Abstract:
An electrical protection circuitry for a docking station base of a hand held meter and method thereof are disclosed. In the event of a short circuit at a meter interface connector, the protection circuitry removes power at the meter interface connector. Similarly, in the event of an applied voltage outside a specified operating range of the base, the protection circuitry removes power to the meter interface connector. These conditions of the electrical system of the base are monitored regardless whether the meter or the meter&#39;s battery is electrically connected to the base. The protection circuitry also provides a visual indication in the event of either the over current and under/over voltage conditions. Additionally, the base is designed to prevent liquid from pooling inside a pocket used to cradle and hold the meter in the base through the use of a drain located at the lowest point in the pocket.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/118,490, filed Apr. 29, 2005, which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to electrical protection, and in particular, to electrical protection circuitry for a docking station base of a hand held meter, such as for example a blood glucose meter, which prevents system damage from liquid contact and unspecified power supply voltages and currents, and the method thereof. 
     In hospitals and out-patient settings that desire to monitor and maintain a patient physiological values, there are a variety of hand held meters involved in bedside testing or near patient testing, which record and transmit such patient data to a remote health information system. One such hand held meter is a blood glucose meter, which in one prior art device, the transmission of patient data between the meter and the remote health information system is via a docking station base. In such a prior art device, the docking station base, in addition to providing a data connection to the information system, also provides power to recharge the battery of the meter. To connect the meter to the base, the meter is seated or docked in a cradle portion of the base having at the bottom thereof a meter interface connector. The meter interface connector provides both the power and data connections to the meter from the base. 
     Due to the use of such hand held meters in the near patient testing setting, customer usage includes periodic cleaning and disinfecting of the exterior surfaces of the meter and base. As the meter and base are not intended to be submerged in any liquid, the recommended cleaning process includes spraying a cloth with a cleaning solution and wiping down the meter and base with the dampened cloth. However, the problem induced to the combined meter and base system is that occasionally customers either clean the meter and base with an over-saturated cloth, or spray the meter and base directly with the cleaning solution which can result in significant residual cleaning solution being left on the meter and base. The excess solution, through help from gravity, collects in the cradle portion of the base or wicks down through the meter interface connector. 
     In such a prior art docking station base, if significant enough solution collects in the base, the cleaning solution can drain inside the base housing in and around the power and data connector and onto a printed circuit board enclosed therein. Once inside the housing, the cleaning solution (specifically bleach based products) can form dendrites on the wiring assembly of the printed circuit board which overtime, can eventually result in unit failure. In some cases, meter and/or base failure occurs almost immediately after solution application and docking of the meter to the base. In other cases, failure of the base is progressive, causing power fluctuations from an electrical short or other compromised circuitry which may unnoticeably jeopardize the operation of the meter. None of the prior art hand held meters having a docking station base addresses the problems of non-recommended cleaning practices and the application of unspecified power supply voltages and currents that may result therefrom. 
     SUMMARY OF THE INVENTION 
     It is against the above background that the present invention provides a number of advantages and advancements over prior art docking station bases for hand held meters. In particular, the present invention incorporates a unique electrical protection circuitry into the electrical system of a docking station base. The electrical protection circuitry, which automatically disconnects power from the meter interface connector under certain detected conditions, mitigates the risk of damage to the meter and base due to non-recommended cleaning practices and the application of unspecified power supply voltages and currents that may result therefrom. As dendrites are encouraged to grow on electrical connectors when a voltage is present, the electrical protection circuitry also provides protection against dendrite growth on the meter interface connector by automatically disconnecting power from the meter interface connector when the meter is undocked and the base is cleaned separately. Should a continued fault condition be indicated by the base, the present invention at a minimum mitigates the risk of damage to the more expensive meter by being also a less expensive replacement item in the combined meter and base system. 
