Universal fuel quantity indicator apparatus
TECHNICAL FIELD 
     The present invention relates to fuel quantity indicator apparatus, and particularly to universal fuel quantity indicator apparatus capable of being reconfigured so as to be usable with a plurality of different fuel tanks having different associated parameters, with the fuel quantity present in such a fuel tank being based on calculations of capacitance of a tank capacitor array and a compensation capacitor, wherein a plurality of different parameters specific to a given tank configuration determine the measurement factors for the calculated fuel quantity determination. 
     BACKGROUND ART 
     Fuel quantity indicators, such as for use on aircraft, which determine the fuel quantity present in a fuel tank based on calculations of capacitance of a tank capacitor array and a compensation capacitor are well known in the art, such as disclosed in commonly owned U.S. Pat. Nos. 4,090,408 and 4,173,893, both of which name Geoffrey S. Hedrick, one of the inventors herein, as the sole inventor thereof. Each of the fuel quantity indicators disclosed in these patents are digital fuel quantity indicators and employ zero set adjustment. However, neither of these prior art systems is able to act as a universal fuel quantity indicator which can be reconfigured for a plurality of different fuel tank configurations such as by reprogramming the fuel quantity indicator constants to conform to varing fuel tank parameters associated with different aircraft or different fuel tank configurations on a given aircraft. This has required that several different fuel gauges be inventoried for a given aircraft and for a given aircraft manufacturer. This is so despite the fact that digital fuel quantity indicators employing a programmable read only memory as an integral part of the fuel gauge are known in the prior art, such as disclosed in U.S. Pat. No. 4,083,248 in which the programmable read only memory is used to compensate for errors in the probe for a specific tank. However, this system does not reconfigure the fuel quantity indicator and is tank dependent as opposed to being tank independent and, moreover, is not an alterable data source of different groups of parameters specific to a plurality of changing fuel tank configurations. This is also so despite the prior art use of a read only memory as a look up table in a digital fuel gauge for transforming the measured signal into an output signal proportional to the amount of fuel in the tank, such as disclosed in U.S. Pat. No. 4,487,066 which, once again, is not a universal fuel gage which may be reconfigured for different fuel tank configurations. Thus, none of the prior art known to applicants, provides a universal fuel gage which may be reconfigured to a plurality of different fuel tank configurations, such as by changing the stored parameters specfic to a given fuel tank configuration which determine the measuring factors for the calculated fuel quantity determination so as to set up the initial conditions for a given fuel tank for a universal fuel quantity indicator, nor such an arrangement in which a removably connectable interface is employed for a universal fuel gauge which, in conjunction with the fuel quantity indicator, can provide an auxiliary data source which is reprogrammable by the user to the particular specific fuel tank configuration with which the universal fuel quantity indicator is being used at that time. These disadvantages of the prior art are overcome by the present invention. 
     DISCLOSURE OF THE INVENTION 
     A universal fuel quantity indicator interface apparatus for providing a variable set of initial conditions for a universal fuel quantity indicator apparatus for variably configuring the universal fuel quantity indicator apparatus for a particular specific fuel tank for determining the fuel quantity present in the specific fuel tank based on calculations of capacitance of a tank capacitor array and a compensation capacitor wherein a plurality of different parameters specific to a given tank configuration determine the measurement factors for the calculated fuel quantity determination is provided as well as an improved fuel quantity indicator apparatus employing such an arrangement. The indicator comprises a display arrangement, such as a conventional digital display, for displaying fuel quantity information with respect to the fuel tank based on the calculated fuel quantity determination. A microprocessor control arrangement, including a central processing unit and static memory, is provided for calculating the fuel quantity determination and for controlling the display based on this calculation. The microprocessor static memory stores the control program information for controlling the operation of the microprocessor and for storing information corresponding to the different parameters specific to a given tank configuration for enabling the microprocessor to provide the calculated fuel quantity determination on the display for a given specific fuel tank. The interface apparatus includes an alterable static memory, such as a non-volatile RAM or NOVRAM or an electrically erasable programable memory or EEPROM, which is operatively connectable to the microprocessor control arrangement for providing an alterable auxiliary data source, to the microprocessor control arrangement, of different groups of the different parameters for a plurality of different specific fuel tanks. The interface also includes an up/down steering control circuit operatively connectable to the microprocessor central processing unit and the various static memories for selecting a particular group of the different parameters dependent on the parameters associated with a particular specific fuel tank. The up/down steering control circuit comprises a pair of switches for variably selecting the value of each of the parameters