High reliability display

A character display in which failures are evident. Selected segments of a seven-segment LCD are activated by control signals supplied through selected fore plane and back plane traces. By coupling specific ones of the fore plane and back plane control signals to selected segments, any failure in the control signal or circuitry providing the signal to the segments of the display character becomes immediately evident to an operator when the character represented by the display is not one of a predefined set of characters.

FIELD OF THE INVENTION 
This invention generally relates to a digital display, and more 
specifically, to a display that includes a plurality of segments, which 
are selectively energized to indicate a specific alphanumeric character. 
BACKGROUND OF THE INVENTION 
In a conventional alphanumeric display, individual segments arranged in 
some predefined pattern are selectively energized to visually represent a 
desired character. The most common type of display includes at least seven 
discrete segments arranged so as to visually represent a numeral eight if 
all of the segments are simultaneously energized. Although this type of 
display can represent a limited number of alpha characters, it is most 
often employed to represent the numbers zero through nine. Substantially 
the same seven segment configuration (although fabricated very differently 
for the two types of display technologies) can be used for both light 
emitting diode (LED) displays and liquid crystal displays (LCDs). 
There are several advantages that justify using LCDs rather than LEDs in 
instrument display panels. Particularly important in portable, battery 
powered instruments is the intrinsically lower power requirement of the 
liquid crystal technology. In addition, relatively complex arrangements of 
graphic icons and alphanumeric character displays can readily be 
configured on a common substrate to produce a complete LCD panel for an 
instrument. By contrast, LED displays are more directed to representing 
characters than graphic icons and are not as easily fabricated in complex, 
integrated panels. 
Each segment of an LCD corresponds to similarly shaped electrically 
conductive regions applied to the front and rear surfaces of the display. 
These electrically conductive regions are coupled to a control circuit 
that supplies a voltage appropriate to modify the optical characteristics 
of a liquid crystal layer disposed between the front and rear surfaces. 
The electrical signal applied to these regions causes the liquid crystal 
layer to become more opaque, so that a character or graphic icon 
corresponding to the shape of the electrically conducting regions is 
visible. Optionally, background lighting can be provided for an LCD to 
produce greater contrast so that the characters or icons are more easily 
visible at low ambient light levels. 
There are certain applications in which a failure of one of the segments 
that defines a character or graphic icon may have life-threatening 
consequences. For example, a display panel on a medical instrument may 
indicate certain critical operating parameters to an operator of the 
instrument. Clearly, in this instance, it is very important to avoid 
errors in reading the displayed data. An error caused by the failure of a 
segment in a numeric display character would be particularly serious if 
the character is the most significant digit of a critical displayed value. 
For instance, failure of the center segment in a seven segment character 
of the display would cause an "8" to visually appear as an "0". A medical 
practitioner relying on the incorrect displayed reading caused by such a 
failure might use the instrument in a manner that hams a patient. 
Accordingly, for any critical displayed parameter on a medical instrument 
or on instruments used in other critical applications, designers have 
recognized the importance of detecting a display failure so that the user 
is alerted and does not rely upon an incorrect value. 
One way to insure that a failure in a critical display character is 
detected is to duplicate the entire displayed parameter, so that two 
nominally identical values for the parameter are indicated in separate 
displays. If the two displayed values are different, the user is supposed 
to recognize that a failure has occurred in one of the duplicated 
displays. However, because the two displayed values are spatially 
separate, such difference may go unnoticed. Moreover, space limitations on 
a display panel often render it impractical to provide duplicate values of 
a parameter, and this solution to the problem is inelegant at best. 
If LEDs are used for such critical displayed parameters, the electrical 
current to each digit of the display can be monitored and compared to an 
expected value in a look-up table that relates each of the possible 
characters to the electrical current draw required for that character, 
based on the number of segments that must be selectively energized to 
represent it. For example, a numeral "1" requires that only two segments 
be energized (in a seven segment display), and the look-up table defines a 
current corresponding to that required by the two segments. If less 
current is detected, at least one of the two segments may have failed and 
a monitoring circuit alerts the operator with a visual and/or audible 
alarm. Similarly, if current to a segment is detected when that segment 
should not be energized, the monitoring circuit also detects a failure. 
