Abstract:
The disclosure describes a computer-aided instructional system for teaching a student to trouble-shoot a piece of equipment, such as a radio receiver, with a tester, such as a volt-ohm meter (VOM). In order to achieve this result, the receiver and VOM are simulated by non-operating components which are controlled by the computer to respond as if they were fully operative and functioning devices. A computer-operated display presents instructions to the student and monitors the student&#39;s responsive use of the simulated receiver and VOM. Additional instructions are selected for display depending on the nature of the student&#39;s responses. In this way, the student learns to analyze the equipment by proper use of the tester.

Description:
This is a continuation of application Ser. No. 508,997, filed Sept. 25, 1974, now abandoned. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to instructional systems and more particularly relates to automated systems for sequentially presenting instructions to a student. 
     Automated devices for presenting instructions to students and for monitoring their responses have been devised in the past. Although these systems have been adequate to teach purely intellectual or abstract reasoning skills, experience has shown that they are inadequate to teach psychomotor skills needed for the trouble-shooting, repair or maintenance of complicated devises, such as electronics equipment. In order to improve psychomotor skills of the type needed to handle electronics equipment, the student must be given the &#34;hands-on&#34; experience of actually working with the equipment. This requirement leads to substantial difficulties if large numbers of students must be trained on large, complex electronics systems. Typically, the systems are not available in sufficient quantity to provide each student with his own piece of equipment and the systems are too valuable, delicate or dangerous to place in the hands of a novice. 
     In order to overcome these difficulties, the applicants have invented an improved system for teaching a student to evaluate equipment, such as a radio receiver, having a plurality of points normally maintained at predetermined conditions, such as particular DC voltages, by means of a tester, such as a volt-ohm meter (VOM), capable of indicating point conditions sensed by a probe connected to the tester. 
     In accordance with a principal feature of the invention, the equipment being taught is simulated by inexpensive, nonoperating components. For example, in the case of a radio receiver, the outer case, control knobs and printed circuit boards holding the electrical chassis are provided, but the electronic chassis is omitted. The tester is also simulated. For example, if the tester is a VOM, the outer case, control knobs and indicating meter are provided, but the electronic chassis is omitted. 
     A test point data storage device, such as a digital memory, stores digital test point condition signals which represent a simulated condition at preselected test points of the simulated equipment, for example, conductors between components and conductors connected to control knobs, indicator lamps and the like. Likewise, probe detection storage means store a digital test point identification signal which represents the test point of the simulated equipment communicated with by the detector of the simulated tester. Instruction means, such as a scope display unit, present instructions and questions to the student. 
     A data processing device, such as a general purpose digital computer, controls the presentation of the instructions to the student and monitors the student&#39;s responsive use of the simulated equipment and tester. For example, in response to a request to place the detector on a particular test point, the data processing means scans the detection storage means to determine which test point, if any, is touched by the detector. The digital test point condition signal corresponding to the touched test point is then obtained from the test point storage device, and a tester indicating signal corresponding to the fetched digital test point condition signal is generated and transmitted to the simulated tester. If the tester simulates a VOM, the display signal causes the needle of the meter in the simulated VOM to indicate a voltage corresponding to the actual voltage which would be encountered in a real piece of equipment, such as a radio receiver, at the test point touched by the detector. In addition, the instruction means, such as the scope display unit, can highlight on the circuit schematic the desired test point location and can outline on the circuit schematic the test point actually touched by the probe. This permits the student to relate the functional electrical operations, illustrated by the circuit schematic, and the actual physical location of the components and test points on the circuit board. 
     According to another feature of the invention, the test point storage device is capable of storing different test point condition signals for each test point. By use of this technique, the invention can simulate both normal equipment operation and operation while certain components of the equipment have failed. Thus, the student is able to learn trouble-shooting under controlled equipment failure conditions without the necessity for actually placing inoperative components in the equipment. 
     A system of the foregoing type offers a number of advantages. Since there is a direct electronic link between the data processing means and the simulated equipment and tester, the data processing means can continuously monitor the reaction of the student to any instruction requiring him to use the tester or simulated equipment. The ability of the applicants&#39; system to continually monitor the student&#39;s performance and responses makes the system virtually &#34;cheat&#34; proof. 