in the particular group and for storing each of the selected parameters in the group in the alterable static memory, with the selected parameter content of the alterable static memory being loaded into the microprocessor static memory for configuring the fuel quantity indicator for a particular specific fuel tank under control of the steering control circuit. Thus, the fuel quantity indicator apparatus is reconfigurable for a different specific fuel tank by varying the selected fuel tank parameters by the up/down steering control circuit. In this regard, the steering control circuit includes mode selection having, having as via a jumper, a first mode in which the parameters are selected and a second mode in which the reconfigured fuel quantity indicator operates without the need of the alterable static memory whose contents have been loaded into the static memory of the fuel quantity indicator. The display of the fuel quantity indicator is used in both modes with the display in the first or calibrate mode being employed to varify the selection of the parameters before they are stored, and with the display being used in a conventional manner in the second or run mode to display the measured fuel quantity information with respect to the particular fuel tank for which the indicator has been configured in the calibrate mode. In utilizing the steering control circuit in the calibrate mode, one of the switches preferably increments the selected parameter value and the other of the switches decrements the selected parameter value, and both switches are operated together in order to store the selected parameter value and advance through a predetermined selection sequence to the next parameter to be configured. In employing the universal fuel quantity indicator apparatus to the present invention in connection with an aircraft fuel tank, the selected parameters may comprise constants for the specfic fuel tank associated with the calculated fuel quantity determination, with with these constants comprising fuel scale display factors for the display device. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a functional block diagram of the presently preferred universal fuel quantity indicator apparatus of the present invention, with the interface device, or digital calibration trim unit, which is illustrated in FIG. 5 not shown; 
     FIG. 2 is an illustrative wiring diagram for the fuel quantity indicator of FIG. 1; 
     FIG. 3 is a schematic diagram of the CPU printed circuit board portion of the wiring diagram of FIG. 2; 
     FIG. 4 is a schematic diagram of the fuel quantity printed circuit board portion of the wiring diagram of FIG. 2; 
     FIG. 5 is a schematic diagram of the presently preferred interface or digital calibration trim unit of the present invention employed with the universal fuel quantity indicator apparatus of FIG. 1; 
     FIGS. 6A and 6B are side elevation views of a typical interface or digital calibration trim unit of FIG. 5 showing the sleeve off and on the unit; 
     FIG. 7 is a logic flow diagram illustrative of the data acquisition program for the interface or digital calibration trim unit of FIG. 5; 
     FIG. 8 is a logic flow diagram, similar to FIG. 7, of the data entry program for the interface or digital calibration trim unit of FIG. 5; 
     FIGS. 9 through 17 are illustrative examples of typical displays which preferably appear on the display device of the fuel quantity indicator of FIG. 1 in the calibrate mode during the configuring of the universal fuel quantity indicator to a specific fuel tank configuration, with FIG. 9 illustrating the entry display for this calibrate mode, FIG. 10 illustrating the normal display indicating the parameter number, FIG. 11 illustrating a normal parameter value display, FIG. 12 illustrating a normal display of parameter 2, FIG. 13 illustrating a normal display of parameter 3, FIG. 14 illustrating a normal display of parameter 4, FIG. 15 illustrating a normal display of parameter 5, FIG. 16 illustrating a normal display of parameter 6 and FIG. 17 illustrating an error display.; 
     FIG. 18 is a schematic diagram, partially in block, illustrating the interface between the fuel quantity indicator of FIG. 1 and the interface or digital calibration trim unit of FIG. 5; 
     FIGS. 19 through 21 are further illustrative examples of typcal display configurations in the calibrate mode with FIG. 19 showing the entry display, FIG. 20 showing a display of parameter 1 and with FIG. 21 showing the display of parameter 4, and 
     FIG. 22 is a block diagram illustrating a general representation of the ideal electrical circuit for the fuel quantity indicator of FIG. 1. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to the drawings in detail, and initially to FIGS. 1 through 4 and 22, the presently preferred universal fuel quantity indicator apparatus of the present invention, generally referred to by the reference numeral 100, is shown. As will be explained in greater detail hereinafter, and as illustrated in FIG. 18, the fuel quantity indicator apparatus 100 of the present invention preferably interfaces with an interface apparatus termed a digital calibration trim unit or DCTU, generally referred to by the reference numeral 102, illustrated in greater detail in FIGS. 5 and 6. As will be explained in greater detail hereinafter, the digital calibration trim unit 102 is preferably employed with the universal fuel quantity indicator apparatus 100 in order to configure and/or reconfigure the universal fuel quantity indicator apparatus 100 for a specific fuel tank configuration, such as for an aircraft, with the fuel quantity indicator apparatus 100 being reconfigurable for a different specific fuel tank by varying selected fuel tank parameters through the use of the digital calibration trim unit 102, as will be explained in greater detail hereinafter. 
