CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Serial No. 60/358,128, filed Feb. 2, 2002. 
    
    
     FIELD OF INVENTION 
     This invention relates generally to diagnostic and control channels for electrical components. More particularly, this invention relates to multiplexing an ignition coil circuit node to perform a diagnostic function and a control function. 
     BACKGROUND 
     Gasoline internal combustion engines now commonly use a single ignition coil for each cylinder. The ignition coil is frequently configured for mounting directly atop a spark plug screwed into the cylinder head. Such an ignition coil arrangement is commonly known as a coil-on-plug arrangement. 
     A power transistor within an engine control module (ECM) generally conducts current flow through the primary winding of an ignition coil during a dwell period, after which the spark plug fires. The ECM also generally contains a microprocessor that executes software to diagnose the performance of the ignition coil. This diagnosis is commonly performed by measuring the voltage across the power transistor, which is representative of the voltage across the primary winding of the ignition coil and indicative of ignition system performance. 
     The ECM power transistor develops heat, however, making it desirable to locate the power transistor outside of the ECM and away from the microprocessor. A common location for the power transistor is on the ignition coil where it is in close proximity to the primary winding. Such an arrangement presents at least two new problems, however. The first problem lies in the course of events should the control wire to the power transistor become shorted to a high or low voltage source, such as battery or ground, respectively. Without additional circuitry, either the ECM, the power transistor, or both, could become damaged and unserviceable by excessive current flow and power dissipation. 
     The second problem lies with reliably diagnosing performance of the ignition coil. A solution to the diagnostic problem has heretofore required diagnostic wiring, additional to the control line for sending a dwell pulse to the power transistor, to be connected between the ECM and the ignition coil/power transistor assembly (hereinafter referred to as an ignition coil assembly). The additional wiring carries an electrical signal from ignition coil assembly back to the ECM so that it may perform diagnostics on the assembly and its performance. This additional diagnostic wiring creates added expense through higher connector pin counts and added conductors. The additional wiring also increases the risk of system failure by failed connections. 
     SUMMARY 
     It is therefore one aspect of the invention to provide an ignition coil assembly having an integrated driver where serviceability of the assembly is tolerant of a driver control line being short circuited. 
     It is yet another aspect of the invention to provide an ignition coil assembly having a common signal line for both a dwell pulse and diagnostic information. 
     In accordance with the aforementioned aspects, the present invention provides an ignition coil assembly for providing ignition energy to a spark plug in accordance with a dwell pulse and transmitting diagnostic data related to an ignition event occurring after the dwell pulse. The assembly has an ignition coil with a spark plug terminal adapted to mate with the spark plug, and a transistor for conducting current flow through the primary winding of the ignition coil. The current flow is in accordance with the dwell pulse, which arrives over a signal line. A diagnostic block receives at least one electrical signal from the ignition coil and derives diagnostic data therefrom for transmitting over the same signal line in the absence of the dwell pulse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 depicts a single wire control-only circuit of the prior art, 
     FIG. 2 depicts a multiplexed single wire system with voltage multiplier, 
     FIG. 3 depicts a single wire system with window comparator, 
     FIG. 4 depicts a multiplexed single wire system with window comparator and low-pass filter, 
     FIG. 5 depicts waveforms of the circuit of FIG. 4, and 
     FIG. 6 depicts an example circuit implementing a diagnostic circuit block for the circuit depicted in FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the invention, its applications, or uses. 
     Open collector (or drain in the case of a field effect transistor) output is a common method for interfacing the output of an ECM to a load with an integrated power driver. FIG. 1 shows such an interface used with an ignition coil assembly  2  having an integrated driver transistor  4 . 