     The applicants&#39; system also gives the student hands-on experience which accurately simulates the training received on real pieces of equipment using real testers. However, since the applicants&#39; simulated equipment and testers employ low voltages and current values, there is no danger that either the student or the equipment will be harmed or rendered inoperative by any mistake of the student. This feature offers a substantial cost saving over the use of actual equipment for training purposes. When real equipment is utilized, a mistake by a novice or careless student could destroy or seriously damage the equipment. Such equipment is generally costly and time-consuming to repair, and the training procedure is interrupted until the repair is completed. Furthermore, introduction of faulty components for practice in malfunction diagnosis often causes other components in the operating circuit to be damaged. Then, even if the device continues to operate, the student is precluded from observing properly functioning equipment. 
     The applicants&#39; invention also increases the safety of the student. Many types of electronic equipment employ high voltages or high currents which are harmful, if not fatal. By using simulated equipment of the type described, the voltage levels are low and undetectable to the touch of the student. 
     The applicants&#39; system also is adaptable for use by large numbers of students in geographically separated classrooms. However, even when the system is used for such group teaching techniques, each student is free to choose his own instructional sequence and to proceed individually at his own pace through the sequence of instructions. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages and features of the present invention will appear in connection with the accompanying drawings wherein: 
     FIG. 1 is an isometric view of a preferred form of student station made in accordance with the present invention, including an exemplary simulated short-wave radio receiver, an exemplary simulated volt-ohm meter, a CRT display, and keyboard input; 
     FIG. 2 is a top plan view of the radio receiver shown in FIG. 1; 
     FIG. 3 is an electrical schematic diagram that is shown to the student on the CRT display of the student station shown in FIG. 1; 
     FIG. 4 is an electrical schematic diagram illustrating a portion of the simulated radio receiver shown in FIG. 1; 
     FIG. 5 is an electrical schematic diagram of the simulated volt-ohm meter shown in FIG. 1; 
     FIG. 6 is a block diagram of a preferred form of computer-aided training system made in accordance with the present invention; FIG. 7 is an enlarged, electrical schematic drawing of a portion of the hybrid link illustrated in FIG. 6; 
     FIG. 8 is an enlarged, electrical schematic drawing of another portion of the hybrid link illustrated in FIG. 6; and 
     FIG. 9 is a block diagram of a preferred form of software system used in connection with the training system shown in FIG. 6. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, a preferred form of computer-aided training system made in accordance with the present invention basically comprises a simulated piece of electronic equipment 10, a simulated piece of test equipment 140, a data processor 180, a hybrid link 190, a display and keyboard unit 200 and a software system used in connection with the data processor. 
     More specifically, referring to FIGS. 1-4, simulated electronic equipment 10 comprises a shortwave radio receiver 12 with parts of the normal chassis removed. The simulated receiver includes a tuning indicator 13, a tuning dial 14, a volume dial 15, a beat frequency oscillator (BFO) dial 16, a mode selector dial 17, a band selector dial 18 and an on-off dial 19. The receiver also includes a printed circuit board 20 which supports simulated components 21 on a frame 22. Of course, the components may be actual instead of simulated. However, since no power is applied to the components, they remain inoperative in any case. 
     Referring to FIGS. 3, 6 and 7, multiple conductors are connected to defined test points of the receiver components. More specifically, test points T1-T12 are connected to conductors 31-42, respectively; test points T14-T17 are connected to conductors 44-47, respectively; test point T18 is also connected to conductor 47; test points T19-T25 are connected to conductors 49-55, respectively; and test points T27, T28 are connected to conductors 57, 58, respectively. Receiver 12 includes simulated resistors R1-R19, R21-R29, R31-R39, and R41, R42; rotary switches BS1F, BS1R, RS2F and BS2R; and transistors Q1-Q9, D9 and D10. 
     Referring to FIGS. 4 and 6, -10 volts is supplied to simulated receiver 12 through a conductor 169, and ground potential voltage is supplied to receiver 12 through a conductor 149. The voltage on conductor 169 is applied to additional circuitry in the receiver through an on-off switch 131 (FIG. 4) which is controlled by on-off dial 19. For example, the voltage is applied to a series of indicator bulbs 132 through a resistor 133. 