     In one embodiment, in the event of a short circuit in the electrical system of the base, the protection circuitry according to the present invention removes power to a meter interface connector. Similarly, in the event of a voltage being outside a specified operating range of the electrical system of the base, the protection circuitry according to the present invention removes power to the meter interface connector regardless if the meter is connected to the base. Essentially, the electrical system of the base is monitored with or without the meter being docked to the base. Additionally, the protection circuitry provides a visual indication of the over-current and under/over voltage fault conditions. 
     In another embodiment, the base shape is designed to prevent liquid from pooling inside a cradle portion used to hold the meter in the base. In particular, the base shape design prevents ingress of a liquid, such as a cleaning solution, which may contact any electronic components inside the base in its intended orientation through the use of a drain located at the lowest point in the cradle portion. Liquid ingress is also addressed by a provided dam to the underside of the meter interface connector which provides an area to house a gasket material to seal the underside of the base housing around the meter interface connector. 
     These and other features and advantages of the invention will be more fully understood from the following description of various embodiments of the invention taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of the embodiments of the present invention can be best understood when read in conjunction with the following drawings and in which: 
         FIG. 1  is a front perspective view of a meter in a docking station base according to the present invention; 
         FIG. 2  is a top view of a docking station base according to the present invention; 
         FIG. 3  is a bottom view of a docking station base according to the present invention; 
         FIG. 4  is an exploded view of components forming a docking station base according to the present invention; 
         FIG. 5  is a schematic of an embodiment of an electrical system with protection circuitry according to the present invention; 
         FIG. 6  is a schematic of an embodiment of a voltage supervisor circuit according to the present invention; 
         FIG. 7  is a schematic of an embodiment of an over-current monitoring circuit according to the present invention; 
         FIG. 8  is a schematic of an embodiment of a latch circuit according to the present invention; 
         FIG. 9  is a schematic of an embodiment of logic circuitry according to the present invention; and 
         FIG. 10  is a schematic of an embodiment of flash circuitry according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Throughout the specification, and in the claims, the term “connected” means a direct electrical connection between the things that are connected, without any intermediary devices. The term “coupled” means either a direct electrical connection between the things that are connected, or an indirect connection through one or more passive or active intermediary devices. The term “circuit” means one or more passive and/or active components that are arranged to cooperate with one another to provide a desired function. The term “signal” means at least one current signal, voltage signal or data signal. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on”. Also, “battery” includes single cell batteries and multiple cell batteries. 
       FIG. 1  is a front perspective view of a hand held meter  10  and a docking station base  20  according to the present invention. In the illustrated embodiment, the meter  10  is a hand held blood glucose meter, such as sold by Roche Diagnostics Corporation, under the trademark ACCU-CHEK® Inform. The meter  10  includes internal data processing and handling hardware, software, and firmware, generally indicated by symbol  11 , which is all powered by an internal rechargeable battery  12 . In other embodiments, the meter  10  may be any hand held patient diagnostic device, which can be connected to a docking station base for battery charging and for coupling the device to a remote system. 
     In the illustrated embodiment, the meter  10  and the base  20  together form a blood glucose monitoring system, generally indicated by symbol  14 . Typically, such a system is used in hospitals and out-patient settings for Bedside Glucose Testing (BSGT) or Near Patient Testing (NPT) to maintain patient blood glucose values and quality control data, and to transmit such data to a hospital information system  16  or remote personal computer  18 . Accordingly, the base  20  is a stationary platform that serves as a battery charging stand for the meter  10 . Additionally, the base  20  serves as a conduit for transmitting data from the meter  10  through the base  20  to one of the remote system  16 ,  18 . As the meter  10 , information system  16 , and personal computer  18  are conventional no further discussion is provided. 