     Preferably, the universal fuel quantity indicator apparatus 100 of the present invention will indicate fuel quantity in both analog and and digital format for a plurality of different fuel tank configurations in conjunction with the presently preferred digital calibration trim unit or interface 102 of the present invention. Preferably, the fuel quantity indicator apparatus 100 determines fuel quantity by first measuring the capacitive values of the conventional tank probe and then performs mathematical computations using the measured capacitances as input data under control of a microprocessor control unit 104, such as one comprising a central processing unit or CPU and associated internal and external static memory 150 and 106, respectively, such as a conventional NOVRAM for the external memory 106 with the microprocessor control unit 104 being illustrated in great detail in the schematic diagram of FIG. 3. The universal fuel quantity indicator apparatus 100 of the presently invention preferably interfaces with the personally preferred digital calibration trim unit 102 which initially stores the capacitance and maximum fuel weight data which are specific to the particular tank and probe assembly to which the fuel quantity indicator apparatus 100 is attached at that time in an alterable static memory 108 (FIG. 5), such as a non-volatile static memory such as a NOVRAM, such as a conventional Xicor X2444, or a conventional EEPROM such as one commercially available from National Semiconductor under the designation NMC 9306. The alterable static memory 108 employed in the DCTU 102 preferably retains any value stored therein on power down, which is important consideration in an aircraft environment. As will be described in greater detail hereinafter, the data or parameters stored in the alterable static memory 108 of the digital calibration trim unit 102 preferably companies the constant values necessary for the microprocessor control unit 104 of the universal fuel quantity indicator apparatus 100 to make the fuel weight calculations referred to above. In this regard, the fuel quantity indicator 100 preferably determines the fuel quantity present in the particular fuel tank for which it has been configured at that time by measuring the capacitances of the tank capacitor array, represented by reference numberal 110 in FIG. 22, and the compensator capacitor represented by reference numeral 112 in FIG. 22, and then conventionally digitally calculates the fuel quantity from these values. 
     Referring to FIG. 22, a general representation of the ideal electrical circuit of the fuel quantity indicator apparatus 100 of the present invention is shown. The capacitor C t , which represents the tank capacitor array 110, and the capacitor C c , which represents the compensation capacitor 112 for a given tank probe 114 determine the fuel quantity based on the following theory of operation. 
     THEORY OF OPERATION 
     In explaining the theory of operation of the fuel quantity indicator apparatus 100 of the present invention, the following terms are employed with reference to FIGS. 1 and 22: 
     v=voltage, c=capacitance, D=DAC counts 
     O=an offset value of DAC counts, (i.e. expected minus actual) 
     c t  =tank capacitor full tank capacitance 
     c t  =tank capacitor empty tank capacitance 
     c c  =compensator capacitor wet capacitance 
     c c  =compensator capacitor dry capacitance 
     D t  =tank capacitor DAC counts 
     D c  =conpensator capacitor DAC counts 
     D tfc  =expected tank capacitor DAC counts at full tank 
     D tec  =expected tank capacitor DAC counts at empty tank 
     O f  =interpolated factory calibration full tank offset 
     O e  =interpolated factory calibration empty tank offset 
     O vcx  =virtual compensator mode calibration offset (x=l, m or h) 
     O vci  =virtual compensator mode offset (interpolated for tank drive) 
     DAC=digital-to-anolog conventer 
     The process of nulling the fuel quantity indicator 100 of the present invention is the process whereby the value of a compensator DAC 116 is adjusted so that a small increment of the value of the compensator DAC 116 will cause the value of the input from a sample/hold circuit 118 to toggle between binary zero and one. Leakage, which is a leakage current passed to the CPU 104 though a leakage amplifier 120, is preferably defined as the binary input of the circuit which measures the electrical resistance of the fuel. The presence of a binary one at the input from this circuit indicates the presence of contamination in the fuel, with fuel contamination being measured as a function of electrical resistance, and also indicates that the fuel measurement is to be considered in error. It should be noted that as used throughout the balance of this this discussion &#34;tank capacitor&#34; is meant to refer to the entire tank capacitor array 110. 
     Thus, given the equation for capacitive reactance, ##EQU1## where i=current; w=2 pi f; v=voltage; C=capacitance; Xc=1/WC=capacitive reactance; and f=frequency. 
     It follows that, 
     If v t  =tank drive voltage, c t  =tank capacitance, and i t  =current in c t  and v c  =compensator drive voltage, c c  =compensator capacitance, and i c  =current in c c , ##EQU2## 
     If it is given that the drive voltages may be adjusted such that the voltage at point &#34;N&#34; in FIG. 22 is zero, then i t  =i c  when this is true. 
     If it is given that v t  and v c  are equal in magnitude and frequency, but 180 degrees out of phase, then at this null point, ##EQU3## Preferably, the value of c c  is a constant for constant values of the dielectric constant of the fuel in the tank being measured. Since the dielectric constant of Jet A fuel varies with density, and the density of the fuel in the tank is not constant from loading to loading, an accommodation must preferably be made for a variance in the value of c c . Preferably, by way of example, the selection of a value of c k  equal to c c  ±2.135479 will produce accurate results, for the entire range of Jet A densities, for the equation, 
     
         vt±c.sub.t =vc±(c.sub.c +c.sub.k)                    (6) 
    
     Where, c t  =tank full capacitance, c t  =tank capacitance, 
     c c  =compensater wet capacitance, c c  =compensator dry capacitance. 
