As is well known to those versed in the art of performing measurements of lengths and distances ranging from the order of tenths of millimeters to several feet, a great variety of instruments with various degress of pre- cision is available. In the order of increasing precision, there are the meter stick or foot ruler, the vernier caliper, the micrometer, and acoustical and electromagnetic wave devices using the principle of wave reflection or used as interferometers. For ordinary measurements of lengths to a precision of tenths of centimeters, the meter stick is used. Although this is the classical and time-honored method of measurement, it is not entirely satisfactory. The user must overcome errors associated with parallax and interpolation between scale divisions. Furthermore, the scale markings are fixed and conversion to other systems of units must be done as a separate operation, introducing additional errors. Previous devices competing with the meter stick have used mechanical switching of electrical current in an electrical resistor to perform measurements of length.

Gallacher et al Patent 3,973,326 is illus¬ trative of ^' such a prior art device, having a, cursor on a movable wand and a resistor extending along the length of the wand. An electrical contact on the cursor contacts the resistor, thereby forming a potentiometer with an output which is variable with cursor -position. A digital voltmeter provides an indication of the distance measured. Such devices are cumbersome to use and the mechanical switching mechanisms cause reliability problems. Moreover, prior devices for performing ordinary measurements of length have not been directly coupled to electron¬ ic calculating devices so as to provide convenience and the greater power of electronic arithmetic. The use of light sensing elements, particu¬ larly fiber optics, to determine distances in con¬ junction with counters and stored program computers is disclosed in Rempert Patent 3,598,978. However, relative movement is required between the object and the light sensing element, and the device does not operate as a replacement for a ruler or meter stick having a scale thereon. Scales are utilized in Zipin Patent 3,748,043, but only for viewing by photosensors to determine a more accurate measure- ent by interpolation. A display is provided for the determined distance which is, however, not under operator control with respect to either scale factor or other functions.

Other devices are known for distance eas- urement by optical means as illustrated by Renner et al Patent 3,965,340 and Kimura Patent 3,784,833. Such devices require the use of gratings to effect light measurement. Renner et al, for example, de¬ tects a change in light transmission due to a

relative displacement between a fixed interference grating (on a fixed caliper) and a movable grating (on a movable caliper) . The changed light trans¬ mission is detected to provide an indication thereof on an associated calculator. Kimura requires a diffraction grating to effect light measurement utilizing LED's and photodetectors by affixing the detectors directly to the rear of an index grating. Such devices rely on complicated optical instru- mentation and do not provide for ordinary scale measurements of distances of either commonplace or arbitrary scale factors.

Lewis Patent 3,515,888 uses a reticle assembly to determine position change with re- spect to a fixed reference by using optical gratings for chopping a light beam. Niss Patent 3,765,764 uses light deflecting means for coordinate meas¬ urements. A source projects light onto a movable deflecting means, thence to a further measuring point. Grendelmeier Patent 3,599,004 determines, scale placement utilizing equi-spaced photocells. A differential amplifier is used to permit measure¬ ment of displacements less than the cell size. Such devices, while utilizing light sensing means do not provide a portable means, under operator control, for obtaining distance measurement comparable to ordinary meter-stick manual measurement, nor do the devices provide for variation of a scale factor under operator control. Moreover, the prior art devices do not contemplate the use of an optically active scale for distance setting and measurement. SUMMARY OF THE INVENTION The present invention overcomes the dis¬ advantages of the prior art and provides distance

measurement and setting utilizing an optically active ^' scale. In accordance with the invention, a high re¬ solution distance measurement device is provided for measurement in the British system, the Metric system, or in any other user-defined system of measurement. The apparatus disclosed herein provides a digital numeric display of the measurement to the user, as well as displaying a length when the numeric distance and basic measurement unit are provided as inputs to the device.

