GB2080593A

GB2080593A - Monitoring circuit for checking the connections of liquid crystal displays

Info

Publication number: GB2080593A
Application number: GB8120445A
Authority: GB
Original assignee: Kienzle Apparate GmbH
Current assignee: Digital Kienzle Computersysteme GmbH and Co KG
Priority date: 1980-07-16
Filing date: 1981-07-02
Publication date: 1982-02-03
Also published as: FR2487099A1; DE3026848C2; SE8104364L; DE3026848A1; FR2487099B1; ATA283281A; GB2080593B; SE443060B; AT382256B

Abstract

In a monitoring circuit for checking the connections of liquid crystal displays and their segment drivers, each segment (3) is connected, e.g. via a conducting elastomer (4), through first lead (5) directly to the driver circuit for normal control and simultaneously via second lead (6) to a checking condenser (9). When segment (11) is activated the condenser (9) in loop (i) is charged and discharged by the control module (8), e.g. a microprocessor in multiplex with the normal activation signals, the condenser current being apparent as a potential drop across resistor 10 for comparison with required values. The circuit may be used in a liquid crystal display in a taximeter. <IMAGE>

Description

SPECIFICATION Monitoring circuit for checking the connections of liquid crystal displays The invention refers to a monitoring circuit for checking the connections of liquid crystal displays and the corresponding segment drivers of a control circuitry having for instance elastic interconnectors connecting at least two separate leads for split contacting the segment electrodes.

By 7-segment-display devices it is in many cases not possible to check whether the indication is correct or incorrect. Surely there will be a selection of segments which when they are dead result in a mutilated figure to be indicated so that in such cases visually a malfunction can be recognized. However, apart from the recognizable wrong indications due to additionally activating or non-activating a segment figures can be displayed which considerably deviate from the originally desired value position of for instance measured values so that a wrong indication is given by the display which is not recognizable as such.

A Amalfunction with respect to the activation of one or several segments will not only result in a wrong figure indication but such a malfunction is absolutely unpermissible when the display is applied for instance for indicating the price of goods to be sold. In this connection there are legal prescriptions that a malfunction of the display must not remain undiscovered.

An arrangement for giving a warning signal when the 7-segment display devices does not function properly has for instance become known through U.S. patent 3943 500. The device shown in this patent refers to a 7-segment display device for quantity and price data at a fuel pump. The expenditure for producing such a warning signal is high with the shown solution. With the known device usable signals can only be derived from active displays by measuring currents in the control connectors of the segments, that means essentially displays prnduc ing light themselves such as for instance light emitting displays. For this reason the known method is right from the beginning not to be used for producing a malfunction signal in checking liquid crystal displays.In such so-called passive displays as the liquid crystal displays the formation of the symbol to be indicated is effected by modulating outside light in a nematic crystal substance due to applying an outside electrical field. Such liquid crystal displays are generally considered to be advantageous because they draw very little current.

However, as a consequence checking the function of the segment by measuring the currents in such segments by the known method does not lead to any useful result.

When analysing the possible causes of malfunc tions in liquid crystal displays it is to be recognized that any deficiencies at the part such as a broken glass, wear out, an untight crystal cell or such like always result in that all segments of such display elements are effected by such deficiencies, that rnnnrin###hI# For visually non-recognizable display deficiencies the cause is almost exclusively to be found in the connecting leads for controlling and signal transmission. The causes which lead to a wrong indication may be found for instance in a defective driver module, in defective connecting leads, and especially in the interruption of individual connecting leads, and in a deficiency in contacting the display module or such like.Especially the last mentioned causes of malfunctions do not lead to any visually recognizable malfunction in the display. For obtaining the permission of the administration for devices to be calibrated such as taximeters, price and volume counters for fuels, transmission lines etc. as practical applications it is therefore an indispensible condition that visually non-recognizable malfunctions of the display must be supervised respectively signalized.

It is therefore the object of the present invention to make a monitoring circuit for checking the liquid crystal connectors and the corresponding segment drivers so that a display to be operated by multiplexing resp. its individual segments connecting leads and drivers may be automatically or optionally checked with the least possible expenditure.

