Abstract:
A simplified user interface for an electronic test and measurement device comprises an arrangement of a segmented LCD device and a plurality of keys used to implement a softkey system, in combination with a single AMPs input connector and a rotary selector having only a single position for each basic kind of parameter to be measured. The segmented LCD device includes, at each location adjacent to a key, a plurality of segments representing a vertically-arranged list of functions to be controlled. In one embodiment of the invention, the housing of the electronic test and measurement device is fabricated to have finger-like projections overlaying a portion of the LCD screen for creating the appearance of the continuation of displayed LCD softkey separation lines.

Description:
FIELD OF THE INVENTION 
     The subject invention concerns the field of user interfaces and control systems for multimeters in general, and specifically concerns multimeters having softkeys. 
     BACKGROUND OF THE INVENTION 
     A multimeter, as the name suggests, is an instrument which is capable of measuring a number of different parameters, such as voltage, current, resistance, and the frequency of an input signal. Modern multimeters have become quite advanced and are also sometimes capable of measuring capacitance, duty factor, and temperature. It is also common for multimeters to automatically select the proper range for display. 
     Unfortunately, the user interface on the front panels of these existing multifunction multimeters is becoming more and more cluttered, confusing, and difficult to use. One reason for this clutter is the number of buttons (i.e., keys) and input terminals required to select and exercise the available features. For example, it is common for selector knobs to have three positions for the AMPS function (i.e., A, mA, and μA), and at least two positions for the Volts function (AC and DC). There are usually two AMPs input connectors (i.e., AMPs and mA) in addition to the VOLTs and COMMON input connectors. Thus, the user is presented with a confusing multitude of legends, buttons and knobs, and inputs, from which he must choose to exercise the desired function. 
     It is known to use so-called “softkeys”, to reduce the number of buttons needed, by changing the legend on a display screen adjacent to the button. The display device has to be versatile enough to display a number of different softkey legends. Such systems typically employ a CRT or a dot matrix LCD so that the area immediately adjacent to the button can be erased and a new legend displayed when the key function is to be changed. Unfortunately, in a handheld device such as a multimeter, a CRT is impractical for reasons of weight, size, and durability, and a dot matrix LCD is impractical for the added complexity and expense that its use would entail. What is needed is a user interface for a multimeter which retains the multifunction capability but overcomes the above noted difficulties. 
     SUMMARY OF THE INVENTION 
     A simplified user interface for an electronic test and measurement device comprises an arrangement of a segmented LCD device and a plurality of keys used to implement a softkey system, in combination with a single AMPs input connector and a rotary selector having only a single position for each parameter to be measured. The segmented LCD device includes, at each location adjacent to a key, a plurality of segments representing a vertically-arranged list of functions to be controlled. 
     Another aspect of the invention is that a single amps connector is used to receive the current to be measured, and the measurement takes place over a range extending from nanoamperes to amperes. 
     In one embodiment of the invention, the housing of the electronic test and measurement device is fabricated to have finger-like projections overlaying a portion of the LCD screen for creating the appearance of the continuation of displayed LCD softkey separation lines. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a digital multimeter (DMM) in accordance with the invention. 
     FIG. 2 shows a segmented LCD display for use with the multimeter of FIG.  1 . 
     FIG. 3 shows an input switching arrangement for the AMPs connector of the multimeter of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a digital multimeter DMM  100  having a simplified user interface comprises a housing or case  110 , and an LCD display  120 , and softkeys  130   a ,  130   b ,  130   c , and  130   d . It is also important to note the DMM of FIG. 1 requires only a single input connector  140 , labelled A, for receiving current to be measured by the AMPs function. It is important to note that the current measurement is made over a range extending from nanoamperes to amperes. Note also that rotary function selection switch  150  has only a single position for each function to be exercised. There are only five other buttons  151 ,  153 ,  155 ,  157 , and  159  which are used for accessing further features of the DMM. A sixth button  160  activates a backlight to provide easier viewing in low light conditions. The combination of these elements provides a clean, easy to use, and non-intimidating human interface. 
     A segmented LCD display  220  showing all segments illuminated is shown in FIG.  2 . LCD display  220  is an illuminated illustration of LCD display  120  of FIG. 1 highlighting the large number and complex arrangement of segments required to implement a modern DMM. In fact, the number of segments is actually greater than the number of addressable points. That is, the addressing arrangement for this particular segmented LCD display is four by forty addressable points, while the actual number of segments needed was one hundred eighty. This problem was overcome with the realization that in certain functions some of the segments are always illuminated in pairs, and that those segments could be connected together. 