A series of optically active elements is spaced along a line in close proximity to the straight edge of the device to serve as a scale. A keyboard permits the user to input numbers and to control the operations and functions performed by the subject device, with a digital numeric display conveying the value of a distance setting, or the results of a length measurement, to the user.

In accordance with the invention, apparatus is provided for distance measurement comprising op¬ tically active scale means having a plurality of optically active elements for measuring a distance; numerical display means for displaying a numerical representation of the distance measured; means for providing operator input; and control means respon¬ sive to said input means and connected to scale means and display means for activating at least one element of said scale means, and for causing said display means to display a numerical representation of the distance measured along said scale means by said at least one activated element.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects, features and ad¬ vantages of the present invention will be made clear, along with other advantages, features and objects thereof, by reading the specification along with the drawings wherein like numbers designate like objects throughout.

FIGURE 1 shows an apparatus embodying the present invention. FIGURE 2 illustrates the internal compo¬ nents of the presently preferred embodiment and the electrical interconnections therebetween.

FIGURE 3.shows the electrical connections for the keyboard and display of the invention. FIGURE 4 shows the electrical connections ^• to the scale of the invention.

FIGURE 5 shows a circuit diagram of sup¬ porting logic used within the invention.

FIGURE 6 is a flow chart illustrating the interaction between the operator and the inventive apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGURE 1 illustrates a preferred embodi- ment of the electronic rule. Three major elements of the device are shown: an optically active LED scale 10, a digital numeric display 20, and a

keyboard 30. The keyboard comprises a plurality of keys 32, including keys for numerical entry and func¬ tion keys.

The electronic rule disclosed herein can be used to perform length measurement or distance setting. In performing a measurement of distance or' length, a linear distance between the left and right endpoint.s of an object is determined. In dis¬ tance setting, the user specifies a measurement unit and a numerical value, d. The electronic rule will then select two points so that the distance between them is d. These operations are defined and further described in conjunction with FIGURE 6, infra.

Turning now to FIGURE 2, the various com- ponents utilized in the preferred embodiment, and the interconnection thereof, are shown as including a microprocessor chip 40, such as is available from MOS Technology, Inc. of Norristown, Pa. under the nomenclature MPS 6502 and two peripheral interface/ memory units of large scale integration (LSI chips available from MOS Technology as chips MCS 6530) shown at 42 and 44. Each LSI unit includes two in¬ put/output registers, two peripheral data buffers, an interval timer, two data control registers, an address decoder, a data bus buffer, a chip select unit and two memory units, a mask programmable 1024 X 8 ROM and a 64 X 8 static RAM.

These units as well as the microprocessor unit are described in Appendices G and H in MOS Technology "KlM-1 User Manual".

Support logic unit 46, shown more fully in FIGURE 5, is connected to the microprocessor and LSI units. Unit 42 interfaces with both keyboard 30 and a seven segment display 20, shown combined in

block 45. Unit 44 is used to interface with scale 10.

A bus structure is utilized for communi¬ cating between the various units as follows. An address bus 47, having lines AB0-AB11 therein, communicates between microprocessor 40 and units 42 and 44. Additionally, lines AB10 and AB11 there¬ in communicate with support logic 46.. A data bus 48, containing lines DB0-DB7 therein, further pro- vides communication between the microprocessor- and the two LSI units. Peripheral pins PA0-PA6 and PB1-PB4 of unit 42 communicate with and drive the keyboard and seven segment display of the invention, and peripheral pins PA0-PA7 of unit 44 control the various elements of the optically active scale 10.

Each peripheral pin may be used either as an input or an output pin, the specific use being determined by the content of a corresponding bit in an I/O direction register contained within the unit, the register contents being under the control of an operating program, which also controls microproces¬ sor 40, the system's central processing unit.

Central-Processing-Unit 40 is capable of performing simple arithmetic logic operations, such as add, subtract, logical AND, OR, EXCLUSIVE OR, NOT, and shifting. Control instructions such as jump, jump to subroutine, return from subroutine, and conditional jumps are also available. The microprocessor uses a stack to store return addres- ses for subroutine calls. The stack is located in the RAM of MCS 6530 chips 42 and 44.