     When switch 131 is closed, -10 volts is also supplied through a resistor 135 to switches 17A, 18A and potentiometers 15A and 16A. The operation of switches 17A and 18A is controlled by mode selector dial 17 and band selector dial 18, respectively. The operation of potentiometers 15A and 16A is controlled by volume dial 15 and BFO dial 16, respectively. The switches and potentiometers are connected to conductors 117-120 in the manner shown in FIG. 4. These conductors act as test points for detecting the condition or state of the control knobs 17-20. 
     When switch 131 is closed, 10 volts also is supplied through a resistor 136 to a potentiometer 14A controlled by tuning dial 14. A conductor 116 is connected to potentiometer 14A and another conductor 115 is connected to resistor 136 in the manner shown in order to test whether switch 131 is in the on or off position. Thus, conductors 115 and 116 are test points for determining the condition or state of tuner dial 14 and switch 131. 
     Of course, simulated equipment 10 can simulate different features from different actual pieces of equipment. For example, the mixer circuit from one radio and the detector circuit from another radio can be simulated by the same piece of simulated equipment, so that the student can learn different types of radio circuits existing in different radios from a single piece of simulated equipment. 
     Referring to FIGS. 5 and 6, simulated test equipment 140 preferably comprises a volt-ohm meter (VOM) 142 including a case 143 and a conventional meter movement 144 that operates a deflection needle 145. Behind the needle are various scales which read in volts DC, volts AC and ohms. Eight volts DC must be applied to the meter movement in order to achieve full scale deflection of needle 145. A tester indicating signal which deflects needle 145 is received over a conductor 147 and through a resistor 148. Conductor 149 is connected from the meter movement to ground potential. 
     A scale selector knob 150 moves a rotary control switch 152 comprising a movable arm 153 which makes contact with conductors 156-167 one-at-a-time. Minus 10 volts is supplied to arm 153 via conductor 169. Conductors 156-167 correspond to various scales inscribed on the face of meter 142. For example, conductors 156-159 are used in connection with the 1 volt DC, 10 volt DC, 100 volt DC, and 1000 volt DC scales, respectively. Likewise, conductors 160-163 are used in connection with the 1.2 volt AC, 10 volt AC, 100 volt AC and 1000 volt AC scales, respectively. Conductors 164-167 are used in connection with the resistance x1, resistance x100, resistance x10K and resistance x1M scales, respectively. Knob 150 is operated by the student in order to identify the desired sensitivity of the tester on the volts DC, volts AC and resistance scales. 
     Plus 10 volts is transmitted over a conductor 170 to a detector probe 171 having a tip 172 that is adapted to comate with the test points in receiver 12. Detector probe 172 is sufficiently large to be handled manually by a student using the system. 
     As shown in FIG. 6, conductors 156-167 are bound together in a cable 174, conductors 31-42 and 44 are bound together in a cable 175, and conductors 45-47 and 49-58 are bound together in a cable 176. 
     Data processor 180 preferably comprises an XDS Sigma-5 computer with a FORTRAN compiler. The processor includes an arithmetic unit 182, a control unit 183, and a memory 184 having logical arrays DI1, DI2, DI3, IDI4, IDI5 and designated memory locations A (340), A (341), A (343) and A (202). Memory 184 also includes array tables V and R. Logical arrays DI1 and DI2 comprise probe detection means for storing a different digital test point identification signal for each test point touched by probe 171. Thus, arrays DI1 and DI2 are responsive to the voltage on probe 171. Array table V comprises test point storage means for storing digital test point condition signals representing a simulated voltage condition at each of the test points. Array table R serves the same function for resistance conditions at each of the test points. Array IDI4 comprises tester control storage means for storing a digital tester control signal representing the state of the VOM control switch 152. 
     Data processor 180 also includes an input-output unit 185 which preferably comprises an XDS model 7929 direct input-output adapter. Additional storage capacity is provided by a conventional 3.3M magnetic disk storage unit 186 with an average access time of 17 milliseconds at a rate of 0.2M characters per second. 
     Referring to FIGS. 6-8, data processor 180 communicates with receiver 12 and VOM 142 through a hybrid link 190 which preferably is an Adage Model 770 computer link. Link 190 comprises: a 14-bit discrete input 121 which receives data from cable 175 and transmits data to memory logical array DI1 over a 14-bit cable 121C; a 14-bit discrete input 123 which receives data from cable 176 and transmits data over a 14-bit cable 123C to memory logical array DI2; a 14-bit discrete input 124 having its inputs disconnected and having its outputs connected to memory logical array DI3 over a 14-bit cable 124C; and a 14-bit discrete input 125 which receives data from cable 174 and transmits data over a 14-bit cable 125C to memory logical array IDI4. Input 125 provides a means for sensing the state or position of VOM control switch 152. Input 124 may be used to expand the number of test points that can be probed on receiver 12. 
