1. Field of the Invention
The present invention relates to a dot-matrix type display device capable of performing large-scale display.
2. Description of the Related Art
A display device using field-effect liquid crystal is effective as a large-scale display device. Particularly, a display device of super-twisted nematic (STN) liquid crystal in which a structure having a large twist angle is given to the liquid crystal is known as a display device using a time-division driving characteristic. The principle of time-division driving in the above dot-matrix type display device will be described hereunder. As shown in FIG. 1, the dot-matrix type display device is provided with linear Y electrodes (signal electrodes) formed on a lower electrode substrate (not shown) and linear X electrodes (scanning electrodes) formed on an upper electrode substrate (not shown), so that display of characters, graphics, etc., is performed through an electric field applied to a field-effect liquid crystal sandwiched between the X and Y electrodes. That is, the display is performed by selective turning-on (or turning-off) of the liquid crystal at points of intersection between the X and Y electrodes.
Scanning is sequentially and repeatedly performed on the scanning electrodes X.sub.1, X.sub.2, . . . X.sub.n in FIG. 1 sequentially line by line to thereby perform timedivision driving. When, for example, the scanning electrode X.sub.3 is selected, display signals from the signal electrodes Y.sub.1, Y.sub.2, . . . Y.sub.n are applied to all pixels Z.sub.1, Z.sub.2, . . . Z.sub.n on the electrode simultaneously so as to perform selection (or non-selection). Points of intersection can be thus turned on (or turned off) by combinations of voltage pulses applied to the scanning electrodes X and to the signal electrodes Y. In this case, the number of scanning electrodes X corresponds to the number of time divisions (the number of scanning lines or the number of display lines).
The number of display lines is, however, naturally limited due to the characteristic of the liquid crystal. For the purpose of increasing the number of display lines, stripe electrodes (X, Y) may be separated into two groups to form two systems, which makes it possible to increase the number of display lines by twice, as typically shown in FIG. 2. For example, 800 display lines can be provided even in the case of a liquid crystal device having the number of scanning lines limited to 400. It is, however, apparent from FIG. 2 that the signal electrodes Y are connected to external driving circuits 2, 2'and the scanning electrodes X are connected to the external scanning circuits 3, 3'through end portions of the substrate 1. Accordingly, it has been impossible to separate the signal electrodes into three or more groups.
As a measure to solve the aforementioned problem there has been proposed a method of separating the scanning electrodes into three or more groups in which holes are formed so as to substantially perpendicularly pass through the substrate on which the signal electrodes are formed, and in which electrical conductive leads are inserted into the through holes respectively, connected to the signal electrodes and led out to the back surface side of the substrate.
According to the proposed method, the signal electrodes can be separated into a desired number of groups, so that the total number of display lines can be increased. In this method, however, occurrence of irregular display called "crosstalk" cannot be suppressed perfectly. The above "crosstalk" means a phenomenon that the display contrast ratio of the whole display screen is reduced because of occurrence of imperfect turning-on at non-display points (non-selective pixels) not to be displayed. This is a weak point of the dot-matrix display device. When, for example, a certain driving voltage is set, the liquid crystal at non-selection points responds, even through imperfectly, to the driving voltage to increase transmittivity because a bias voltage is applied also to the non-selection points. That is, the difference between the transmittivity in this state and the transmittivity in the case of attaining a perfect non-selection state is called "crosstalk". The crosstalk phenomenon changes depending on the number of time divisions, the electrode resistance and the connection resistance. Generally, crosstalk is apt to occur as the number of time divisions increases, and it becomes large as the sharpness of the threshold characteristics of the voltage and transmittivity becomes low as the electrode resistance and the connection resistance become large.