The previously mentioned operating pro¬ gram is used to support functions of the Electronic Rule and is stored in the ROM portion of the two

MCS 6530's. The program can be activated by an RST (reset) interrupt to the MPS 6502. When the RST in¬ terrupt occurs, (when the user of the electronic rule pushes the f RS 1 key) , the microprocessor fetches from a fixed location in the operating program an address, which points to the beginning of the oper¬ ating program. The address is loaded into the pro¬ gram counter and execution of the operating program begins. As previously mentioned, each of the two

MCS 6530 chips contains 64 bytes of RAM, IK bytes of ROM and 64 locations for input-output ports and timers. The RAM portion is used to store variable data such as user-defined measuring units, digits to be displayed, locations of the reference point ^' (RP) and cursor (X) , etc. The RAM is also used by the microprocessor as the system stack.

Each I/O port in a MCS 6530 chip is asso¬ ciated with an I/O data register and an I/O direσ- tion register. Both registers can be accessed by providing a unique address, which is part of the entire memory address space. That is, the I/O ports are treated like memory storage locations.

Referring now to FIGURE 3, the keyboard 30 and display 20 are shown in greater detail. Spe¬ cifically, seven-segment display units 51-56 are shown, each being driven by lines 57. A specific display element is chosen for activation by one of transistors Q1-Q6, activated and deactivated by signals output by decoder 60. Decoder 60 comprises a BCD to decimal decoder such as commonly available under the designation SN 74145 from Texas Instru¬ ments. The input lines 62 to decoder 60 are connec¬ ted to the peripheral bus for.connection to pins

PB1-PB4 of unit 42.

Additional output lines 64 from decoder 60 are connected to the three rows of the keyboard in the presently preferred embodiment. Pins PA0-PA6 of unit 42 are connected to the columns of the key¬ board and also, through open collector inverters 66, to display units 51-56.

Under control of the operating program, display 20 is activated by scanning the several dis- play elements. The scan is achieved, by the proper coding of pins PB1-PB4, resulting in sequential selection of transistors Q1-Q6 by the action of decoder 60. For each selected transistor, and cor¬ respondingly selected display element, the number to be displayed is obtained from a table in unit 42 and converted to a code suitable for seven segment display which is placed in a register. Pins PA0- PA6, caused by the program to act as output pins, convey the proper seven-segment code to the dis- play element. The operating program also selects the proper scale elements to display RP and X. After performing its display function, the operating program causes keyboard 30 to be scanned.

Keyboard scanning is achieved by providing sequentially the codes for activating output lines 00-02 of decoder 60. The codes are applied sequen¬ tially by pins PB1-PB4 of unit 42. For each code, one of lines 00-02 is chosen, and one of rows 0-2 of keyboard 30 is activated. During each such acti- vation, pins PA0-PA6, now caused to act as input pins, are sequentially selected, thus scanning each key in the selected row of the keyboard. If a key de¬ pression is detected, the operating program executes the function represented thereby. If not, the next

row is scanned. After execution of the function (or determining that no key was depressed) , the program again energizes the display and scale. The -functioning of the main operating program is more clearly shown in Appendix A, which includes a flow chart representation thereof. The various subroutines are shown in Appendix B. As is apparent from the preceeding description, the in¬ ventive device thus includes a means for multiplex- ing the display and keyboard sensing, thereby pro¬ viding for the use of fewer pins, lines, connec¬ tions, and other hardware. In view of the high operating speeds available in digital computers, such multiplexing does not adversely affect the user's perception of the display and scale. Spe¬ cifically, objectionable flicker and other disad¬ vantages do not result from the approach used here¬ in.