According to the invention each segment electrode is directly connected by means of a first connector directly to the driver for regular call up and that simultaneously by means of a second connector forming an own current loop a separate connection to each segment is created and in that in each current loop a checking condensor is provided which when the corresponding segment is called up via the control circuitry in the multiplexing procedure is loaded and unloaded.

In a preferred embodiment the loading and unloading current for the checking condensor are connected to the same voltage supply as the segment drivers a resistor being connected within the unloading current loop to all segment connections to be included in the checking process at which resistor a voltage drop can be measured due to the unloading current.

In an advantageous manner in the circuit according to the invention the driver for regularly controlling the liquid crystal display can be additionally used as uncoupling circuit for the testing process. When deriving a checking signal the checking means, according to the invention the checking condensor, can be retained in the control cycle without any disadvantageous effect since by timing the checking process and the driving cycle in sequence there will be no interferences. The checking process is initiated independently of the normal driving control of the segments for instance by initiating it every 10 sec. or by allowing for a manual release by operation of a key or another suitable process within the control logic. The checking process is effected as a sequence of five steps and is advantageously controlled by a microprocessor.For realizing the checking circuit standardized modules such as for instance commercially available CMOS shift registers or latches may be used. The circuitry allows for a selective checking of individual segments resp. of their connectors, leads, and drivers. Due to the use of the driver modules also for the checking circuitry the expenses are greatly reduced. Also the arrangement of the corresponding leads can be very much simplified due to the corresponding arrangement of the checking condensors which are commonly connected to the voltage source.

In the following specification and the drawings an embodiment of the invention has been shown. In the drawings Figure 1 is a diagrammatic representation of the checking circuitry for individual segment connectors shown in section of a liquid crystal display, Figure 2 is a simplified representation of the checking circuitry in the loading phase of the checking condensors, Figure 3 is a simplified representation of the checking circuit in the unloading phase of the checking condensorforthe purpose of checking the function of the segment included in this current loop.

As a rule there are measured values or results of calculations available in a form as suited for electronic processing which finally have to be visually shown by means of a display in the form of decimal figures. In known manner these decimal figures are mostly transmitted as binary decimal coded values which by means of a decoder driver via seven control leads are indicated by means of for instance a 7-segment-display. By lighting individual segments in a corresponding configuration the figures "0" to "9" within one decimal may be represented.

In Figure 1 a section of the segment control of a liquid crystal display 1 (LCD) is shown in simplified manner. In a partial section from the liquid crystal display 1 on a glass plate 2 leads 3 for contacting the individual segments of the display modules are shown. The leads 3 are connected for instance by a conductive elastomer 4 or a similar interconnector with at least two separate leads 5, 6 of a circuit plate 15 which can simultaneously be the carrier. Lead 3 which is normally also called the segment electrode is therefore connected by a first lead 5 directly to a control lead 7 with a driver of a control module 8.

There are in known manner seven control leads 7 required for activating the 7 segments of a liquid crystal display element by means of a combination of selected segments to represent the figures from 0 to 9. Simultaneously by means of a second lead 6 the lead 3 of each segment is additionally contacted so that for each segment control an own current loop i is formed. In each current loop ithere is provided a checking condensor 9 which when checking the segment included in the current loop ivia the control module 8 in the multiplexing procedure is loaded and unloaded.The checking condensors 9 are commonly connected to a voltage source Vlss. As may be seen also from Figure 2 and 3 all loading current loops it and unloading current loops 1E are connected to a voltage source Vss, V'ss which is the same for all segment drivers. In a common part of the unloading current loop iE of the segment connectors 11 there is a resistor 10 at which a voltage drop may be measured by the unloading current. As a segment connnection 11 the electrical connection may be understood comprising the first lead 5 with the conductive elastomer 4, the segment itself resp.

the electrode 3, the conductive elastomer 4 to the second lead 6. The segment connection 11 is shown in Figure 2 and 3 in the form of a simplified equivalent circuit.