     Focusing now on the present invention, the base  20  extends the functionality of prior art bases by providing extra safeguards against product malfunction in certain non-recommended usage conditions, such as cleaning the base with a non-recommended solution which can damage the internal circuit board if contact is made for extended periods of time. In particular, the base  20  provides fault protection for both improper voltage and current conditions. The base  20  provides a visual indicator  22  which functions to indicate to the user the detection of a fault condition, which is explained more fully in a later section. 
     Base Design 
     With reference made also to  FIGS. 2-4 , the base  20  is designed to be stable on a relatively horizontal desk-top. The base  20  is further designed such that the placement of the meter  10  into the base  20  does not allow the assembled components to topple in its intended orientation. The base  20  also includes a key-hole shaped opening  24  ( FIG. 4 ) on a rear surface thereof such that the base may be fastened on a vertical wall surface, if desired. 
     In the illustrated embodiment, the base  20  includes a meter interface connector  26  ( FIGS. 2 and 4 ) compatible with a base interface connector  28  ( FIG. 1 ) of the meter  10 . The meter interface connector  26  provides power for recharging the battery  12  and a data connection for coupling the meter  10  to the remote system  16  or  18 . Also, the base  20  includes in its form a base drain  30  ( FIG. 2 ) and connector dam  32  ( FIG. 4 ). The base drain  30  and connector dam  32  address part of the problem of excessive cleaning solution being applied to the meter, the base or both. 
     As best shown by  FIG. 2 , which is a top view of the base  20 , the base drain  30  provides an integral cross-shaped slew portion  34  at the bottom of a cradle portion  36  of the base  20  which supports the meter  10 . With the help of gravity, the slew portion  34  as shaped does not allow liquid to pool at the bottom of the cradle portion  36  as liquid is directed downward towards the rear of cradle portion  36  to a drain hole  38 . The drain hole  38  ensures that any liquid exits away from the base  20  and provided data port and power connector socket portions  40  and  42 , respectively ( FIGS. 3 and 4 ). 
     Furthermore, the connector dam  32  prevents liquid from entering the internal cavity  44  of the base  20 , which houses an electrical system  46  ( FIG. 4 ) of the base. The connector dam  32  provides a gasket material (not shown) to seal the internal cavity  44  from ingress of liquid from around the portion of the meter interface connector  26  that protrudes upwards from the bottom of the cradle portion  36 . Accordingly, together the base drain  30  and connector dam  32  prevent liquid pooling and liquid ingress should the meter or base be improperly cleaned. Reference is now also made to  FIG. 5 , which is a schematic of an embodiment of an electrical system  46  with protection circuitry  54  according to the present invention. 
     Power 
     The electrical system  46  of the docking station base provides a conduction path to an output terminal  66  ( FIG. 5 ) of the meter interface connector  26  which connects to the meter  10  to recharge the battery  12 . With the meter  10  cradled or docked in the base  20 , the electrical system  46  supplies current to the battery  12  such that it may be recharged. In one embodiment, the electrical system  46  is provided power via a power connector socket  48  which is compatible with a power cord  50  from a power supply  52 . In one embodiment, the power supply  52  is a voltage converter which converts energy from AC mains into sufficient DC current for charging the battery  12  of the meter  10 . In another embodiment, the power supply  52  may be an external source providing DC power supply, such as for example, a battery pack. In other embodiments, the electrical system  46  may include an internal voltage converter such that AC main may be connected directly to the electrical system, or include an internal DC power supply. 