Referring now to FIGURE 4, scale 10 is shown comprising a plurality of active optical ele¬ ments disposed at intersections of addressing lines for the elements emanating from decoders 68 and 70. The decoders are available, for example, under the label SN 74154 from Texas Instruments, and comprise four-to-sixteen decoders. A particular optical ele¬ ment is activated by selecting the specific row and column output lines at whose intersection the ele¬ ment sits. The presently preferred embodiment con¬ templates the use of light emitting diodes as the scale elements, but it is recognized that any ele¬ ments which affect the absorbance, reflectivity, or emission of visible light may be used. Thus, while it is known to use optically passive scales for conventional measurement, including scales which

are inscribed for absorbing, or reflecting light with greater intensity than the medium in which the scale is embedded, the present invention utilizes optically active scales. It is contemplated that any scale comprising electronic components which can emit vis¬ ible light or absorb visible light from another light source under the control of the user may be used. More particularly, the optically active scales con¬ templated herein include any scale having components wherein one or more of the following properties of light are controlled whether by electrical, electro¬ magnetic, thermal or magnetic fields: emission, ab¬ sorption, reflection and transmission, for example. Such elements include light emitting diodes, as pre- sently selected for use in the scale, liquid crystal . elements, etc.

The specific elements of the scale are se¬ lected by LSI unit 44 providing output signals on peripheral lines PA0-PA7 to decoders 68 and 70. While the presently preferred embodiment contemplates the use of 121 elements in scale 10, it is clear that with no modification of hardware design the eight out¬ put lines from LSI unit 44 in conjunction with de¬ coders 68 and 70 may equally address 256 components of a scale. Similarly, with slight modification, such as increasing the storage available to unit 44, and with the use of different decoders 68 and 70, virtually any number of elements may be incorporated within the scale. In the presently preferred embodi- ment, two elements are activated to display two points. Conceivably, scale displays of more than two elements, or of variable numbers of elements or of a fixed num¬ ber of variable elements may be desired. For any of these possibilities the elements to be activated may

be selected either by scanning and multiplexing ele¬ ments along a row, or by simultaneous activation of all selected elements. The preferred embodiment provides multiplexed activation of the LED's selec- ted to represent RP and X.

Turning now to FIGURE 5, the supporting logic shown in FIGURE 2 is illustrated as comprising a timer 72 connected to CPU 40 as well as to LSI units 42 and 44. Additionally, a crystal circuit is utilized in the timing connections, thereby pro¬ viding a separate phase for the logic circuitry. In operation, the use of an optically active scale permits, inter alia, provision of a standout optical contrast for the endpoint optical elements in comparison with the interval being measured. That is, ^" while the two LED's at the end- points of a line interval being measured are acti¬ vated, remaining diodes are not and the endpoints only are made conspicuous, thereby decreasing the chance for measurement error. The system may simi¬ larly operate by activating the optical elements along the entire interval, or by activating all elements except those along the interval being meas¬ ured, rather than only the endpoint elements. Either of these alternatives also provides enhanced con¬ trast and reduction in measurement error. In per¬ forming a measurement, three phases of operator- machine interaction are contemplated. The following discussion may best be understood with reference to FIGURE 6 where circles correspond to operations by user and rectangles to values of registers in¬ ternal to the apparatus.

In a first phase, a reference point is selected. As discussed above, only two scale

positions are activated. The two positions may co¬ incide. One position is called the reference point (hereinafter RP) , and the other is called the cursor (or X) . When the power is turned on, RP is auto- matically set to the left margin of the LED scale and the cursor position will be the same as RP. When the buttons and are depressed, the cursor position will be changed. The new cursor po¬ sition depends on the previous cursor position, the button depressed, and the length of time a button remains depressed. The cursor position will propa¬ gate to the right (or left) as long as , (or ) is depressed. When the user positions the cursor adjacent to one endpoint A of the object to be measured, may be depressed to indicate that this position has been selected as the new RP. The light for the old RP will be turned off and, until the next time RP is changed, any later measurement will be relative to this point. That is, a dis- tance to the left of RP will be displayed as a negative number on the digital numerical display and a distance to the right as positive. The con¬ vention can be reversed by depressing (flip sign) key. When is depressed twice, the de- fault convention will be used. It is noted that the user can also choose to physically translate the whole rule so that any lighted LED can serve as the RP.