Unfortunately, the low current requirements of LCDs make it practically 
impossible to detect a failed segment in an LCD character based on 
electrical current measurements. Consequently, although LCDs are almost 
uniformly preferred for display panels because of the variety of graphic 
options that can be included and because of their low power requirement, 
LEDs are often used for the display of critical parameter values, simply 
to ensure that the failure of a segment in the critical display can be 
detected by the current monitoring method. Therefore, both an LCD display 
panel and LEDs must be used on such instruments. It should be evident that 
it would be preferable to use only LCDs in a display panel, if failure of 
a segment of any LCD characters was clearly evident. 
In a copending, commonly assigned patent application, Ser. No. 08/125,508, 
filed on Sep. 21, 1993, entitled "Display Having Redundant Segments" (R. 
Poli and A. Ravid), a solution to the above-noted problem is disclosed. In 
the invention disclosed in this reference, at least some of the segments 
comprising each display character are configured as pairs of segments, 
wherein one of the pair of segments is redundant, extending either 
parallel or end-to-end relative to the other segment of the pair. If one 
of the pair of segments should fail to be visually perceptible when 
activated or by remaining visually perceptible when de-activated, the 
failure will be immediately visually evident to the user because the other 
segment of the pair will continue to operate properly. Any difference 
between the operation of the two segments comprising each pair is visually 
evident and thus serves to warn a user that a problem or failure in the 
display has occurred. If fewer than all of the segments are formed as 
pairs of segments, the segments selected for redundant configuration are 
those in which failure of the segment, if not evident, could cause the 
resulting character to visually represent an incorrect character that 
might not otherwise be evident as an incorrect character. 
Although the display circuit comprising the above-noted invention 
represents a solution to the problem of detecting failed segments in a 
display and yet representing a desired character even when a failure of 
one segment has occurred, there are some applications in which it may be 
too expensive to fabricate pairs of redundant segments, or in which the 
display panel does not have sufficient area or edge connect terminations 
to support the greater number of conducting traces and terminals required 
for such a display circuit. Accordingly, another approach that is simpler 
to fabricate and requires fewer conductors would be preferable. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a display circuit is defined that 
comprises a plurality of conducting traces. A plurality of segments are 
electrically coupled to the plurality of conducting traces and are 
configured in a predefined pattern on the substrate. The pattern is 
capable of visually representing a character from a set of predefined 
characters. Specific segments are electrically coupled to selected ones of 
the plurality of conducting traces. The conducting traces that are thus 
coupled to specific segments are selected so that a failure of any one of 
the conducting traces to electrically actuate a single segment to which it 
is electrically coupled, in response to an externally supplied electrical 
signal that should have actuated the segment, does not cause a different 
character of the set of predefined characters to be visually represented 
by the display circuit than would have been if the failure had not 
occurred. Thus, any failure that causes a non-recognized character (not 
from the set) serves to warn a user that this type of failure has 
occurred. 
The display circuit also includes display control means that respond to the 
externally supplied electrical signal to provide an actuating voltage to 
specific segments that must be actuated to visually represent a selected 
character. 
A fore plane electrode and a back plane electrode preferably define a 
visually perceived shape of each segment. The actuating voltage applied 
between the fore plane and back plane electrodes activates a liquid 
crystal region that is disposed between the fore plane electrode and the 
back plane electrode to change an optical property of the liquid crystal 
region so that the segment is visually perceptible, unless a failure has 
occurred. 
The plurality of conducting traces comprise a set of fore plane traces and 
a set of back plane traces. The actuating voltage applied to activate the 
liquid crystal region for each segment is coupled to the fore plane 
electrode for that segment through one of the fore plane traces and to the 
back plane electrode for that segment through one of the back plane 
traces. 
In one preferred form of the invention, one of the sets of the fore plane 
traces comprises two conducting traces, and the other set comprises four 
conducting traces. In addition, the set of predefined characters 
preferably includes at least the numbers zero through nine. It is further 
intended in at least some embodiments, that the substrate includes areas 
in which a plurality of characters are defined and are visually 
perceptible when selected segments are actuated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a generally conventional seven-segment LCD display 
character 10. Display character 10 includes segments 12a-12g configured so 
that when all of the segments are actuated, a numeral "8" is visually 
indicated. By controlling the specific segments 12a-12g that are 
activated, each of the numerals 0 through 9 can be selectively represented 
by display character 10. Alternatively, many of the alpha characters, such 
as A, C, E, F, etc. can also be represented by selective activation of 
segments 12a-12g. However, as noted above, failure of one of the segments 
to be activated in response to a control signal applied to display 
character 10, for example, due to an open circuit that prevents an 
actuating voltage from being applied to the segment, can cause an error in 
the character represented. Thus, a "7" can appear as a "1" if segment 12a 
fails to activate, or an "8" can appear as a "0" if segment 12g fails. 