     While the prior-art interface of FIG. 1 has the advantages of low cost and simplicity in design, it also suffers from inherent weaknesses. Once such weakness relates to control lead  8  becoming undesirably shorted to either battery voltage B+ or to ground  10 . If control lead  8  is shorted to B+, the output driver  12  of the ECM  14  will pass unimpeded current to ground  10  and will likely fail. Similarly, should the control lead  8  become shorted to ground  10 , the driver transistor  4  will turn on unintentionally and pass unimpeded current to ground  10  through the primary winding  16  of ignition coil  18 . In this aspect, either the ignition coil  18  or driver transistor  4 , or both, may be damaged by the unintended current flow. 
     Another inherent weakness of the interface of FIG. 1 is the limited ability of the microprocessor to diagnose the performance of the system. In this arrangement, the microprocessor may be limited to diagnosing the operation of output driver  12  and be unable to diagnose the operation of ignition coil assembly  2  or electrical characteristics of dwell and ignition. 
     Turning now to FIG. 2, an aspect of a first improved ignition coil assembly  20  is shown in combination with an ECM. A microprocessor  24  executes software for controlling and diagnosing the performance of an ignition coil assembly  20 . The microprocessor  24  has an output for controlling an output driver  26  and an input for receiving a diagnostic signal from a voltage comparator  28 . Comparator  28  is referenced to battery supply, B+, and receives an input signal from multiplexed (MUX) signal line  32 , which is also connected to the collector of output driver  26 . 
     The MUX signal line  32  connects to ignition coil assembly  20 . Assembly  20  has an integral driver transistor  22  that is controlled by a pre-driver  34 . The input to pre-driver  34  is pulled-up by pull-up resistor  50  through diode  48 . Transistor  22  passes current that travels from B+, through the primary winding  42  of coil  40 , and then through shunt resistor  46 . A charge pump  38  produces a voltage greater than B+ and powers a diagnostic block  38  therewith. Diagnostic block  36  has a voltage measuring input  52 , or current measuring input  54 , or both, for detecting electrical signals of the primary winding  42 . The diagnostic block  38  sends diagnostic information to the ECM through MUX signal line  32 . A series resistor  56  may be used to protect the diagnostic block output from excessive current flow. Diode  48  operates to prevent the diagnostic block output from coupling to the charge pump  36 . 
     In operation, MUX signal line  32  carries control and diagnostic data. The microprocessor  24  begins dwell by sending a dwell pulse via output driver  26 , thereby pulling the MUX signal line  32  down to ground  58  potential for the duration of the pulse. With the MUX signal line  32  pulled low, the pre-driver  34  turns on driver transistor  22 , thereby allowing dwell current to begin to flow from B+, through the primary winding  42 , the driver transistor  22 , and, if used, the shunt resistor  46 . The diagnostic block  38  determines diagnostic current information from a signal at current input  54 . While coil  40  is in dwell, the MUX signal line  32  is pulled near ground  58 , momentarily precluding the transfer of diagnostic data over the line  32 . 
     Once the coil  40  has been in dwell for the desired duration of the dwell pulse, the microprocessor  24  turns off output driver  26 , allowing MUX control line  32  to be pulled to B+ by pull-up resistor  50 . With the MUX control line  32  potential at B+, pre-driver  34  turns off driver transistor  22 , thereby stopping current flow through the primary winding  42 . A high voltage is created in the secondary winding  44  when the current through the primary winding is turned off, thereby causing a spark across the gap between spark plug electrodes  60 . The spark plug is connected to the secondary winding  44  via a terminal  45  adapted to mate with the spark plug  60 . The high voltage is reflected from the secondary  44  to the primary winding  42 , and attenuated by a turns ratio of the ignition coil  40 . The diagnostic block  38  detects the reflected voltage at voltage input  52  and determines diagnostic voltage information therefrom. This diagnostic information may also include spark-pulse duration, or burn time, information determined from the reflected voltage. The output driver  26  is off in the absence of the dwell pulse and the diagnostic block  38  transmits diagnostic information over the signal line  32  during the absence. The information is encoded as pulsed data, with pulses having a high voltage approximately equal to the output voltage of the charge pump  36 . Voltage comparator  28 , which is connected to the signal line  32 , receives these pulses. The output  30  of the voltage comparator produces a digital pulse, compatible with the microprocessor  24 , for each period the voltage of the signal line  32  exceeds the reference voltage (B+ in this example) of comparator  28 . The digital pulses are representative of the diagnostic pulses sent by the diagnostic block  28 . Since both the voltage comparator  28  and charge pump  36  are referenced to B+, signals sent from the diagnostic block  38  may be resolved by the voltage comparator  28  regardless of the magnitude of B+. 