Having determined a new RP, the cursor is moved to a second endpoint of the object to be meas¬ ured and the distance between RP and cursor dis¬ played. It is possible, however, that different measurement units might be desired by the user. Accordingly, a second phase is provided wherein the

specific unit to be used is determined. When enter¬ ing the second phase, the cursor position coincides with RP. If the user selects a conventional distance unit such as inches (or centimeters) , he can de- press , (or ) , which will move the cursor position to the right by one inch (or one centi¬ meter) . The display will have a value 1. If the units are arbitrary then the calibration point CP must be defined. The user then depresses or to move the cursor position, similar to the steps in the first phase, until it is adjacent to the calibration distance. He further depresses and enters the numerical value C of the cali¬ bration distance by depressing the digits in se- quence. It is noted that C may be negative. The value of C will be displayed. The user then de¬ presses to indicate that the calibration dis¬ tance is to be taken as C units. At this time, the calibration phase is complete and the display shows the distance C between the RP and the cursor posi¬ tion.

The third phase is the measurement phase. When the user positions the cursor adjacent to the other endpoint B of the object, the distance bet- ween A and B will be converted to proper units and displayed. The orientation of the vector from A to B will also be displayed with a negative number to mean left-going and a positive number to mean right- going unless a reverse orientation has been selected by depressing .

The distance setting process can also be described as a 3-phase procedure with the first two phases identical to those of the length measuring process. During the third phase, the user enters

a number, j, which may be negative, by pressing the digits of j in sequence followed by • The num¬ ber j will be displayed and the electronic rule will move the cursor (in the direction depending on the sign of j) to a position such that the dis¬ tance between RP and the cursor position, in the units specified by the user, is j.

In both the length measuring and the dis¬ tance setting processes RP is defaulted to the left- most end of the rule if the user chooses not to set his own RP position. The measurement unit is de¬ faulted to the distance between two neighboring LED lights if the user does not specify his own. • In FIGURE 6, the control sequences described above are summarized as a flow chart. At each circle the specific user operation represented by the circle will cause the actions described in the associated rectangle. Each circled operation represents pres¬ sing of the similarly labeled button by the user on the keyboard. The registers represented by the rect¬ angles include:

RP - position of the reference point X - position of cursor D - number displayed U - the unit conversion factor in terms of number of LED' s IN,CM - constant unit conversion factor for inch or centimeter. Any sequence of operations not included in the flow chart will be considered illegal. The display will blink all the seven segment lights when an illegal operation is detected. The user can press 3 to clear the display. In this case, RP and the meas¬ urement unit will not be changed.

As an illustrative example, where it is de¬ sired to determine the length in centimeters of an object, the scale is placed against the object. If the object extends, for example, from the 10th LED through the 63rd, the operator would follow the pro¬ cedure outlined above and, after turning on the de¬ vice, move the cursor to the right until it is ad¬ jacent to one of the object endpoints, for example the left endpoint at LED 10. Depresseing the RP button will select that position as the reference point. Since the distance is desired in centimeters, the user may depress the button which moves the cursor one centimeter to the right and displays a 1 in the digital display. Further depressing the button to move the cursor to the right, to the 63rd LED, in this example, the user then ob¬ serves the distance in centimeters on the numerical display.