     Since DAC counts are preferably considered proportional to voltage, the above equation (6) may be expressed as follows: 
     
         D.sub.t ±c.sub.t =D.sub.c ±(c.sub.c +c.sub.k)        (7) 
    
     Where Dt+number of tank DAC counts, and Dc=number of compensator DAC counts. At null, the compensator DAC 116 shouild preferably, by way of example, be set to approximately 3600 counts. This would allow for sufficient &#34;headspace&#34; in the calculation. The number of tank drive counts that will produce a null (with a full tank) at 3600 counts of compensator drive is given by the equation, ##EQU4## Since the result of equation (8) is not a whole number and the DAC inputs must preferably be integers, DT must be rounded. After this rounding, the effect of the rounding may be quantified by, ##EQU5## The number of DAC counts (theoretical) when the tank is empty and at null can be now given by, ##EQU6## If desired, in order to compensate for internal gain and bias errors, offset values can be calculated for known capacitances, stored in the indicator 100 in non-volatile memory 106 and used to calculate the constants of the fuel weight equation. In addition the indicator 100 is preferably conventionally initialized and errors which would be introduced to the aircraft environment are compensated for. Thereafter, the following expression is employed in the indicator apparatus microprocessor control unit 104 to calculate the fuel weight: 
     
         Fuel Weight=((D.sub.c -O.sub.tot)-D.sub.cet)/DAC Range±Max Fuel Wt. (11) 
    
     In the normal operating mode, 
     O tot  =zero, DAC Range=(D cft  -(O f  +O r ))-(D cet  -(O e  +O z )) 
     In VCOMP operating mode, 
     O tot  =O vci , DAC Range=(D cft  -(O f  -O r ))-(D cet  -(O e  +O z )) 
     By way of example, the microcomputer or microprocessor control unit 104 employed in the fuel quantity indicator apparatus 100 of the present invention is a conventional CMOS microcomputer, such as a Hitachi HD63P01M1, also conventionally referred as the Hitachi 6301, preferably operating at a clock rate of one megahertz in the example herein. By way of example when a Hitachi 6301 microcomputer 104 is employed the following port assignments may be used: 
     
         ______________________________________                                    
TABLE OF HD6301 PORT ASSIGNMENTS                                          
                                      DE-                                 
                               INPUT/ FAULT                               
      PIN                      OUT-   ON                                  
PORT  #      FUNCTION          PUT    RESET                               
______________________________________                                    
P10   13     DCTURAM chip select                                          
                               O      0                                   
P11   14     Tank DAC chip sel not                                        
                               O      1                                   
P12   15     Excessive leakage I      x                                   
P13   16     DCTU clock        O      0                                   
P14   17     display driver CS not                                        
                               O      1                                   
P15   18     Disable sample not                                           
                               O      0                                   
P16   19     DCTU data for &#34;CAL mode                                      
                               O      0                                   
P17   20     Display backplane O      0                                   
P20    8     Mode select       I      x                                   
P21    9     Mode select/periph clk                                       
                               I/O    x                                   
P22   10     mode select/VTO   I/O    x                                   
P23   11     DCTU chip select  O      0                                   
P24   12     DCTU data for &#34;RUN mode                                      
                               O      0                                   
P30   37     D0 data/serial input                                         
                               I/O    0                                   
P31   36     D1 data/serial output                                        
                               I/O    0                                   
P32   35     D2 data           I/O    0                                   
P33   34     D3 data           I/O    0                                   
P34   33     A0 address bit 0  O      0                                   
P35   32     A1 address bit 1  O      0                                   
P36   31     A2 address bit 2  O      0                                   
P37   30     Repeater input    I      x                                   
P40   29     Output Low        O      1                                   
P41   28     Regression cap in O      1                                   
P42   27     Regression cap ground                                        
                               O      1                                   
P43   26     Compensater drive disable                                    
                               O      1                                   
P44   25     DCTU &#34;UP&#34; not button                                         
                               I      x                                   
P45   24     DCTU &#34;DOWN&#34; not button                                       
                               I      x                                   
P46   23     Deadman output    O      0                                   
P47   22     Signal comparator input                                      
                               I      x                                   
______________________________________                                    
 
    
     The functions of the microcomputer 104 I/O signals and the digital calibration trim unit or DCTU 102 listed in the above table are defined in greater detail as follows: 
