deflection means including -a binary data translating mean-s to convert input data thereto of a first set of values into a second set of values having more orders than said first set,

said second set including a predetermined value not included in said first set,

and means to set and hold said translating means at said predetermined value during graphic mode of operation of said system,

4. Apparatus in accordance with claim 3 wherein,

said translating means comprises logical matrix means to selectively convert said input data into said second set and an additional set having a different extent about said predetermined value,

whereby a change in size of a character delineatable by said device is eifected.

5. Apparatus in accordance with claim 3 wherein,

said input data to said translator comprises three binary orders and said second set comprises the input data plus three lower orders.

6. Apparatus in accordance with claim 3, wherein said input data to said translator comprises three binary orders and said second set comprises the input data plus three lower orders,

said translating means comprises means to selectively convert said input data into said second set and an additional set having a diflerent extent about said predetermined value,

whereby a change in size of a character delineatable by said device is effected.

7. Apparatus in accordance with claim '6, including control means for switching between graphic and character modes of operation and separate data channels for each.

8. In a digital data display system having a display delineating device and non-character and character modes of operation of the same,

deflection means for controlling the movement of said delineating device,

said deflection means including primary deflection signal means to etfect non-character mode display traces and supplementary signal means to efiect character delineating display traces for superposition on said non-character traces,

said supplementary signal means including a data translating means to convert input data thereto of one number set into a second number set,

said second set being characterized by symmetry around a predetermined value in said second set not present in said one set,

and means for holding said translating means at said predetermined value during non-character mode operation of said system.

References Cited UNITED STATES PATENTS 3,256,516 6/1966 Melina et al. 340324.l X 3,267,454 8/1966 Schaaf 340324.l 3,281,822 10/ 1966 Evans. 3,293,614 12/1966 Fenmore et al. 3,325,802 6/1967 Bacon 340324.1

RODNEY D. BENNETT, Primary Examiner. BRIAN L. RIBANDO, Assistant Examiner.

US. Cl. X.R. 340-324.l

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