     Moving to FIG. 3, an aspect of a second improved single-wire system is shown. A microprocessor  60  executes software for controlling the performance of an ignition coil assembly  62 . The microprocessor  60  has an output for controlling an output driver  64  and an input for receiving diagnostic information from a diagnostic interface circuit  66 . This diagnostic interface circuit  66  is application specific and operates to shift the voltages on signal line  70  to voltages compatible with the input of microprocessor  60 . A resistor R1, in series with the collector of the output driver  64 , operates, in part, to limit current through the driver  64  in the event signal line  70  becomes shorted to B+. Resistor R1 also operates in conjunction with resistor R2 to create a voltage divider having the signal line  70  at the voltage divider tap. 
     The signal line  70  operates to provide a dwell pulse to a window comparator  74 . The window comparator turns on driver transistor  76 , via predriver  78 , when the voltage across the signal line  70  is within upper and lower voltage thresholds of window comparator  74 . When output driver  64  is turned on, the voltage across the signal line  70  is approximated by the equation 
     
       
           V   s   =V   batt   *R   1 /( R   1   +R   2 )  Eq. 1  
       
     
     where 
     V s =voltage of signal line  70  with respect to ground  80 , 
     V batt =B+ in volts, 
     R 1 =ohmic value of resistor R1, and 
     R 2 =ohmic value of resistor R2. 
     The upper and lower voltage thresholds of the window comparator  74  may be set such that 
     
       
           V   H   =V   S +Delta1  Eq. 2  
       
     
     and 
       V   L   =V   S −Delta2  Eq. 3 
     where 
     V H =upper voltage threshold of window comparator  74 , 
     V L =lower voltage threshold of window comparator  74 , 
     Delta1=positive voltage, and 
     Delta2=positive voltage&lt;V s . 
     The resistors R1 and R2 may be simply set equal to each other so that the signal line  70  is at V batt /2 while the output driver  64  is turned on. The window comparator  74  thresholds, V H  and V L , may simply be set ratiometrically to B+. For example, V H =2V batt /3 and V L =V batt /3. 
     The circuit of FIG. 3 advantageously operates to protect output driver  64  when signal line  70  is shorted to either B+ or ground  80 . As mentioned previously, the output driver  64  is protected by R1 when signal line  70  is shorted to B+. The circuit also advantageously operates to protect the ignition coil assembly  62  when signal line  70  is shorted to ground. Such protection is achieved by the voltage of the signal line  70  being outside of the voltage thresholds of window comparator  74 . Since the signal line voltage is outside of the thresholds, the window comparator turns off the driver transistor via predriver  78  thereby precluding unintended current flow through the driver transistor  76  and its associated primary winding of the ignition coil. 
     Turning now to FIG. 4, an aspect of a third improved single-wire multiplexed system is shown. In addition to the functionality of the circuit of FIG. 3, the circuit of FIG. 4 adds the capability of transmitting diagnostic information back to the ECM  92 . A diagnostic circuit  84  has a voltage input  90  and a diagnostic output  88 . The diagnostic circuit  84  may also have a current input arrangement similar to the circuit of current input  54  shown in FIG.  2 . The voltage input  90  detects voltage reflected through the ignition coil (as discussed earlier) and determines diagnostic voltage information therefrom. This diagnostic information may also include spark-pulse duration information as determined from the reflected voltage. Similarly, the diagnostic circuit may determine diagnostic current information from a current input, if so equipped. 