     &#34;DCTURAM chip select&#34; is active high and enables the DCTURAM to accept serial data on the next active clock edge. &#34;Tank DAC chip select not&#34; is active low and selects the signal DAC as the active device with address and data information to be clocked into the device on the next rising clock edge. &#34;Excessive leakage&#34; is an input polled by the CPU 104 and indicates if there is an excessive resistive component between the plates of the sensing capacitors. The resistive element causes high signal levels 90 degrees out-of-phase from the true capacitive signal and is isolated and detected on this input. &#34;DCTU clock&#34; is the clock input to the DCTU 102 with data being synced to the rising edge of this clock at the DCTU. &#34;Display drivers chip select not&#34; is active low and enables the LCD driver pair to accept serial data on the next active clock edge. &#34;Disable zero-cross one-shot timer&#34; is active low and inhibits a conventional one-shot timer 124 from triggering a sample cycle by holding the clear pin low. This is preferably required when sending serial data on the compensator line and when executing a signal check space since both conditions could throw the signal null to a state that would require a software settling routine or wait loop. Thus, to avoid this condition the sampling is preferably inhibited by a low state on this pin. &#34;DCTU data for CAL mode&#34; is the calibration mode data input to the DCTU 102. Restraints on the content of the command strings would preferably require that a write or store command sent to the DCTU 102 must be transmitted on a pin that can be physically removed from the circuit during normal operation, which is accomplished in the presently preferred embodiment of the invention. &#34;Display backplane frequency 60 Hz (±25%) is an input which is toggled at, preferably, approximately 60 Hz to supply the needed AC frequency to a liquid crystal display 130 via an LCD driver pair 132, 134 comprising digits driver 132 and segments driver 134. Preferably this is a priority routine, as failure to execute a 50/50 duty cycle frequency could result in permanent plating damage to the LCD display 130. &#34;Mode Select&#34; is for entry into the single chip mode of the microcomputer 104&#34; with program pins P20, P21 and P22 in the logical one state immediately after the reset pin goes to a high state. Port P20 is preferably used exclusively for this purpose and has a pull-up resistor to ensure the proper logic level. &#34;Mode Select/Peripheral clock&#34; at P21 has two purposes. First, immediately following a reset it is read as a program pin, a pull-up resistor being used to ensure that a proper logical `1` is present. Its second purpose is that of providing a constant frequency clock output for the peripheral devices. Using the CPU 104 internal timer, a constant frequency pulse train will preferably be generated at all times, independent of all CPU states except reset. &#34;Mode select /VTO driver&#34; is a dual purpose pin. On reset, P20, P21 and P22 determine the mode of operation. The passive pull-up on this line ensures this function. The second function is the repeater mode VTO relay function. A logic `high` output will turn an FET 136 `on`. &#34;DCTU chip select&#34; is the active high chip select for the DCTURAM component of the DCTU 102. The clock input to the DCTU should preferably be low at the time of application of this signal. Data will preferably be clocked into the device on the rising clock edge after the chip select is applied. &#34;DCTU data for &#34;RUN&#34; mode&#34; is the run mode data input to the DCTU 102. Once again restraints on the content of the command strings preferably require that a write or store command sent to the DCTU 102 be transmitted on a pin that can be physically removed from the circuit during normal operation. The data transmitted to the DCTU 102 during the &#34;RUN&#34; or normal operation mode is preferably screened and transmitted in a very strict, straightforward manner that can be easily audited. &#34;D0 data bit zero&#34; for the tank and compensator drive DAC, 116, 122 is the parallel bus data bit zero. For the serial bus devices, this is the data input to the CPU 104 data output of the peripheral devices. When used as a parallel bus bit, it can be either an input or an output. &#34;D1 Data bit 1/serial data output&#34; for parallel data bus applications, acts, as the D1 data bit. The bit also functions as the serial bus output pin. &#34;D2 Data bit 2&#34; is used as a parallel bus data bit only for interfacing to the signal DAC. &#34;D3 Data bit 3&#34; is used as a parallel bus data bit only for interfacing to the signal DAC. &#34;A2 Address bit 2&#34; is used as a parallel bus address bit for addressing internal registers of the signal DAC. &#34;Repeater input&#34; is the input for the data when the gauge 100 is acting as a repeater. &#34;Regression capacitor in&#34; is active low, and switches the regression capacitor into the null point. &#34;Regression capacitor to ground&#34; is active low, and switches the regression capacitor to ground to reduce its stray capacitance effects. P41 is preferably in its inactive state when th P42 function is active. &#34;Compensator drive disable&#34; is active low, and switches the compensator 112 out of the circuit when it detects that a shorted condition exists. &#34;DCTU UP not input&#34; is the &#34;UP&#34; button 140 (FIG. 5) interface from the DCTU 102. The operation of this button 140 as well as the general DCTU 102 operation will be described in greater detail hereinafter. &#34;DCTU DOWN not input&#34; is the &#34;DOWN&#34; button 142 interface from the DCTU 102. Again, the operation of this button 142 will also be described in greater detail hereinafter with respect to the DCTU 102 operation. &#34;Deadman output&#34; is from a deadman timer circuit 144 and is preferably, by way of example, toggled at 120 Hz±10%, or the deadman circuit 144 generates a system reset. &#34;Signal comparator input&#34; is tied to the comparator which indicates the null point condition. A one on this line indicates that the compensator is higher than the null point, and the compensator DAC 116 counts should be decreased to approach a null point. A zero on this line indicates that the compensator is lower than the null point and that the compensator DAC 116 counts should be increased to approach the null point. 