Were the user to desire a measurement in some arbitrary scale, as would be the case in read¬ ing a map, for example, and assuming the distance is still between the 10th and 63rd LED, the follow¬ ing procedure will be followed: After selecting the reference point, the cursor is moved to the right from the reference point a particular distance. In the case of map reading, the cursor is moved to the right by a distance corresponding to the particu¬ lar map-scale factor. Thus, where 1/8 inch equals one mile and a map scale is provided, the cursor may be moved to the right by 1/8 inch. At this point, the button is depressed and the numerical value being measured is entered. In this example, the num¬ ber 1 would be entered if the desired distance is in miles. Depressing of the button indicates the

calibration distance as being 1 unit. Finally, with the" scale against the distance being measured, the cursor is displaced to the 63rd LED and the position between the reference point and cursor is displayed numerically in miles. As a further example of the effectiveness of the present invention, in a map- scale having 1/8 inch equivalent to ten miles, the calibration step described above would be modified by entry of the number 10 rather than 1 from the keyboard after depressing the fC l button. The dis¬ play would then provide the distance in miles.

Clearly, the scale need not provide a linear measurement. Thus, it is contemplated that scales having optically active components which are themselves non-linearly distributed along the scales . may be used. The elements may be logarithmically spaced, for example. Similarly, the elements may be linearly spaced but the measurement phases, under the control of the operating program, may provide displays corresponding to non-linear distances. Thus, for example, the exact distance along a logarithmic chart might be measured using the present electronic rule under a logarithmic subroutine in the operating program. Additionally, it is recognized that the scale need not be linear but may be curved, and may, for example, be provided along a French curve.

As an example of the distance setting pro¬ cedure, a user may follow Phases 1 and 2 as previous¬ ly outlined, and may provide a distance either in centimeters, inches, or some arbitrary unit. Thus, once a reference point is determined, the inch or centimeter button may be depressed or an arbitrary unit may be entered by moving the _. cursor to a cali¬ bration distance and entering the numerical value

of that distance. The third phase of distance set¬ ting, however, requires entry of a number and de¬ pressing the button. Thus in the map example previously used, once the scale had been entered as 1/8 inch per ten miles, for example, by displacing the cursor 1/8 inch from the reference point and by depressing the button followed by entry of the number 10 from the keyboard and by depressing of the button, the user may choose to display a distance representative of 42.5 miles. To do this, the num¬ ber 42.5 would be entered by the keyboard, the depressed, and the cursor would be moved by the operating program to 42.5 miles (at a scale of 1/8 per ten miles) from the reference point. The user would then have the reference point and cursor sepa¬ rated by the distance equivalent to 42.5 miles at that scale factor.

The software used to support the functions of the electronic rule is the operating program. The program is stored in the two IK byte ROM of the

MCS 6530 chips. Basically the program follows the flow of control as shown in FIGURE 6.

The primary concept used in monitoring the keyboard in driving the LED's in scale display is to alternate between reading the ke ^' yboard and. writing to the display and scale at such a speed that the multiplexing is not discernible.

The operating program consists of a main program and several subroutines. The main program and the major subroutines are described briefly here:

(1) MAIN - This program can be entered as a result of an RST interrupt. Upon entering the pro¬ gram, data variables will be initialized by calling subroutine INIT. SCAN will then be called to execute

the scan cycle, i.e., to display the scale LED, to activate the numerical display, and to read the key¬ board in a multiplexed manner. When a key in the keyboard is depressed by the user, the program will also branch to a segment of code labeled EXEC, in which the function associated with the key is exe¬ cuted. If no key is depressed, the program will re¬ peat the scan cycle.

(2) INIT, INIT1 - This is the initializa- tion routine which sets up the initial values of all variables used in the operating program and moves RP to the left margin of the scale. INITl is a dif¬ ferent entry point to the subroutine.

(3) SCAN - The routine first selects each of the six seven segment units in sequence to dis¬ play a number (with possibly a sign and a unit) , then turns on the two LED lights in the scale that cor¬ respond to RP and X (cursor) , and checks to see if any key in the keyboard is depressed. If no keyis depressed, the same control sequence is repeated, that is, displaying a number, turning on RP and X, and checking the keyboard. If a key is found de¬ pressed, the control will set up a nonzero value in the accumulator A. Otherwise, A will be cleared to zero. The routine calling SCAN can check the con¬ tents of A to determine if a key is depressed.