     Link 190 includes a 14-bit discrete input 126 which receives data from mode switch 17a and band switch 18a and transmits data over a cable 126C to memory logical array IDI5. Link 190 also comprises an analog-to-digital converter 127 (FIG. 6) which receives data over conductors 116-118 from potentiometers 14a-16a and transmits corresponding digital data over conductors 127A-127C to memory locations A (340)-A (342), respectively. Input 126 and converter 127 comprise equipment control detection means for sensing the position of the receiver control knobs 14-29 and for communicating this information to processor 180. Likewise, memory array IDI5 and memory locations A (340)-A (342) comprise equipment operating control storage means for storing digital equipment control condition signals representing the positions of the receiver control knobs 14-19. 
     Link 190 also includes a digital-to-analog converter 128 which receives digital data from memory location A (202) and transmits a corresponding tester indicating signal over conductor 147 to VOM 142. 
     Link 190 provides a source of plus 10 volts 129 and a source of minus 10 volts 130 to conductors 170 and 169, respectively. Conductor 149 is grounded through link 190 as shown in FIG. 6. 
     Referring to FIGS. 1 and 6, the training system includes a student scope display and keyboard input unit 200 which preferably is a Honeywell Information System Model S-50. Unit 200 comprises a cathode ray tube (CRT) display 202 and a conventional keyboard input 204. Unit 200 is connected to other parts of the system through a cable 206. 
     The training system also comprises an instructor scope display unit 210 which preferably is a Control Data Corporation Model DD-40 display unit. Unit 210 is connected to other parts of the system by a cable 212. Data processor 180 is programmed by a unique software system. The function of the software system is to: 
     (1) Display text material explaining the operation of VOM 142 and receiver 12; 
     (2) Provide a simple structure for a student question-answer dialog; 
     (3) Drive and monitor VOM 142; 
     (4) provide a short summary of results; 
     (5) Display the student&#39;s progress on instructor CRT 210; and 
     (6) Monitor the receiver switch and knob settings. 
     In this system, computer 180 is programmed to instruct a student by displaying on CRT 202 both text material and the schematic of receiver 12 shown in FIG. 3. The student responds via keyboard 204 or by measuring a specified test point using VOM 142. Radio receiver 12 is made of simulated components, control knobs and test points. The control knobs are monitored by the computer for proper setup. The test points are also monitored to sense if VOM probe 171 contacts them. 
     VOM probe 171 has a low voltage (+10 volts) on tip 172 which completes a circuit when it touches a test point. By monitoring all test points, the computer &#34;knows&#34; which point is touched. VOM needle 145 is deflected according to the VOM setting of control knob 150, the test point touched and the test point condition data prestored in the memory tables V and R. This test point condition data can simulate normal receiver operation or receiver operation under different types of component failures. The present system is programmed to simulate dc voltage and resistance-to-chassis-ground measurements for normal operation and abnormal operation due to one type of failure. In addition, the test probe location can be superimposed on the schematic displayed on the CRT. 
     Instructor display 210 displays time and the location of the student in the programmed learning and testing phase. In addition, if the student requests help, a message appearing on the instructor display notes that assistance is required by the student. 
     Student responses from keyboard 204, as well as data from VOM 142 and receiver 12, are input and output via the I/O 185 and hybrid computer link 190. As previously described, this link consists of an analog-to-digital converter 127, a digital-to-analog converter 128 and discrete input and output words 121, 123, 125 and 126, all under computer control. the computer monitors the receiver test points and VOM control knob settings through the discrete input words. Upon computing the proper VOM needle deflection, converter 128 sets deflection needle 145 accordingly by transmitting a tester indicating signal over conductor 147. 
     By using the disk storage unit 186, text material displayed on CRT&#39;s 202 and 210 is brought into the computer memory. As each instructional module is accessed, a new block of text is brought in from disc 186 in 6.4K character blocks. Each new text block overlays the previous block. In addition, a common 6.4K character text block is used by all modules and is not changed. This latter block contains messages indicating student errors, representative system responses shared by all modules, the student&#39;s name, etc. This method of storing and swapping text information consumes only 0.5 sec. for each 6.4K block of data swapped, and does not appear to influence performance. 