These types of failures can have severe consequences if display character 
10 is used on the display panel of medical apparatus or in other 
applications in which an improperly represented character causes the 
display to improperly represent critical information to an operator that 
can have grave, possibly life threatening consequences. 
In the prior art, each of segments 12a-12g is separately and independently 
controlled to represent a desired alphanumeric character. As a 
consequence, failures of certain of the segments due to interruption of 
the actuating voltage normally provided to a segment can cause erroneous 
characters to be represented without any indication to the user that a 
failure has occurred. However, the present invention substantially reduces 
the risk of such failures going undetected, by selectively controlling the 
segments with fore plane and back plane signals selectively connected to 
specific segments so as to make such failures visually evident. Details of 
this technique are explained below. 
Turning now to FIG. 2, an appropriate circuit is shown for driving one or 
more of the display characters used in implementing the present invention. 
A data source 40, which may comprise a central processing unit (CPU) or 
the controller of an instrument produces a binary signal that is coupled 
through data lines 42 to a display driver 44. The binary signal conveyed 
on data lines 42 indicates a desired character or characters to be 
visually represented on a display 50. In response to the signals supplied 
to it, display driver 44 interprets the binary signal provided by data 
source 40 and produces appropriate fore plane signals and back plane 
signals that are conveyed to display 50 on lines 46 and 48, respectively, 
to cause the display to visually indicate the character(s). Display driver 
44 comprises, for example, a Motorola type MC145000 Master LCD driver 
circuit. If more than one character is included in display 50, the 
additional characters can be controlled using, for example, a Motorola 
type MC145001 Slave LCD driver circuit for each additional character used 
in the display. Such display drivers or their equivalents are readily 
configured to provide the multiplexed fore plane and back plane signals 
described below. 
As used herein, the term fore plane signals and back plane signals 
specifically refer to segment actuation voltages at levels sufficient to 
cause one of the segments comprising a character on display 50 to be 
actuated so that it changes optical characteristics and is visually 
perceptible. Preferably, display 50 comprises an LCD panel. 
As is well known to those of ordinary skill in the art, an LCD region 
becomes visually perceptible when, in response to an appropriate voltage, 
an electric field is developed across the LCD region that causes the 
region to darken, in contrast with a lighter, reflective background. Under 
low ambient light conditions, the background may optionally be lighted to 
contrast with the segments of each LCD character that are activated. The 
voltages required to activate specific regions corresponding to segments 
of character(s) in display 50 are provided by display driver 44 in a 
generally conventional manner. However, unlike the prior art, the fore 
plane and back plane signals are selectively coupled to specific segments 
so that failure of any one of the signals to actuate the segment or 
segments to which the signals are supplied becomes immediately evident 
because a character that is not part of the set of characters normally 
represented on display 50 becomes visually perceptible instead of the 
desired character. 
The interconnections between specific segments and the fore plane signals 
and back plane signals to produce a display character that indicates when 
such a failure has occurred are shown in FIGS. 3 and 4, respectively. FIG. 
3 shows the fore plane electrodes 56a-56g, corresponding to the seven 
segments a-g of the display character in accordance with the present 
invention. Fore plane electrode 56a thus defines the shape of segment "a" 
corresponding to segment 12a in prior art display character 10 (FIG. 1), 
and in similar fashion, fore plane electrodes 56b-56g each defined the 
shape of corresponding segments "b" through "g," i.e., corresponding to 
segments 12b through 12g in prior art display character 10. Fore plane 
electrodes 56a-56g are applied proximate the front surface of a substrate 
(not shown) comprising display 50. 
Although the fore plane and back plane electrodes for only one character 
are represented in FIGS. 3 and 4, it will be understood, that each 
character of display 50 includes fore plane and back plane electrodes that 
operate in substantially the same manner. Thus, FIG. 4 shows that the back 
plane signals input to the illustrated display character are conveyed to 
another character in a plurality of characters that may comprise display 
50. In the event that only a single character is required, these back 
plane signals would be provided only to the one character. 