     As shown and preferred in FIG. 3, the microcomputer or microprocessor control unit 104 preferably includes the aforementioned internal memory 150 for carrying the operating or control software. Given the example of a Hitachi 6301 microcomputer 104, such a microcomputer 104 accomodates piggyback PROM assemblies, such as a 2764 series 8K-by-8 EPROM, with this EPROM static memory 150 (FIG. 3) preferably appearing to be part of the memory map of the microcomputer 104. In this regard, the operating control program which interfaces the fuel quantity indicator 100 with the digital calibration trim unit or DCTU 102 of the present invention and which is resident preferably in static memory 150 of the microprocessor control unit 104 is given by way of example below for the embodiment illustrated herein: 
     OPERATING CONTROL PROGRAM ##SPC1## 
     With respect to the above control program, the microcomputer or microprocessor control unit 104 preferably supports software interfaces with four programmable logic devices in the universal fuel quantity indicator apparatus 100; namely a display driver, a dual digital-to-analog converter 16,122, the internal non-voltile random access memory 150 termed CALRAM and the external non-volatile random access memory 108 termed DCTURAM which is housed in the digital calibration trim unit or DCTU 102. All of these software interfaces are preferably supported by a multiple purpose internal communication bus which, in the above example, is defined on port lines P30 through P36. The display driver is preferably a conventional display driver such as a Holt Intergrated Circuits Inc. PN: HI-8010, with the indicator apparatus 100 software preferably providing all of the necessary timing, formatting, data translation and control necessary for serial communication with the display driver. 
     The display driver is preferably treated as a pair of shift registers with data being strobed into the input shift register by first presenting one bit of the data on the DATA LINE and toggling the CLOCK LINE once. After the entire shift register is filled, the LOAD LINE is preferably toggled to move the contents of the input shift register into the display register. This shift is preferably performed internally and continues to drive the display with the correct character pattern regardless of the state of CS not. 
     The previously referred to digital-to-analog converters or DACs associated with Tank Drive 122 and the Compensator Drive 116, as was previously mentioned with respect to FIG. 1, are preferably contained in a single dual 12-bit DAC such as an Analog Devices PN AD7549 dual 12-bit DAC. The universal fuel quantity indicator 100 software preferably provides all the necessary timing, formating, data translation and control necessary to support, in the above example, 12-bit multiplexed parallel communication with the DACs. 
     The previously mentioned CALRAM 150 is a conventional static memory such as a National Semiconductor PN 9306 E. Once again, the associated control software for the universal fuel quantity indicated apparatus 100 provides all the necessary timing, formatting, data translation and control necessary to support serial communication with the CALRAM 150. 
     With respect to the DCTURAM 108, such as a Xicor Inc. PN X2444, the afore mentioned control, software resident in memory 150 of the universal fuel quantity indicator apparatus 100 preferably provides all the necessary timing, formating, data translation and control necessary to support serial communication with the DCTURAM 108 on two separate conductor paths. These two separate conductor paths are preferably used, alternately, in the running and calibrating modes of operation which are selected by a mode selection jumper 154 (FIG. 5). By way of example, the data in the DCTURAM 108 is preferably organized as indicated in one of the following memory map tables. 
     
                       TABLE                                                       
______________________________________                                    
DCTURAM MEMORY MAP                                                        
Data Word #                                                               
         Contents           Valid Range                                   
______________________________________                                    
00       bank checksum word 0000H to FFFFH                                
01       LB/KG indication   0 or 1                                        
02       maximum fuel weight                                              
                            10 to 8000                                    
03       dry tank capacitance                                             
                            399 to 65,000                                 
04       wet tank capicitance                                             
                            799 to 20,000                                 
05       dry compensator capacitance                                      
                            199 to 1490                                   
06       wet compensator capacitance                                      
                            399 to 1490                                   
07       VCOMP zero-adjust offset                                         
                            0-1024                                        
       (represents values from -512 to 512)                               
08       range adjustment   0-1024                                        
       (represents values from -512 to 512)                               
Words 09-13                                                               
         Not used - set to 0000H.                                         
14       Number of writes to the                                          
                            1-1000                                        
         device                                                           
______________________________________                                    
 
    
     Preferably, the universal fuel quantity indicator apparatus 100 of the present invention has the ability to communicate with external devices by modulating the amplitude of the compensator drive. By driving the compensator to full scale for a logical 1 and to 0 for a logical 0, the indicator 100 will transmit data. When communications to the repeater are occuring, perferably the sample/hold amplifier 118 will be disabled to avoid inducing off scale bursts of signal data into the null point. Preferably, by way of example, one bit can be sent every two miliseconds with the stream being transmitted at an interval of no less than once every 1.5 seconds. 