     In one embodiment, the electrical system  46  provides the meter  10  with an operating voltage ranging from about 8.5 to about 9.5 VDC, with less than about 100 mV peak to peak noise in order to supply sufficient output current to charge the battery  12 . In other embodiments, other operating voltage ranges are possible and will depend on the power requirements of the hand-held meter and included battery. Additionally, although the voltage output available at the meter interface connector  26  is a relatively narrow voltage range, it is to be appreciated that the protection circuitry  54  of the electrical system  46  in one embodiment will still function properly with DC voltages from the power supply  52  ranging from 6 VDC to 15 VDC, and over a current range of 0 to 1.5 A with less than 100 mV peak-to-peak noise. In other words, as will be explained in greater detail in a later section, the protection circuitry  54  will provide a fault indication and disable the output voltage to the meter interface connector  26  should the operating voltage range from about 6.00 to about 8.49 VDC and about 9.51 to about 15.00 VDC. 
     In one embodiment, the output current to the meter  10  from the electrical system  46  via the meter interface connector  26  ranges from 0 to about 1.2 A. In another embodiment, the electrical system  46  via the meter interface connector  26  supplies three output current levels to the meter  10  under different operating states. In such an embodiment, the operating states include a battery constant-current (CC) charge mode, a battery constant-voltage (CV) charge mode, and a charger disable safe mode. In all modes, the meter  10  may still be operated when connected to base  20 . The following conditions cause the current output of the electrical system  46  to go to the charger disable safe mode at any time: the meter  10  is not connected to the meter interface connector  26 ; the input voltage from the power supply  52  is out of a specified operating range high, which in one embodiment is greater than 9.5 V; the input voltage from power supply  52  is out of a specified operating range low, which in one embodiment is less than 8.5 V; or the current draw of meter  10  from the base  20  is greater than a specified operating range, which in one embodiment is greater than 1.2 A. 
     In one embodiment, when the electrical system  46  is in safe mode and senses that the meter  10  has been connected to the meter interface connector  26 , i.e., docked, the electrical system goes to CC charge mode. After docking, the following conditions cause a transition to CV charge mode of electrical system  46 : the time of the electrical system in CC charge mode is over a predetermined period, which in one embodiment is greater than 1 hour; or the output current is below a predetermined value, which in one embodiment is less than 800 mA. When the electrical system  46  is in CV charge mode, the following conditions cause a transition to safe mode: the meter  10  is disconnected from the base  20 , i.e., undocked, or the time in CV charge mode is greater than a predetermined period, which in one embodiment is greater than 3 hours. In another embodiment, the internal hardware, software, and/or firmware  11  of the meter  10  is in control of the CC and CV charge modes. In such an embodiment, as long as the meter  10  is docked and the voltage and current are within limits, the electrical system  46  of the base  20  will not go to SAFE mode. 
     In one embodiment, the electrical system  46  will indicate the operating state with the visual indicator  22  on the front of the base  20 . In one embodiment, CC and CV charge modes are indicated with the visual indicator  22  continually on with full illumination. In another embodiment, the CC charge mode may be indicated with the visual indicator  22  continually on but with less than full illumination, thereby providing a dimmed appearance to distinguish between the CC and CV modes. In the charger disable safe mode, the visual indicator  22  is either off or flashing if indicating a fault condition. As the fault conditions are explained more fully in a later section, attention is now drawn to the data connection of the base  20 . 
     Data Connection 
     As mentioned previously above, the base  20  provides to the meter  10  a data connection to the remote system  16  or  18 . This data connection is provided by an included data port connector  56 . In one embodiment the data signals are passed between the meter  10  and the remote system  16  or  18  through the data port connector  56  without modification from the base  20 . In one embodiment, the data port connector  56  provides a serial connection, such as a RS232, RS485, or USB compatible connector, and in other embodiments may provide a network interface card from a direct network connection. A more detailed discussion on the electrical system  46  and the included protection circuitry  54  is now provided hereafter. 
     Electrical System 
     With reference made mainly to  FIG. 5 , the general operation of the electrical system  46  and the included protection circuitry  54  is as follows. As illustrated, power from the power supply  52  is delivered to the electrical system  46  via the power connector  48  and is routed to a voltage regulator  58 , which in one embodiment is rated at 5 volts. Power from the power supply  52  is also connected to the input of a current shutoff switch (Q 1 )  60  of the protection circuitry  54 . When enabled, switch Q 1   60  connects power from the power supply  52  to the meter interface connector  26 . The output of the voltage regulator  58  provides a constant operating voltage for the internal circuitry in the base  10 . As mentioned above, the meter interface connector  26  is the interface point between the electrical system  46  and the battery  12 , as well as a providing a data interface between the serial connector  56  and the meter  10 . 