Referring first to FIG. 3, fore plane signals are selectively applied to a 
specific character 56a to activate selected segments of the character 
through traces 60 and 62, which are also applied to the substrate, 
proximate its front surface. A first fore plane signal FP1, conveyed 
through trace 60 is coupled to a fore plane electrode 56c, and through 
interconnecting traces 68, 70, and 72, this signal is also coupled to fore 
plane electrodes 56d, 56e, and 56f, respectively. Similarly, the voltage 
comprising the other of the fore plane signals, FP2, is conveyed to fore 
plane electrode 56a through trace 62 and is coupled by interconnecting 
traces 64 and 66 to fore plane electrodes 56b and 56g. 
As shown in FIG. 4, traces 74, 76, 78, and 80 convey back plane signals, 
BP1-BP4, respectively, to corresponding selected back plane electrodes 
58a-58g. These back plane electrodes are disposed proximate the rear 
surface of the substrate, generally underlying the corresponding fore 
plane electrodes 56a-56g. Consequently, when the appropriate voltage is 
applied between fore plane electrode 56a and back plane electrode 58a, the 
segment represented by the shape of the two electrodes is visually 
activated as the liquid crystal region disposed between the two electrodes 
experiences a change in optical characteristic so that it is visually 
perceptible. 
Back plane signal BP1 conveyed over trace 74, is coupled to back plane 
electrode 58b. The same voltage is conveyed through an interconnecting 
trace 84 to back plane electrode 58f. Assuming that additional characters 
exist in display 50, the voltage comprising back plane signal BP1 is 
conveyed to the corresponding back plane electrode of the next character 
(not shown) through a trace 74'. Back plane signal BP2 is coupled through 
trace 76 to back plane electrode 58c, and through an interconnecting trace 
82, to back plane electrode 58a. A trace 76' conveys back plane signal BP2 
to the next character, if used. Back plane signal BP3 is conveyed through 
a trace 78 to back plane electrode 58g and then through an interconnecting 
trace 86, to back plane electrode 58e; a trace 78' conveys the signal to 
the next character. Finally, back plane signal BP4 is conveyed through 
trace 80 to back plane electrode 58d and is applied to the next character, 
if used, through a trace 80'. 
If four display characters are used in display 50, the same four back plane 
signals BP1-BP4 are simultaneously applied to each of the display 
characters, however, fore plane signals FP1 and FP2 are applied to only 
one character at a time. The four back plane signals are changed as 
appropriate to represent successive characters on the display, and the 
appropriate fore plane signals for the character in question are then 
activated. Each of the numerals preferably comprising the four digits 
indicated on display 50 are repetitively visually activated at a 
multiplexing speed sufficiently fast so that the observer sees the four 
digits as if each display character were continuously indicated. The 
persistence of the LCD regions causes the display character to be visible 
briefly after the voltages activating each of the segments comprising the 
character have been deactivated, virtually eliminating any flicker due to 
the time multiplexing of the display characters. 
The advantage of selecting the specific fore plane electrodes 56 that are 
coupled to fore plane signals FP1 and FP2 and the specific back plane 
electrodes 58 that are coupled to the back plane signals BP1-BP4 is 
indicated in Table 1. 
TABLE 1 
______________________________________ 
Failure Analysis 
In- Desired Characters vs. 
activated Indicated Characters 
Failure Segments 0 1 2 3 4 5 6 7 8 9 
______________________________________ 
BP1 Open 
d 
##STR1## 
##STR2## 
##STR3## 
##STR4## 
##STR5## 
##STR6## 
##STR7## 
##STR8## 
##STR9## 
##STR10## 
BP2 Open 
f, b 
##STR11## 
##STR12## 
##STR13## 
##STR14## 
##STR15## 
##STR16## 
##STR17## 
##STR18## 
##STR19## 
##STR20## 
BP3 Open 
e, g 
##STR21## 
##STR22## 
##STR23## 
##STR24## 
##STR25## 
##STR26## 
##STR27## 
##STR28## 
##STR29## 
##STR30## 
BP4 Open 
a, c 
##STR31## 
##STR32## 
##STR33## 
##STR34## 
##STR35## 
##STR36## 
##STR37## 
##STR38## 
##STR39## 
##STR40## 
FP1 Open 
f, c, e, d 
##STR41## 
##STR42## 
##STR43## 
##STR44## 
##STR45## 
##STR46## 
##STR47## 
##STR48## 
##STR49## 
##STR50## 
FP2 Open 
b, a, g 
##STR51## 
##STR52## 
##STR53## 
##STR54## 
##STR55## 
##STR56## 
##STR57## 
##STR58## 
##STR59## 
##STR60## 
______________________________________ 
As shown above in Table 1, failure of any one of the four back plane 
signals or the two fore plane signals causes one or more specific segments 
to be inactive, but the resulting displayed character is either the 
intended display character because the affected segment(s) are not 
involved in visually representing the intended display character, or, the 
resulting displayed character is not one of the digits 0-9 that is 
expected to be displayed. For example, if back plane signal BP2 is open, 
the segments corresponding to fore plane electrodes 56f and 56b (or back 
plane electrodes 58f and 58b) are inactive. Segment f is used in 
representing numbers 3, 4, 5, 6, 8, and 9; and segment b is used in 
representing numbers 1, 2, 4, 7, 8, and 9. With both segments f and b 
inactive, however, the resulting visually perceptible character is not any 
number in the set of numbers from 0 through 9, inclusive. The same type of 
failure analysis is applied to each of the other back plane and fore plane 
signals that can be open. 