     With respect to the CPU 104 software requirements, there are certain overhead processing requirements that should be considered. Thus, with respect to power-on initialization, the reset line of the micro computer 104, which is pin 6 in the above example, is Active low. While this pin is held low, the microcomputer 104 is held in a reset state. When a positive transition is detected on this line, the micro computer 104 fetches the reset address as defined by the operating mode, which is, in turn, defined by the state of port Lines, 20, 21, and 22. Preferably, in the indicator apparatus 100 of the present invention, all three mode select lines are pulled high which forces the micro computer 104 into mode 7 in the above example. In this mode, microcomputer 104 fetches the contents of memory location FFFEH: FFFFH, stores this value in the program counter, and begins program execution. This value is the address, in hexadecimal notation, of the first executable instruction executable instruction of the initalization routine illustrated above, which will also be the lowest address of the EPROM. To initialize the microcomputer 104, the stact pointer is set to FFH. All of the memory locations from 00H through SSH will be loaded with 0 and all of the port lines will be set to the I/O states referred to above. As for memory tests, the RAM will be check summed and the RAM will be tested by writing and reading test patterns to and from each location with a failure of either memory test preferably causing the software to enter a wait loop to reset. The software is also preferably designed to test the DCTURAM 108 communication line to see if it is present in which instance control passes to the field calibration routine forming part of the above illustrated control program. As for the operating constant calculation, which is where the digital calibration trim unit 102 in the calibrate mode configures the Fuel quantity indicator 100 for the specific fuel tank by inserting the specific parameters associated with the given fuel tank configuration, the above illustrated initialization routines calculate the constants necessary for the fuel weight equation, with the following constants or parameters specific to a given tank configuration which determine the calculated fuel quantity determination being required; 
     DTCCCK=NCOMPT tank drive DAC counts 
     DTCK=VCOMP tank drive DAC Counts 
     DKEMPTY=compensator drive DAC counts for empty tank 
     TKRANGE=DAC counts representing the span of compensator drive DAC count values (from empty to full tank) for the current tank 
     VCOFF=DAC count bias (offset) for VCOMP mode 
     The operating constants will be calculated in the above example, using the following equations: ##EQU7## 
     Once a tank DAC value has been determined, the tank is placed either in the high or low range. The offsets for that tank can be found by a straight linear interpolation between two points. ##EQU8## 
     TKEMPTY is calculated using the theoretical value minus the empty tank offset. ##EQU9## 
     TKRANGE is calculated using the theoretical value minus the full offset minus a full scale adjustment. ##EQU10## 
     The virtual compensation offset, VCOFF, is preferably a combination of the biasing deviation found in calibration of the apparatus 100 and a zero adjust entered in the DCTU. ##EQU11## 
     Preferably, in addition to the overhead functions already specified, the main execution loop continuously provides a calculation of the fuel weight using the following algorithm: 
     RUNNING AVERAGE OF COMPENSATOR DAC COUNTS--CALIBRATION OFFSET 
     Empty Tank Compensator DAC Counts)/Valid Range of Compensator DAC Counts * Maximum Fuel Weight=Actual fuel weight present 
     Using mnemonics, this may be expressed more simply by, 
     
         WEIGHT=((AVG-TKEMPTY)/TKRANGE) * TOTFUEL 
    
     Preferably, the calibration routines can adjust the offset values such that the accuracy of the above calculation is by way of example, 0.5% of the actual fuel weight present in the specific tank for which the constants have been configuraled. 
     Digital Calibration Trim Unit 
     Although the previously referred to digital calibration trim unit or DCTU 102 may be permanently connected to a universal fuel quantity indicator 100 if desired, it is most preferably removably connectable as an interface to universal fuel quantity indicator 100, such as illustrated in FIG. 18 and in FIGS. 1 and 5. In such an instance, the DCTU 102, such as illustrated in FIG. 6 contains a multi-pin connector 160 which is plug compatable with the universal fuel quantity indicator 100 and another electrical connector 162 which is preferably plug compatable with the tank probe located in the fuel tanks for which the universal fuel quantity indicator 100 is to be configured such as an 18 pin connector to mate with the indicator 100 and all 11 pin connector to mate with the aircraft wiring by way of example. For ease of access, by way of example, the up and down button switches, 140 and 142, respectfully, extend through the outside of the housing 164 which is illustrated as being cylindrical although any type of shape desired may be employed. In addition, as illustrated on FIG. 6B, preferably a removable sleeve 166 is located on the digital calibration trim unit 102 so as to cover the select buttons 140 and 142 when the unit 102 is in the run mode as opposed to the calibrate mode and prevent inadvertent operation thereof. As will be explained further hereinafter, by pressing and releasing the switches 140 and 142 in the calibrate mode, an operator is able to enter any value for any constant maintained by the DCTU 102 which is in the valid range for that parameter. In the calibrate mode, after successful initialization, as referred to in the above control program, the display 130 of the fuel quantity indicator 100 is preferably illuminated with the message &#34;CAL&#34; to indicate the calibrate mode and the above illustrated control program starts scanning the buttons or switches 140 and 142 for transitions. The appearance of the display 130 at this time is illustrated in FIGS. 9 and 19. As was previously mentioned, preferably if both buttons or switches 140 and 142 are depressed simultaneously, the number of the DCTU 102 parameter being selected will increment by one and the associated parameter number which will be referred to in the following example, is displayed for a predetermined time, such as at least one second after the buttons 140 and 142 are released. Such a display is shown by way of example in FIGS. 10, 20 and 21. It should be noted that preferably the parameter number is the same as the DCTURAM address number and parameter number zero is the &#34;CAL&#34; message. Preferably after the parameter number display, the display 130, which preferably includes an arctic type of display, will show the current value of the current parameter being selected. By way of example, for parameter 1, 1 arc tic of a display and the weight unit legends will be displayed such as shown by way of example in FIG. 20. For parameter 2 in the above example, maximum fuel weight values are preferably displayed in actual amounts with the &#34;FUEL&#34;, the appropriate weight unit legends, and 2 arctics active. For parameters 3 through 6, the number of tenths of a picofarad of capacitance and a number of arc tics equal to the parameter number will be displayed, such as the 4 arc tics shown by way of example in FIG. 21 for parameter 4. For parameters 7 and 8, in the above example, the weight units legend will be lit, the current value of pounds or kilograms will be indicated, and the number of arctics equal to the particular parameter will be lit. As was previously mentioned, preferably, releasing one button or switch 140 or 142 while the other is unpressed cause in increment for the up button 140 or a decrement for the down button 142 of the value of the current parameter being selected. Holding either button 140 or 142 depressed will preferably cause a scroll of current parameter values with the scrolling preferably being slow for a predetermined period such as 2 seconds, and rapid after this period for as long as the selected button is held depressed. Maximum fuel weight will preferably vary in increments or decrements of 100 pounds or kilograms by way of example, and compensator capacitor values will preferably increment by one tenth of a picofarad for values below 1000 pf, by way of example, and one picofarad for values above 1000 pf, by way of example. This data entry routine is illustrated by way of example in the logic flow diagram of FIG. 8 with the data acquisition routine being illustrated by way of example in the logic flow diagram of FIG. 8. In this regard, it should be noted that preferably every time both buttons 140 and 142 are depressed simultaneously, but before the number of the DCTU parameter is incremented by one, the number of writes to the device will be updated and checked for overrange, the current value of the current parameter will be written to the DCTURAM 108, and the checksum will be updated and verified. If the number of writes to the device is overrange or if the checksum test fails, the control program will preferably flag a DCTU error. 