     As noted above, a dot matrix LCD can be erased and rewritten in the area adjacent to a button to create a “softkey” which varies its function in accordance with a displayed legend. A segmented display cannot be erased; its segments are fixed in place. In order to solve this problem, it was decided to provide a vertically-arranged list of softkey legends  221 ,  223 ,  225 , and  227  in the area immediately above the softkey buttons  130   a ,  130   b ,  130   c ,  130   d . Only one legend in each vertical group would be illuminated at a given time. In order to associate these groups of legends with a particular key, one would think that one could encircle the legends with a line segment. Unfortunately, this is not possible because of the fact that in a segmented LCD display device, lines cannot cross. That is, both the segment address lines and the segment display lines must be carefully routed so as not to touch. Thus, if the group of legend segments were encircled by a line segment, the address lines to the individual legend segments would be unable to cross the encircling line segment, resulting in inoperative legend segments. 
     This obstacle was overcome by drawing a line segment  231 ,  233 ,  235 ,  237  around the softkey legend segments on only three sides, as shown in FIG.  2 . This enables the address lines to reach the softkey legend segments without crossing any display lines, while still providing a grouping symbol around the legends to associate them with the same key. Unfortunately, the open bottom of the partially encircling line segment was felt to be less aesthetically pleasing than a fully encircling line segment. This problem was overcome by employing finger-like projections  170   a ,  170   b ,  170   c  which slightly overlay the screen area, and mask the fact that the encircling is incomplete. The result is that the finger-like projections in combination with the observable portion of the LCD partially-encircling line segments “fool the eye” and present a more aesthetically pleasing appearance. 
     The use of a segmented LCD device caused yet another problem to arise in association with the refresh rate of the LCD. When considering the use of a segmented LCD for a multimeter display device, the large number of segments required quickly becomes apparent. Each segment must be refreshed repeatedly, and the large number of segments requires a substantial amount of time to completely refresh all the segments and return to the first one to begin again. Moreover, it was found that care had to be taken to provide a suitable refresh rate in order to avoid an annoying beat between the illumination of the segments and the pulsation of normal room lighting due to the 50 Hz or 60 Hz power line frequency. 
     Referring to FIG. 3, a rotary switch  305  for selection of measurement functions can be simplified to select only the basic kind of function (for example, AMPs, VOLTs), instead of the more usual specific function (for example, AMPs, mA,), because the signal to be measured is not coupled through the rotary switch. Rather, rotary switch  305  is an input to a control microprocessor  310  which controls the actual measurement. Four softkey buttons (i.e., switches)  370   a ,  370   b ,  370   c , and  370   d  are coupled to microprocessor  310  and are used to further refine the input to microprocessor  310  to select specific desired measurement functions. In response to rotation of rotary switch  305  to the AMPs position, and the pressing of one of softkey buttons  370   a ,  370   b ,  370   c , and  370   d , control microprocessor  310  controls switching FETs  320 ,  322 ,  324 , and  326  to select the proper measurement range in cooperation with scaling resistors  330 ,  332 , and  336 . The signal current applied at the single AMPs input flows through fuse  350 , one or more of switching FETs  320 ,  322 ,  324 , and  326  and one or more of scaling resistors  330 ,  332 , and  334  to ultimately develop a voltage at the input of A/D converter  340  that is representative of the magnitude of the current being measured. The digital representation of the A/D input signal is coupled to microprocessor controller  310  for display on display screen  220  of FIG.  2 . 
     Each of the FET switches of FIG. 3 includes a bypass diode for input protection. It is a characteristic of these diodes that they are of high quality in that they can handle high currents, and that they exhibit extremely low reverse leakage. Note that switching FETs  320  and  322  are both N-channel FETs, but that FET  322  is connected in reverse (i.e., oppositely) to its usual orientation. That is, the source terminal of FET  320  is coupled directly to the drain terminal of FET  322 , the drain terminal of FET  320  is coupled directly to the source terminal of FET  322 , and the gate terminals are coupled together and driven by microprocessor controller  310 . The purpose of this reverse orientation of FET  322  is to utilize the internal protection diode to protect against the opposite polarity surges from those handled by the diode in FET  320 . The circuit of FIG. 3 also has the added benefit that FETs  320  and  322  are operated in parallel and thereby lower switch resistance in the high current range. A prior art version of this circuit used a power diode in place of FET  322 . The power diode of this prior circuit did not contribute to the lowering of the switch resistance, as mentioned above, because it was not activated in parallel with FET  320 . Moreover, power diodes having the capability to handle surge currents of amps, undesirably exhibit relatively high leakage currents of approximately 100 nA, which would adversely affect current measurements on the lower ranges. As noted above, the subject apparatus is capable of measuring in the nanoampere to ampere range, and the use of the oppositely-connected FET, rather than the power diode, allows the nanoampere measurement without the adverse effects caused by reverse current leakage. 
     It is important to note that the cooperation between a rotary switch having only a single selection for each major function, a segmented display with attendant softkeys, and a single AMPs input provides the user with a desirably uncluttered, and particularly easy-to-use interface. 
     The term “control microprocessor”, as applied to element  310  is intended to encompass microcomputers, and other forms of controllers such as dedicated hardware controllers including ASICs. 
     While the invention has been described with respect to a digital multimeter, it is herein recognized that it would is also applicable to other types of test and measurement equipment, and such modification is deemed to lie within the scope of the following claims.