(4) CONV - This routine uses an internal table to convert a number into bit patterns which correctly select the segments of one seven segment display unit, causing the number to be displayed in the selected unit.

(5) LED - This routine outputs a value to the MCS 6530 unit 44 peripheral pins, causing one of the LED's in the scale to turn on.

(6) ,KEY, ONEROW - These routines read the keyboard to determine which key is depressed. ONEROW checks only one row of the keys. KEY calls ONEROW repeatedly to check every row. (7) DIVD - A division routine.

(8) MULT - A multiplication routine.

(9) BCD - This routine converts a binary number to its binary-coded-decimal (BCD) equivalent, which will then be used to drive the seven segment display.

(10) ERROR - This routine blinks all the lights in the seven segment LED's to signal an error. The routine is activated only when illegal operations are detected or when an operation exceeds the pre- cision or margins of the electronic rule.

When the power is initially turned on and the fRSl is depressed by the user, an RST interrupt takes place, which starts the SCAN sequence. The SCAN routine will drive the display and the scale and monitor the keyboard repeatedly. When a de¬ pressed key is detected, its function is then exe¬ cuted by EXEC. If an error is detected, ERROR rou¬ tine will flash the display, which can only be cleared by pushing the [RSI key. Each time the [RSI key is depressed, the electronic rule is ini¬ tialized and the operating program re-started.

The various subroutines outlined above are shown in flow chart format in Appendix A and B.

APPENDIX A OPERATING--PROGRAM DESCRIPTION

Background

1. When the power is turned on and the IRSI key is. de¬ pressed, an RST (Reset) interrupt is generated which will start the operating program.

2. Each 6530 chip includes a 1024 byte Read-only Memo¬ ry (ROM) . The operating program is to be stored permanently in the 2048 bytes of ROM in the two 6530 chips (6530-X and 6530-Y) .

3. Each 6530 chip includes a 64-byte Random Access Me¬ mory (RAM) . The total 128 bytes of RAM is to be used for (a) the storage area of the stack to be used by the microprocessor 6502 to save return ad¬ dresses in subroutine calls and (b) the data areas storing data variables related to the user opera¬ tions. The data variables related to the item (b) are listed under "Data Constants and Variables".

4. In the flow-chart description of the operating pro¬ gram, "JSR XXX" is used to mean "Jump to subroutine whose name is XXX". This has the affect of saving the return address (RA) in the stack (which is mentioned in 3 (a) above) . The symbol "RTS" is used to mean "return from subroutine", which is the last instruction in execution in a subroutine and has the effect of directing the control to the address (RA) saved on the top of the stack as well as popping RA off the stack.

5. The entire operating program is described as a col¬ lection of routines with each routine represented as a flow-chart.

6. An italic name followed by a colon (:) is used as a label for the nearest statement.

7. DISPLAY, KEYBOARD, and SCALE refer to the three

corresponding componentsof the Electronic Rule. All numbers are in decimal unless specified other¬ wise. Hexadecimal numbers are indicated with a subscript H.

9. Beginning of a routine, name of the routine is enclosed.

An Action, a program statement

A decision box

Branching of control, the subroutine calls and returns.

A "go to" statement, the destination is indicated in¬ side the circle.

10. A is the accumulator of the 6502 microprocessor. 18 is the BCD-decimal decoder 60 shown in FIGURE 3,

DATA CONSTANTS AND VARIABLES

(Note: The size (number of bytes) of the variables is not specified) .

Variables (located in RAM) :

RP Reference point position

XCUR (same as X in Cursor position the previous discussion)

Numerical value of the distance to be displayed, represented in BCD form with each segment of 4 bits for a decimal digit. The leftmost byte of D, called sign byte of D

stores the sign (.<- or no display or +) of.-the. distance. All segments are index pointers to TABLE.