     Summarizing the presently preferred method of configuring the universal fuel quantity indicator 100 of the present invention for a specific fuel tank, the outer sleeve 166 is preferably removed from the digital callbration trim unit 102 to expose the switch buttons 140 and 142 and the mode selection jumper 154 which is then moved from the &#34;RUN&#34; to the &#34;CAL&#34; position. The fuel quantity indicator display 130 will then preferably indicate this change of mode by displaying the word &#34;CAL&#34; such as illustrated in FIGS. 9 and 19. As was previously mentioned, push buttons 140 and 142 may then be pressed simultaneously to prompt the display of the first &#34;DCTU&#34; calibration parameter number on the display 130 with the arc tic corresponding to the parameter being displayed also preferably being active. The following table preferably defines the parameter numbers by way of example: 
     
         ______________________________________                                    
parameter # definition      valid range                                   
______________________________________                                    
1           max quantity of fuel                                          
                            00000-65000                                   
2           lb or kg        lb or kg legend                               
3           dry tank capacitance                                          
                            00000-6500.0 pf                               
4           wet tank capacitance                                          
                            00000-6500.0 pf                               
5           dry compensator cap                                           
                            00000-6500.0 pf                               
6           wet compensator cap                                           
                            00000-6500.0 pf                               
______________________________________                                    
 
    
     As was previously mentioned, the normal display indicating these parameter numbers is illustrated, by way of example, in FIGS. 10, 20 and 21. Preferably after releasing the two buttons 140 and 142, the parameter number will remain displayed for a predetermed period, such as one second, and then the value of the associated parameter will then be displayed on display 130, such as illustrated in FIG. 11. If the data for the particlar displayed parameter is not required to be altered, then the operator would press both push buttons 140 and 142 simultaneously and, thus advance the display 130 to the next DCTU calibration parameter number. FIGS. 12 through 16 illustrate typical normal display parameters for parameters 2 through 6 in the above example. In the event of the occurrence of an error, an error display would appear on display 130, such as shown by way of example in FIG. 17. If, however, the displayed parameter is not the desired parameter for the specific tank configuration with which the universal fuel quantity indicator 100 is to be employed, then the operator would press the corresponding &#34;UP&#34; or &#34;DOWN&#34; push button 140 or 142, respectively, to either increment or decrement, respectively, the displayed value until the desired value is displayed. The displayed value will preferably wrap around if the valid range is exceeded. When the displayed value of the parameter is the desired parameter for the specific fuel tank configuration, then both push buttons 140 and 142 are pressed simultaneously to enter the displayed value into the DCTURAM 108. The next parameter number is then displayed on display 30. If desired, all parameters can be viewed without change by repeated pressing of both push buttons 140 and 142 simultaneously. Preferably, after the operator verifies that the parameters are all now properly configured for the specific fuel tank configuration with which the universal fuel quantity indicator 100 of the present invention is being employed, the DCTU calibration mode is exited by moving the mode selection jumper 154 to the &#34;RUN&#34; position and the fuel quantity indicator 100 immediately returns to the fuel measurement mode in accordance with the above control program. 
     With respect to the balance of the circuitry of the fuel quantity indicator 100 illustrated in the drawings herein, this circuitry is self-explanatory and will be readily understood by one of ordinary skill in the art based on the foregoing information taken in conjunction with these drawings and need not be described in greater detail herein after. Suffice it to say, that with the apparatus of the present invention, a universal fuel quantity indicator apparatus may be provided which is reconfigurable for different specific fuel tanks by varying the selected fuel tank parameters so that different constants for different fuel tanks may be programmed into the fuel quantity indicator and changed as the specific tank configuration with which the indicator is to be employed changes.