     The protection circuitry  54  is arranged to monitor both the voltage deliverable to the meter  10  and the current drawn by the meter  10  from the electrical system  46 . If either the voltage or the current is out of their specified ranges, the protection circuitry  54  will disconnect power to the meter interface connector  26 , via the current shutoff switch  60 . The protection circuitry  54  further comprises a voltage supervisor circuit  62  and a current monitoring circuit  64 . It is to be appreciated that the protection circuitry  54  may be implemented as an analog circuit, a digital electronic circuit, and combination thereof. Additionally, the protection circuitry  54  is arranged to actuate the current shutoff switch  60  when one or more “fault” conditions are detected. The switch  60  provides a conduction path to shunt current away from the power output terminal  66  of the meter interface connector  26 . 
     Voltage Supervisor 
     From the power connector  48 , voltage is also connected to the voltage supervisor circuit  62 . If voltage from the power connector  48  is outside a specified range, the voltage supervisor circuit  62  disconnects power to the meter interface connector  26  via enabling a shutdown circuit  68 . When the shutdown circuit  68  is enabled, the current shutoff switch (Q 1 )  60  is disabled. Additionally, when the shutdown circuit  68  is enabled, a logic circuit  70  is enabled which controls the power to the visual indicator  22  to indicate a fault. When voltage from the power connector  48  returns to a normal, specified range, the power to the meter interface connector  26  is restored by the shutdown and logic circuits  68  and  70 , respectively, returning the electrical system  46  to its original functional state. This restoration of power to the meter interface connector  26  in a voltage fault condition will occur when or if the meter  10  is connected thereto. It is to be appreciated that although power may be cut-off to the meter interface connector  26  when the meter  10  is not connected to the base  20 , monitoring by the electrical system  46  for an out-of-range voltage condition will continue regardless of the meter  10  being docked to the base  20 . The voltage supervisor circuit  62  also includes hysteresis to prevent the power from oscillating due to noise on the supply line. 
     In one embodiment, illustrated by  FIG. 6 , the voltage supervisor circuit  62  is composed of two comparators U 1  and U 2 , which controls an NV_FAULT output signal connected to the logic circuit  70  ( FIG. 5 ). In particular, the upper comparator U 1  will pull the nV_FAULT output signal low if the input voltage Vin from the power connector  48  falls below a predetermined threshold value, while the lower comparator U 2  will pull the nV_FAULT output signal low if the input voltage Vin rises above a predetermined threshold value. The reference voltage VREF is supplied by a precision voltage reference (not shown). The pair of resistor dividers R 1   a  and R 2   a , and R 1   b  and R 2   b , set the nominal Vin divide rations for comparison voltages V 1  and V 2 , respectively, while the feedback resistors R 3   a  and R 3   b  provide the hysteresis. 
     Resistors R 4   a  and R 4   b  are large enough in comparison to the feedback resistors R 3   a  and R 3   b  such that they must also be figured into the hysteresis equations. Op-amp U 3  is set up as an inverter to flip the polarity of the hysteresis on the lower comparator U 2 , making the threshold go up when voltage V 3  goes low. Voltage V 5  is set by a voltage divider from the voltage regulator  58  ( FIG. 5 ). In one embodiment, the inverter U 3  requires a comparison voltage greater than half the voltage V 5  because the hysteresis of low comparator U 2  will not engage until inverter U 3  can switch. The higher the voltage V 5 , the shorter the delay before inverter U 3  can change polarities. By choosing a voltage V 5  higher than the AND gate (not shown) that accepts nV_FAULT output signal as an input to the logic circuit  70 , oscillations in voltages are blocked from reaching the current shutoff switch  60 . Additionally, by choosing a high value for voltage V 5 , the time duration during which oscillations are possible is limited to just a few μs. 
     In one embodiment, components of the voltage supervisor circuit  62  were selected to make the V 1  voltage equal to the reference voltage Vref when the supply voltage Vin was equal to a nominal trip point determined by equations (1)-(3), whereby when the output is high, V 1 =V 1 H, and when the output is low V 1 =V 1 L. 
     