     FIG. 9 illustrates the software organization of the computer. The control hierarchy starts at the MAIN routine, which in turn calls the system routines (EXEC) or the instructional modules (INTRO, SETUP, VOM, FDES, EXAM): These in turn access lower level routines which perform special purposes. For example, KEYIN monitors the student keyboard and stores keyboard characters in a buffer in data bus STEXT. 
     Communication between modules or routines is primarily through common data blocks in memory. These blocks are denoted by THRUST and STEXT. Routines which access these data blocks or buses are shown by the open (dotted) lines. 
     During operation, a student proceeds through a series of instructions and questions programmed in the instructional modules and displayed on CRT 202. Based on his response, the routines set pointers denoting text to be displayed, instructional paths, repeat paths for errors, etc. For example, when an instruction requests the student to perform a step of a test procedure, the system pauses to give the student time to attempt to perform the step. A variety of responses can then be displayed on CRT 202 depending on the manner in which the student performs the step. 
     Use of this pointer method permits a simpler coding form and ease in allocating instructional paths. The pointer system is based on a section-chapter-paragraph order. An example is found in the functional descriptive routine FDES (which will be disclosed in detail later). In this case, FDES is denoted by the section pointer being 4. The routine contains 16 chapters. Initial entry to this routine is with the chapter and paragraph equal to 1. Each time the student sets the keyboard carriage return key, the paragraph counter is incremented. The chapter counter has a variable increment dependent upon the response. For some errors, the counter may be reset to one causing a repeat of a chapter, or going to the next chapter for a correct response. 
     The software routines and their respective functions are shown in the following Table 1. 
     
                       TABLE 1______________________________________System Routines  MAIN     Basic Routine to call other subroutines.  EXEC     Provides input of trainer parameters and - data dump. Also      provides time counter.Input/OutputRoutines  CONVERT  Provides analog-to-digital (A/D) and      digital-to-analog (D/A) control and scaling.  DISPLAY  Controls both instructor&#39;s and student&#39;s CRT      display. For instructor&#39;s CRT, provides      time and location of student in program.      For student&#39;s CRT, displays text statements      from text library, based on text pointers,      and outlines circuits, components or probe      test points on the schematic.  KEDISP   Samples student keyboard input and stores      input text in memory. Also sets carriage      return flag, decodes messages like &#34;help&#34;      and sets &#34;help&#34; flag.  DSTORE   Inputs test sentences and test point data,      and stores in specified files.  QUE      Inputs text data to memory from files.      Text is automatically input every time      a new section module (FDES) is referenced      by program. Common text occupies 1st      100 locations of ITLIB. Section dependent      text, which is overlayed for each section,      occupies the upper 100 locations of ITLIB.,      i.e., ITLIB (17,101) to ITLIB (17,200).      Text is assessed by DISPLY.      QUE routine also provides a student monitor      function to note gross error, like making      resistant measurements with radio on.  DCOM     Data command routine, brings test point      data into memory, calls DSTORE. Also      computes test point student is probing and      VOM needle deflection voltage to D/A 1.InstructionalRoutines  INTRO    Describes system operation and command      structure to student, and provides student      computer dialog.  SETUP    Instructs student on proper radio receiver      setup, and provides correction logic and      text to correct improper setup.  VOM      Instructs student as to VOM operation and      set up. Provides text for student to per-      form simple VOM measurements. Monitors      student test responses and provides error      detection and corrective text logic.  FDES     Instructs student on function and operation      of radio receiver. Provides test logic for      student to probe circuit board and display      probe position on CRT with schematic over-      lay. Also provides series of tests designed      to evaluate student&#39;s knowledge of circuit      board and functional operation.  EXAM 1   Instructs student on typical trouble-shooting      techniques for given failure. Provides sym-      ptoms and typical procedure for this type of      failure.  KEYIN    Checks for errors made by student. Deter-      mines whether VOM input voltage is within      the proper range and whether dial 150 is set      properly.Library Routines  FUND     Provides data for outlines of predetermined      areas of CRT display. The FUND routine is      the data source used to outline a functional      block of the radio receiver with a schematic      overlay on the student CRT display.Additional library routines for utilizing link 190 are executed:    (1) to input to processor 180 a discrete    (14-bit) word in hybrid link 190 (See FIG.    9);    (2) to output a discrete word (IWORD) from    processor 180 to link 190;    (3) to input word I (I=123, 124, etc.); and    (4) to set a 14 element logical array to true    or false depending on the bits being on or off.    (Link bit 0 is the most significant link bit, 13    is the least significant, i.e., if link bit 0 is    true, IA(I) is true.)Other library routines for utilizing display units 200 and 210are executed:    (1) to display a hollerith character on CRT    display 202 at a position specified by X and    Y;    (2) to display a hollerith character string    on CRT display 202, starting at a position    defined by X and Y;    (3) to draw a line from the present raster    located on CRT display 202 to that defined    by X and Y; and    (4) to specify if scope commands DCHAR,    DTEXT and DLINE are displayed on the    instructor scope display 210 or the stu-    dent scope display 202.______________________________________ 
    
     Instructions for preparing these additional library routines generally are provided by the link and display unit manufacturers and are well within the skill of persons trained in computer programming. 