Thus, by carefully selecting the segments energized by specific ones of the 
fore plane and back plane signals, display 50 has been made incapable of 
representing an incorrect number from the set of ten numeric display 
characters expected to be displayed thereon. Consequently, an operator 
seeing a character that is not among the expected set of numeric 
characters 0 through 9 is alerted that one or more of the segments has 
failed due to an open back plane or fore plane trace or some other failure 
that has interrupted the fore plane or back plane signal for the display 
character. Note however, that, for example, if signal BP1 is open so that 
segment d is inactive, the digits 1, 4, 7, and 9 are still properly 
displayed, because segment d is not used in visually representing these 
display characters. 
Another type of failure that can occur may leave a segment continually 
energized. The present invention will not necessarily warn of such a 
failure unless the segment that is continuously energized appears in a 
position where it does not visually represent a segment of one of the 
characters in the set of characters normally expected to be displayed when 
other segments are energized. In order to handle a failure of the type in 
which a segment is always active, the method used in the above-referenced 
copending application in which redundant pairs of segments are employed 
can be used, in conjunction with the present technique. By combining the 
present invention with display characters in which at least some of the 
segments are redundant, it is possible to provide both a visual indication 
that one segment has failed and to still indicate the intended display 
character. 
One advantage of the present invention is that it requires relatively few 
traces to convey the fore plane and back plane signals of the display. For 
example, as shown in FIG. 5, a relatively complex LCD panel 90 comprising 
a substrate 96 includes four digits 92 having segments coupled to fore 
plane and back plane signals in accordance with the present invention. For 
purposes of simplifying the figure, edge connect terminals, fore plane and 
back plane traces, and fore plane and back plane electrodes are not 
discretely shown in FIG. 5; however, given the above disclosure and FIGS. 
3 and 4, these details should be evident to those of ordinary skill in the 
art. 
Four digits 92 on LCD panel 90 are used to display information that is 
relatively critical in nature and should not be misread. In addition, LCD 
panel 90 includes a plurality of additional alphanumeric and graphic 
display areas 94 on which information that is less critical is indicated. 
Thus, four digits 92 are combined with more conventional LCD character and 
graphic information in an integrated manner. 
While the present invention has been disclosed in connection with the 
preferred embodiment in which the display is implemented using LCDs, those 
of ordinary skill in the art will appreciate that it can also be 
implemented using other display technologies, such as LEDs. It should be 
emphasized that the present invention is also usable with displays having 
more or less than seven segments for each character, to provide visual 
evidence that a segment has not been properly activated due to a failure. 
By selectively distributing the control signals between seven or more 
segments of a display, failures that occur because of an open circuit in 
the control signals for the selected segments become apparent when the 
character visually indicated is not one of the characters in a predefined 
set of characters, e.g., not one of the numbers in the set of numbers from 
0-9 or not one of the alpha characters in the set of letters A-Z. The same 
approach can thus be extended to sets of other display characters so that 
a failure of the control signals supplied to actuate a segment becomes 
immediately evident when the indicated character does not match any of 
those in the selected set of characters. These and other modifications to 
the preferred embodiment disclosed above will be apparent to those of 
ordinary skill in the art. Accordingly, it is not intended that the 
present invention in any way be limited by the disclosure, but instead 
that its scope be determined by reference to the claims that follow.