     The diagnostic circuit transmits the diagnostic information over the signal line  70  while the output driver  64  is turned off. The diagnostic circuit transmits the information by turning on transmit transistor  86 , which pulls the signal line  70  to ground  80  potential, thereby sending information data. 
     Signal line  70  voltage rises to B+ when transmit transistor  86  turns off. During this voltage rise, however, the voltage passes through the voltage thresholds of window comparator  74 , thereby possibly causing the window comparator to inadvertently attempt to turn on the driver transistor  76 . Low-pass filter  82  may be placed between the window comparator  74  and driver transistor  76  to prevent transistor  76  from turning on during this transient voltage rise. Similarly, the low-pass filter prevents the transistor  76  from turning on while the voltage of signal line  70  passes though the window comparator voltage thresholds as it decreases from B+ to ground  80  potential. 
     The circuit of FIG. 4 may be better understood by referring to the time-correlated waveforms of FIG.  5 . The y-axis of traces  100  and  110  represent voltage, whereas the y-axis of trace  108  represents current flow through the primary winding  96  of the ignition coil  94 . The x-axis represents time. Trace  110  shows the voltage of the signal line  70  during one cycle of firing the spark plug. Prior to start of dwell  102 , signal line  70  is pulled up to B+ by resistor R2. During this time, the driver transistor  76  is off because the signal line  70  voltage is outside of window comparator voltage thresholds V H  and V L . Trace  100  represents the voltage at the collector of driver transistor  76  and shows that the collector is at B+ while the driver transistor  76  is off. At the start of dwell  102 , output driver  64  is turned on, thereby bringing the control line  70  voltage within the comparator voltage thresholds. The window comparator  74  then causes driver transistor  76  to turn on as indicated by drop in voltage of trace  100  and the rise in current of trace  108 . The current continues to rise until the end of dwell  104 . At the end of dwell  104 , output driver  64  is turned off as evidenced by the control line  70  voltage going to B+. Control line  70  stays at B+ after the dwell pulse until the diagnostic transmit transistor  86  pulls the control line low to send diagnostic information back to the diagnostic interface circuit  66 . In trace  110 , the diagnostic information is a low pulse representative of the burn time  106  of the spark event. The low-pass filter  82  prevents the driver transistor  76  from turning on while the control line  70  passes through the voltage threshold window at the end of dwell  104  and during switching transitions of the transmit transistor  86 . 
     By way of non-limiting example, a diagnostic circuit  84 , which determines burn time, is shown in FIG.  6 . Voltage detected at input  90  is converted to a current by transistor Q1. The diagnostic data, in the form of a pulse having duration equal to the burn time, appears at the output  88  of comparator stage  114 . At the beginning of the burn time  106 , the output  88  will initiate a diagnostic pulse due to current flow through R7 and R11. Once the diagnostic pulse is initiated, threshold stage  116  turns off the output of comparator U2, thereby effectively removing R11 from the collector of Q1 and reducing the Q1 collector current needed to keep the output of comparator U1 turned on. The output of U1 therefore remains on for the duration of the burn time and derives diagnostic data from the voltage of the ignition coil primary winding  96 . It must be restated that this implementation of a diagnostic circuit  84  is merely an example. Other functions may also be implemented as indicated in this specification. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.
An ignition coil assembly for providing ignition energy to a spark plug in accordance with a dwell pulse and transmitting diagnostic data related to an ignition event occurring after the dwell pulse. The assembly has an ignition coil with a spark plug terminal adapted to mate with the spark plug, and a transistor for conducting current flow through the primary winding of the ignition coil. The current flow is in accordance with the dwell pulse, which arrives over a signal line. A diagnostic block receives at least one electrical signal from the ignition coil and derives diagnostic data therefrom for transmitting over the same signal line in the absence of the dwell pulse.