     The SETUP routine is an important feature of the system, because it alters the instructions to the student depending on the manner in which the receiver control knobs 14-19 are adjusted by the student. As a result, the system is capable of training the student how to operate the receiver as well as to repair it. 
     The DISPLY routine is also an important feature because it enables a schematic diagram of receiver 12 (FIG. 3) to be displayed on CRT 202 and, more importantly, it identifies the test point to be touched by probe 171 on the displayed schematic. 
     One of the principal features of the system is its ability to simulate the operation of electronic equipment and of a test instrument useful in trouble-shooting the electronic equipment. In the preferred embodiment, the electronic equipment is simulated radio receiver 12 and the test instrument is simulated VOM 142. Through conductors 115-120 and cables 174-176, computer 180 monitors the setting of receiver knobs 14-19, VOM scale selector dial 150, and the receiver test points touched by probe 171. 
     In order to determine the particular test point touched by VOM probe 171, the probe has a +10v potential conducted to its tip 172. When this tip touches a receiver test point, it raises the voltage of the test line connected between link 190 and the touched test point. About 10 times per second, computer 180 monitors these lines and notes if a test point line is true (maintained at 10 volts) or false (maintained at 0 volts). If all test lines are maintained at zero volts, the probe is not touching a test point. If a line is true, the computer notes the test point touched and accesses a corresponding value from a pre-stored data table (either table V or R). This data then is scaled to a value corresponding to the proper deflection of VOM needle 145 and is converted to an analog voltage used to deflect the needle. 
     As noted in Table 1, the software routine which provides the simulation capability is DCOM. This routine is accessed by the primary calling routine MAIN. The routine CONVERT, also called from MAIN, causes digital data to be converted to an analog voltage in order to deflect needle 145. A preferred form of the DCOM routine programmed in FORTRAN is given in Table 2. ##SPC1## 
     An illustration of how the system simulates receiver 12 and VOM 142 by means of routine DCOM is given in the following example. During the course of instruction, the student is requested, by the computer, to enter on keyboard 204 the dc voltage at a specified receiver test point. 
     Assume in this case the test point is T1, and its simulated normal voltage is 5.0 volts, dc. Assume also the student sets VOM control knob 150 to the 10v dc scale. As seen in FIG. 5, the setting of knob 150 results in +10 volts being applied to VOM sense line 157. Line 157, as well as lines 156 and 158-167, are connected between VOM 142 and link 190. The electrical voltages on these lines are converted to computer data in subroutine DCOM. Routine DCOM inputs the data from link 190 and stores the voltage control levels in a memory array IDI4 in response to the CALL DINI4 (125, IDI4) command. At line nos. 102 to 112 of DCOM (Table 2), each array location is examined in a sequential manner to see which VOM line was selected by knob 150. IDI4 (1) corresponds to line 156, ..IDI4 (10) corresponds to line 165 etc. Hence the variable IMETER (line nos. 111, 112 of DCOM) is set to 2, since line 157 was on, causing IDI4 (2) to be logically true (less than zero). 