INCM An index pointer to TABLE, 11 for i (inch) . 12 for c (cm) or 10 for null.

UNITSIGN Sign of UNIT, either 1 for negative or 0 for positive. UNIT Unit factor, indicating the number of lights for each basic measurement, unit selected by the user.

CPMODE A flag indicating whether the user is defining his own measurement unit, either 1 for CP mode or 0 for distance setting.

DIGIT The most recent digit entered by the user. MINUSFLAG A flag indicating whether the user has depressed the - key, either 1 for depressed or 0 otherwise.

DIGIT* Number of digits entered by the user. .FLAG A flag indicating whether the user has depressed the .key (decimal) point) , either 1 for depressed or 0 otherwise.

SUM Integer portion of the number entered by the user.

FSUM Fractional portion of the number entered by the user.

DA, DB, Q Divident, divisor, and quotient, respectively.

MA, MB, P Multiplicand, multiplier, and product, respectively.

Constants (located in ROM) :

INFACTOR Number of lights for one inch. CMFACTOR Number of lights for one cm. TOTAL Total number of lights in SCALE. BOUND Maximum number of digits user can enter (integer or fractional portion) .

TABLE (entry 0) - 7-segment code for 0

(entry 1) - 7-segment code for 1

(entry 2) - 7-segment code for 2

(entry 3) - 7-segment code for 3

(entry 4) - 7-segment code for 4

(entry 5) - 7-segment code for 5

(entry 6) - 7-segment code for 6

(entry 7) - 7-segment code for 7

(entry 8) - 7-segment code for 8

(entry 9) - 7-segment code for 9

(entry 10) 7-segment code for No display (null)

(entry 11) 7-segment code for i

(entry 12) 7-segment code for c

(entry 13) 7-segment code for —

About sign convention:

1. By default, if X ≥ RP then the distance is displayed as positive. If X < RP, then it is negative. X is the cursor position.

2. The user can reverse this convention by hitting the F key. This will cause no change in the positions of RP and X per se. But this will cause the reversal of the signs. Hitting the F key again will return to the default convention. X is the cursor position.

3. The - (minus) key by itself does not reverse the sign convention. It merely causes the minus sign to be displayed.

4. During the distance setting operation, one can depress the minus key, followed by a series of numeric keys to cause the distance to be displayed. The following pos¬ sibilities may occur in a distance setting operation. X is the cursor position.

Sign convention Minus key Relative positions depressed ^' of RP,X default (UNITSIGN=0) YES X to the left of RP default (UNITSIGN=0) NO X to the right of RP reverse (UNITSIGN=1) YES X to the right of RP reverse (UNITSIGN=1) NO X to the left of RP

5. When the user is defining his own measurement unit using CP key, the sign of the unit depends on the relative po¬ sitions of RP and X, and whether the minus key is de¬ pressed or not. The following cases are possible. X is the cursor position.

Explanation of MAIN:

There are three JSR SCAN statements in the main program MAIN. If the power is just turned on and no key is depressed, the control stays in the loop of the sec¬ ond JSR SCAN (starting with Repeat) . When a key is de¬ pressed, the control enters into the third JSR SCAN which double checks if the key is indeed depressed

(not noise) . If so, go to EXEC. Otherwise, the con¬ trol returns to the second JSR SCAN. After EXEC, which performs the function associated with the key depressed, the control returns to the loop of the first JSR SCAN

(starting with Start) and waits until key is released.

APPENDIX B

Having thus described the objects, features, and ad tages of the present invention, and having provided a preferr embodiment thereof which is to be -used for illustration and n limitation, it is appreciated that many variations of the dis closure will be apparent to those of ordinary skill in the ar Such variations do not depart from the spirit of the inventio and are included within the scope of the appended claims.