Figure 2 shows diagrammatically the connections between the checking circuit in the loading phase of the checking condensors 9. As may be seen as control module 8 CMOS elements are used which according to the invention are not only used for segment control but simultaneously also as checking circuitry so that the expenses for realizing the checking circuit are greatly reduced. The process of checking the function of the individual segment connections 11 may be subdivided into a sequence of five steps the control of such sequence being effected by a microprocessor 16 otherwise not explained in detail. The loading of all checking condensors 9 is effected as a first step as may be seen from Figure 2 by switching all driver-CMOStransistors 12 to VDD.As a second step the segment connection 11/1 chosen to be checked (see Figure 3) is also connected to Vss. The checking condensor 9/1 arranged in the corresponding current loop XE is unloaded via a CMOS-transistor 13 connected to Vss via a common lead 14 and finally via the resistor 10 to VSSB Due to the unloading of the checking condensor 9 there is in the corresponding unloading current loop 1E a current pulse declining in an e-function when the unloading circuit 1E is in order. In the third step of the checking sequence the unloading current within the declining time of the current pulse produces a corresponding voltage drop at the resistor 10.This voltage drop is the control criterion and its quality may be tested with respect to its admissible amplitude within the declining time. In a simple manner checking is possible for instance by a transistor in emitter circuit. In this phase of the checking the contacts are tested with respect to correct functioning and also simultaneously the checking condensorfor its proper functioning.

In a fourth step after the condensor unloading current has declined there is another checking of the resistor 10 for a voltage drop. For instance if there is a segment short circuit between the segments 11 and 11/1 (see Figure 3) such a state will lead to an irregular static voltage drop at resistor 10 thereby producing a signal serving as fault recognition. 5 When there is a short circuit connection between the segments 11 and 11/1 the unloading circuit loop 1E is simultaneously statically at positive voltage via the connection to the activated driver CMOS-transistor 12 of the neighbouring short circuited segment 11.

For this case of short circuiting as shown in Figure 3 there is a current in the current loop which causes a static voltage drop at the resistor 10 for a defined fault recognition.

The fifth step finally of the checking process consists in that checked segment connection 11/1 according to the example of Figure 3 is again connected to VDD. This serves the purpose that when the other segment connections 11 are submitted to continuous checking there may be no parallel connection of checking condensors 9. Through the application of CMOS elements for control and checking the field effect transistors 12,13 are a nearly ideal switch combination for relatively small currents. In their form as standard CMOS-modules there is also the possibility of individually activating a pairoftransistors 12. 13.

Claims

1. A monitoring circuit for checking the connections of liquid crystal displays and the corresponding segment drivers of a control circuitry including for instance elastic interconnectors connecting at least two separate leads for split contacting the segment electrodes characterized in that each segment electrode (3) is connected by means of a first connector (5) directly to the driver for regular call up and that simultaneously by means of a second connector (6) forming an own current loop (i) a separate connection to each segment is created, in that in each current loop (i) a checking condensor (9) is arranged which when the segment (11) in this circuit loop (i) is called up is loaded and unloaded by multiplexing via the control circuitry (8).

2. A monitoring circuit according to Claim 1, characterized in that a loading circuit loop (ii) and an unloading circuit loop (iE) for the checking condensor (9) are connected to the same voltage supply (V55, V'ss) as all segment drivers and that a resistor (10) is arranged in the deloading circuit (iE) of all segment connectors (11) to be checked at which resistor a voltage drop is to be measured when the condensor is unloaded.

3. A monitoring circuit according to Claims 1 and 2, characterized in that the circuit loop (i) with the checking condensor (9) for regularly calling up the liquid crystal segments is part of the control circuit (8, 16) and may be switched over to become an uncoupling circuit for the checking.

4. A monitoring circuit according to Claims 1 to 3, characterized in that a checking procedure via the uncoupling circuit is carried through separate in time from the normal multiplexing of the segments.

5. A monitoring circuit according to Claims 1 and 4, characterized in that for controlling the individual steps of a checking procedure a microprocessor (16) is used.

6. A monitoring circuit substantially as described herein with reference to and as illustrated by the accompanying drawings.