     The test point touched by VOM probe 171 is computed in line nos. 61 through 101 of DCOM. If probe 171 touches a test point, the test point line associated with that test point is raised to +10 volts. This causes an electronic element to be set in computer link 190 which can be input into computer 180 by the CALL DINI4 (I, DI) command in line nos. 70-74 of routine DCOM. Link 190 is made up of 14 lines per word, where words are designated as units 121, 123, etc. The variable DI is a data array in memory 184 in which each line state is stored. Hence the command &#34;CALL DINI4 (121, DI1)&#34; stores the first 14 test point lines (lines 31-44) in DI1. The next 14 test point lines (lines 45-58) are input through word 123 and are stored in DI2. The values stored in DI1 and DI2 are true if the line has 10 volts. (i.e., if the probe is applied), or false if the line has 0 volts (i.e., if no probe is applied). For computer 180, true is a number less then zero. 
     In line nos. 75-101 of DCOM, the program checks through the test point data arrays in a sequential fashion to determine which, if any, test point has the probe on it. If the test probe is not touching any test point, the data array elements DI1, DI2, DI3 are false resulting in the test point indicator ITP being equal to zero. In this example, the student applies probe 171 to test point T1, so that line 31 is raised to +10 volts and the variable ITP is set to 1. 
     The system is capable of simulating two different voltage conditions (or resistance values) at test point T1, or any other test point. In the present example, data from voltage and resistance measurements are prestored in array tables V (I,J) and R (I,J). I is a failure flag: if I=1, normal operation is simulated, and if I=1, failure type 1 operation is simulated. J is the test point designator. Hence location V (1,1) contains the voltage value at test point T1 for a normal or no-failure condition, and location V (2,1) contains the voltage value at test point T1 for a failure condition. During initialization of the run, both normal and failure simulation data is read into tables V and R of the memory by the routine DSTORE. The failure flag is set through the EXEC routine. In addition to use by the DCOM routine, the failure flag is used by routine KEYIN (line no. 81) for making sure that the student enters the proper VOM value on keyboard 204 and for cuing the student when he starts reading abnormal test values. 
     Line nos. 123 through 139 of DCOM illustrate how VOM 142 is driven from the computer based on the VOM meter setting (IMETER), the test point touched (ITP), and the failure condition or flag. Basically, computer 180 calculates the proper voltage needed to deflect VOM needle 145 in order to simulate the voltage present at test point T1. Of course, resistance measurements also can be simulated. 
     Since meter needle deflection is a function of the VOM control knob setting, the meter setting (IMETER) is checked to identify the appropriate data table or array. If IMETER is greater than 9, resistance or R data is evaluated; if IMETER is less than 9, voltage data or V data is evaluated. After the correct data is accessed, it is scaled appropriately and stored in variable ATD. ATD is stored in the common memory area A(202). The value of ATD is converted to an analog voltage in routine CONVRT. 
     The scale factors used in determining ATD in DCOM are based on the assumption that an 8 volt input to meter 144 causes a full scale deflection of needle 145. If IMETER=2, indicating that the 10 volt dc VOM scale was selected, a 5 volt dc reading would produce a 50% scale deflection. In order to achieve a 50% scale deflection, VOM drive line 147 must be raised to 4 volts. Since IMETER=2 in this example, ATD is calculated at line no. 125 of DCOM by the equation: 
     
         ATD = SFACT*V (IFAIL, ITP)* 0.1. 
    
     since SFACT is the full scale to ATD unit conversion factor (8 volts) and V (IFAIL, ITP)=5 volts (i.e., the voltage to be simulated at test point T1, 
     
         atd=8x (5) x0.1=4. 
    
     The 0.1 term refers to the fact that the meter deflection scale is 10 times greater than the unit&#39;s scale when IMETER=2. Hence, the driving voltage when IMETER=2 is one-tenth the driving voltage when IMETER=1. For resistance measurements, a log VOM scale is used. The log scale is approximated by the equation: 
     
         ATD=SFACT × R/ (scale+R) 
    
     where SCALE is a variable dependent on the resistance VOM scale selected. (See line nos. 134-137 of DCOM). 
     In summary, if the student sets VOM 142 to the +10 volt dc scale and places probe 171 on test point T1, needle 145 reads 5 volts, just as a real VOM would read if its probe were placed on test point T1 of a real receiver. 
     Additional features of the system will be apparent to those skilled in the art from the following detailed listing of the above-described software routines in FORTRAN ##SPC2## ##SPC3## ##SPC4## ##SPC5## ##SPC6## ##SPC7## ##SPC8##