       
         
           
             
               
                 
                   
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     Equations (4) and (5) may be used to calculate the voltage Vin from the resistor and reference values. It is to be appreciated that VinL corresponds to V 1 L, and VinH corresponds to V 1 H (referenced to output state), but because of an inversion in the equations VinL is at a higher voltage than VinH. 
                   VinL   :=     VREF   ⁢           ⁢         R   ⁢           ⁢   1     +     R   ⁢           ⁢   23         R   ⁢           ⁢   23                 (   4   )               VinH   :=         VREF   ·     [             R   ⁢           ⁢     1   ·     (       R   ⁢           ⁢   3     +     R   ⁢           ⁢   4       )         +     R   ⁢           ⁢     2   ·                     (       R   ⁢           ⁢   3     +     R   ⁢           ⁢   4       )     +     R   ⁢           ⁢     1   ·   R     ⁢           ⁢   2             ]       -       5   ·   R     ⁢           ⁢     1   ·   R     ⁢           ⁢   2         R   ⁢           ⁢     2   ·     (           ⁢       R   ⁢           ⁢   3     +     R   ⁢           ⁢   4       )                   (   5   )               
Current Monitoring
 
     Turning back now to  FIG. 5 , the current monitoring circuit  64  disconnects power to the meter interface connector  26  if the current drawn by the meter  10  excesses a specified threshold value. The present inventors have found that power can short to ground in the situation when cleaning fluid contacts one of the interface connectors  26  or  28 . In such a situation, cleaning fluid will be detected by monitoring the filtered DC value of the current flowing through the meter  10 , wherein exceeding the specified threshold value is considered to be a fluid-induced current fault. However, it is also to be appreciated that a fault condition may also result from semiconductor processing defects such as a shorted resistor, mechanical stress, thermal stress, misuse such as utilizing a non-compliant power adapter, as well as others, which the protection circuitry may also detect. 
     From a ground terminal  72  of the meter interface connector  26 , a current to voltage transformation occurs at a current sense amplifier  74  of the current monitoring circuit  64 . The voltage output of the current sense amplifier  74  is connected to an over-current detector  76 . If the specified threshold value is exceeded, the over-current detector enables the shutdown and logic circuits  68  and  70 , respectively. As before, when the shutdown circuit  70  is enabled, the current shutoff switch  60  is disabled, thereby removing power from the meter interface connector  26 . When the logic circuit  70  is enabled, the visual indicator  22  is controlled to indicate a fault. Additionally, when the over-current detector  76  is enabled, both the shutdown and logic circuits  68  and  70  are latched via enabling a latch circuit  78 . When the latch circuit  78  is enable in the case of a current-induced fault, power is not restored to the meter interface connector  26  until power to the electrical system  46  is removed and reapplied, and the current fault no longer exists. 
     Current Sense Amplifier 
     In one embodiment, illustrated by  FIG. 7 , the current sense amplifier  74  has a low side current sense amplifier topology, and converts an input current I in  through resistor R sense  to an appropriately scaled amplifier output voltage V amp     —     out . As shown, resistors R 5  and R 6  set the gain of the op-amp, and resistor R 6  and capacitor C 1  limit the bandwidth. Additionally, the current sense amplifier  74  is configured such that the return current flows through a current sense resistor R sense  from the positive input to the negative input, thereby providing a positive output from the amplifier. 
     When the meter  10  is connected to the base  20  with power available at the meter interface connector  26 , the current may briefly spike, exceeding the current limit threshold. However, the amplifier roll-off, as set by resistor R 6  and capacitor C 1 , slows the signal enough to prevent such a current spike from triggering the over-current shutdown signal. Voltage output V amp     —     out  of the current sense amplifier  74  is dependent on the R 6 /R 5  ratio as well as the value of the current sense resistor R sense , and is defined by equation (6). 
     
       
         
           
             
               
                 
                   
                     
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     It is to be appreciated that the current sense resistor R sense  is chosen as a small value to prevent a large voltage drop from disrupting circuit operation. In one embodiment, the gain resistors R 5  and R 6  are chosen to set the amplifier output voltage V amp     —     out  to full scale for input current values around 2 A. In one embodiment, the nominal gain is about 2.1 V/A. 
     Over-Current Detector 
     In the illustrated embodiment of  FIG. 7 , the over-current detector  76  comprises a comparator  80  with a threshold voltage V t     —     hold  set with a reference diode  82 . The reference diode  80  is chosen such that the output signal nOCFAULT of the comparator  80  will go low when the output of the current sense amplifier  74  corresponds to a predetermined input current limit. In one embodiment, the nominal trip current is 1.19 A. 
     Latch 
     In one embodiment shown by  FIG. 8 , the latch circuit  78  comprises a pair of AND gates U 4  and U 5 , and a memory  84 . In the illustrated embodiment, the memory is a D flip-flop, and in other embodiments may be any type of memory, bistable multivibrator, and the like. The latch circuit  78  is designed to hold its output signal nSD low after a current fault is signaled by the output signal nOCFAULT from the over-current detector  76  going low. It is to be appreciated that the output signal nSD will remain low even if the fault is cleared as indicated by nOCFAULT going high unless power to the power connector  48  ( FIG. 5 ) is cycled off and on. The truth Table for the D flip-flop  84  is shown below in Table 1. 
     The latch truth Table 1 is referenced for the following explanation of the various latch states. In the illustrated embodiment, the underlines states will never occur, as the clock and D inputs to the D flip-flop  84  are pulled low. Additionally, as shown in  FIG. 8 , the Q output is unused. 
                                                                         TABLE 1                   Latch truth Table                Inputs       Outputs                    _PRE   _CLR   CLK   D   Q   _Q (nSD)                       L   H   X   X   H   L           H   L   X   X   L   H           L   L   X   X   H   H             H       H       ↑       H       H       L               H       H       ↑       L       L       H             H   H   L   X   Qo   _Qo                        
When power is turned off, C 2  is discharged as D 2  and R 8  ensure quick discharge as the +V supply goes to zero. Control_CLR will be held low until C 2  can charge past the input threshold of AND gate U 5 .
 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 D flip-flop states on power up, no current fault condition 
               
             
          
           
               
                 Condition 
                 _PRE 
                 _CLR 
                 _Q (nSD) 
               
               
                   
               
               
                 At power up, no fault 
                 H 
                 L 
                 H 
               
               
                 C2 charges, U5 out goes high 
                 H 
                 H 
                 H 
               
               
                 Current fault occurs (nOCFAULT low) 
                 L 
                 H 
                 L 
               
               
                 Current fault is cleared 
                 H 
                 H 
                 L 
               
               
                 (nOCFAULT high) 
               
               
                 Current fault is reintroduced 
                 L 
                 H 
                 L 
               
               
                 (nOCFAULT low) 
               
               
                   
               
             
          
         
       
     
     Table 2 shows the state of the output signal nSD of the D flip-flop  84  upon the occurrence of a fault condition being indicated by the output signal nOCFAULT from the over-current detector  76  going low. As shown above, Table 2 demonstrates that latch circuit  78  will hold the output signal nSD low after it is first pulled low despite any activity on the input line _PRE of the D flip-flop  84  by the output signal nOCFAULT from the over-current detector  76 . 
     If there is a persistent current fault detected at power up, the latch circuit  78  will engage as shown in the Table 3 below. Table 3 shows the state of the output signal nSD of the D flip-flop  84  upon the occurrence of a fault condition being indicated by the output signal nOCFAULT from the over-current detector  76  going low at power up. Additionally, Table 3 demonstrates that the latch circuit  78  will hold the output signal nSD of the D flip-flop  84  low after it is first pulled low despite any activity on the input line _PRE of the D flip-flop by the output signal nOCFAULT from the over-current detector  76 . The output signal nSd of the latch circuit  78  is then used by the logic circuit  70  for current shutoff determinations as well as fault indication, which a discussion regarding is provided hereafter. 
                                       TABLE 3                   D flip-flop states on power up with current fault condition            Condition   _PRE   _CLR   _Q (nSD)               At power up, fault is present   L   L   H       C2 charges, U5 out goes high   L   H   L       Current fault is cleared   H   H   L       (nOCFAULT low)       Current fault reintroduced   L   H   L       (nOCFAULT low)                    
Logic and Shutdown
 
     In the illustrated embodiment shown by  FIG. 9 , the logic circuit  70  comprises an inverting buffer  86  and a first AND gate  88 , and the shutdown circuit  68  is a second AND gate. When the meter  10  is connected to the meter interface connector  26  ( FIG. 5 ), an XGND signal from the meter interface connector is low such that the output signal DOCKED of the inverting buffer  86  is high. The output signal DOCKED going low would indicated that the meter  10  is not connected to the meter interface connector  26 , e.g., not docked to the base  20 . As mentioned above in a previous section, the nV_FAULT signal is the output signal of the voltage supervisor circuit  62 . A high signal output nV_FAULT indicates that the voltage of the electrical system is within tolerance, and that a low signal output nV_FAULT indicates the voltage is outside of tolerance. Also as mentioned above in a previous section, a high output signal nSD from the current monitor circuit  64  indicates that the current draw is not excessive, whereas a low output signal nSD indicates excessive current draw. This signal is latched low after a failure until power is cycled and no fault remains. 
     The CURRENT_ENABLE signal controls the current switch Q 1  as well as the visual indicator  22  via enabling the LEDCTRL output signal of the NOR gate  92  to go high. The CURRENT_ENABLE signal is high when the meter is docked (DOCKED=1), the voltage is in the proper range (nV_FAULT=1) and the over-current detector is not activated (nSD=1). Table 4 is a truth Table of the output of the shutdown circuit  68 . 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 CURRENT_ENABLE truth table 
               
             
          
           
               
                 DOCKED 
                 nV_FAULT 
                 nSD 
                 POWER_GOOD 
                 CURRENT_ENABLE 
               
               
                   
               
               
                 0 
                 X 
                 X 
                 X 
                 0 
               
               
                 1 
                 0 
                 X 
                 0 
                 0 
               
               
                 1 
                 X 
                 0 
                 0 
                 0 
               
               
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
             
          
         
       
     
     Accordingly, output signal POWER_GOOD of the first AND gate  88  will be high when both current and voltage are within tolerance. Should the meter  10  be connected to the base  20  (DOCKED signal high) with the voltage and current within tolerance, a high output signal CURRENT_ENABLE of the shutdown circuit  68  (second AND gate) will provide power to the collector of the current shutdown switch  60 , thereby enabling power to be provided to the meter interface connector  26 . Should either of the signals DOCKED or POWER_GOOD go low, the CURRENT_ENABLE signal goes low, thereby removing power from the collector of the current shutdown switch  60  and cutting power to the meter interface connector  26 . 
     Visual Indicator 
     The only operator interface with the base  20  is the visual indicator  22  ( FIG. 5 ), which indicates power and meter docked status. If the meter  10  is docked and there is no power fault, the visual indicator  22  will be on continuously. If the meter  10  is not docked, the visual indicator  22  will be off, again if there is no fault. If a voltage or current fault is detected, then the visual indicator  22  will blink, regardless of whether the meter  10  is docked. In one embodiment, the visual indicator  22  is a single LED, and in other embodiments, may be any suitable illumination means. 
     To provide the above visual indications, as illustrated by  FIG. 9 , the logic circuit  70  further includes a flash circuit  90  and a NOR gate  92  which provides logic control to the visional indictor  22  ( FIG. 5 ). As shown, the flash circuit  90  provides output signal BLINK. Output signal BLINK is low when POWER_GOOD signal is high, and will oscillates when POWER_GOOD signal is low. Table 5 is a truth Table of flash circuit  90 . It is necessary to differentiate between the blinking state of the BLINK circuit and the solid on or off state. In Table 5, the BLINK output will be labeled 0B and 1B when blinking, 0 or 1 when not blinking 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Flash circuit truth table 
               
             
          
           
               
                 DOCKED 
                 POWER_GOOD 
                 CURRENT_ENABLE 
                 BLINK 
                 LED 
               
               
                   
               
               
                 0 
                 0 
                 0 
                 0B 
                 Off (0) 
               
               
                 0 
                 0 
                 0 
                 1B 
                 On (1) 
               
               
                 0 
                 1 
                 0 
                 0 
                 Off (0) 
               
               
                 1 
                 0 
                 0 
                 0B 
                 Off (0) 
               
               
                 1 
                 0 
                 0 
                 1B 
                 On (1) 
               
               
                 1 
                 1 
                 1 
                 0 
                 On (1) 
               
               
                   
               
             
          
         
       
     
     Accordingly, if the meter  10  is not docked to the base  20 , resulting in DOCKED signal going low and the output signal BLINK is also being low due to the POWER_GOOD signal being high, the control signal output LEDCTRL of the NOR gate  92  is low, thereby turning off the visual indicator  22 . It is to be appreciated that since the protection circuitry  54  ( FIG. 5 ) is powered before the current shutoff switch  60 , should the POWER_GOOD signal go low, the LEDCTRL signal will go high in step with the BLINK signal oscillations, thereby flashing the visual indicator  22  to indicate a fault condition. It is also to be appreciated the protection circuitry  54  will continue to operate after fault until power is removed from the power connector  48 . In one embodiment, the output signal BLINK will oscillate such that the visual indicator  22  will flash at a rate of 2 Hz+/−20%. 
     Flash Circuit 
     In the embodiment shown by  FIG. 10 , the flash circuit  90  comprises a transistor Q 2 , and op-amp  94 . As shown, when the POWER_GOOD signal is high, the collector of the transistor Q 2  will be turn on and voltage V 6  will be pulled low. Pulling voltage V 6  will result in a continuous low signal out of the op-amp  94  (BLINK signal is low). When the POWER_GOOD signal is low, the collector of the transistor Q 2  is open, and output signal BLINK of the op-amp  94  will begin to oscillate, as voltage V 7  is pulled down with the charging cycles of capacitor C 3 . The frequency of the oscillation frequency is defined by equation (7). 
     
       
         
           
             
               
                 
                   f 
                   := 
                   
                     1 
                     
                       2 
                       · 
                       
                         ( 
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           4 
                           ⁢ 
                           C 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             1 
                             · 
                             ln 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             ( 
                             
                               
                                 V 
                                 ⁢ 
                                 
                                     
                                 
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                                 7 
                               
                             
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     The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the present invention. Modification and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims.