Abstract:
A display system comprises a first image memory storing first image data, a second image memory storing second image data operable independently from the first image memory, a display selection and combination circuit, and two display units. The display selection and combination circuit connected to the first and second image memories is formed such that each of the display units displays a combination of data from the image memories according to a display request. Normally, the display units are cathode ray tubes or liquid crystal displays. Usually, one of the first and second image memories stores character information, such as letters and numerals, and the other stores graphic information, such as figures and curves.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a display system capable of selectively displaying graphic display data and character display data and a combination of the two types of data on two display units. 
     2. Description of the Prior Art 
     In recent years, display systems have become increasingly advanced. Even personal computers are now being provided with display systems capable of the display of not only character information, such as letters and numerals, but also graphic information, such as figures, curves, circle graphs, and histograms. 
     The display of an actual graph requires the combination of graphic and character information. In the prior art, this has been achieved by storing the combined graphic display data and character display data in an image memory having a capacity sufficient for a display picture and by displaying the data on a single cathode ray tube (CRT). The prior art, however, has suffered from the problem of inadvertent deletion of characters due to interference from the graphic display when adjusting the display, for example, changing colors, enlarging figures, and changing positions, and the problem of an overly crowded display picture when there are too many characters and symbols for graph notations. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a display system able to display graphic data and character data individually or in combination, preventing inadvertent deletion of character data due to interference from graphic data or vice versa, and presenting a visually simple display picture. 
     In accordance with the present invention there is provided a display system which includes a first image memory storing first image data; a second image memory storing second image data; and a display selection and combination device connected to the first and second image memories. The display selection and combination device has two output terminals connectable to two independent display units, through which the display selection and combination device outputs two display signals selected from the first and second image data in response to a control signal or at least one display signal mixing the first and second image data in response to the control signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other object and features of the present invention will be clear from the following detailed description with reference to the accompanying drawings, in which: 
     FIG. 1 is a block diagram of an embodiment of a display system according to the present invention; 
     FIGS. 2(a) , 2(b), and 2(c) illustrate displays on display units in the display system of FIG. 1; 
     FIG. 3 is a circuit diagram of the connection between the control register and the signal mixing circuits illustrated in FIG. 1 and used in a color CRT display system; 
     FIG. 4 is a detailed circuit diagram of an example of the signal mixing circuits illustrated in FIG. 3; and 
     FIG. 5 is a circuit diagram of another embodiment of the signal mixing circuits illustrated in FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in FIG. 1, a display system in accordance with the present invention comprises a central processor unit (CPU) 1, a timing controller (CNT) 2, a graphic image memory (GM) 3, a character image memory (CM) 4, a character generator (CG) 5, a display selection and combination circuit including a control register (REG) 6 and a display selection circuit (SW) 7, CRT display units CRT1 and CRT2, and video signal converting circuits (VD) 8A and 8B. The above elements are connected to each other by means of a data bus DB, address buses AB1 and AB2, and other buses illustrated in FIG. 1. In this embodiment, data to be displayed are first processed in the CPU. Graphic data, such as figures are stored in the graphic image memory 3 through the timing controller 2. Character data, such as letters, symbols, and numerals are stored in the character image memory 4. Both image memories store the above data in the form of a dot-pattern. 
     It should be noted that, in the case of character data, only codes corresponding to the characters are stored in the character image memory 4. The above codes are transformed into the dot-pattern signals corresponding to the actual characters recorded in the character generator 5, for example, a read only memory (ROM). The data stored in the image memories 3 and 4 are serially output, in response to addresses on the address buses AB1 and AB2 supplied by the timing controller 2, to the display selection circuit 7 and finally displayed on the CRT display unit CRT1 and/or CRT2 through the video signal converting circuits 8A and/or 8B. 
     The display content can be selected or changed by keyboard operation or the like. In response to the keyboard operation, the CPU 1 sends a selection signal to the control register 6 through the data bus DB or a display command to the timing controller 2. The storage operation of the image memories 3 and 4 may occur at the desired times, such as at predetermined intervals, at the time of data changes, or only upon request. Therefore, the display may be updated at any desired time. 
     A feature of the present invention is that any display can be obtained on the CRT displays by changing the selection data in the control register 6 provided by the CPU 1. This will be made clear through the following explanation. 
     In this embodiment, the control register 6 is a four-bit register and, therefore, can be used for the selection of 16 combinations of displays in accordance with to its control status, as listed in Table 1. In Table 1, &#34;G. ACT&#34; and &#34;C. ACT&#34; respectively stand for a graphic display activate command and a character display activate command. When G. ACT is &#34;1&#34;, a graphic display may be provided to the corresponding CRT. On the other hand, when C. ACT is &#34;1&#34;, a character display may be provided to the corresponding CRT. In Table 1, furthermore, GRAP. designates a graphic display, CHR. a character display, and GRAP. +CHR. a combined graphic and character display. For example, when the control status is &#34;1001&#34;, a graphic display can be presented on the CRT display unit CRT1 and a character display on CRT2. When the status is &#34;1101&#34;, a combined graphic and character display can be presented on the CRT display unit CRT1 and a character display on the CRT display unit CRT2. 
     
                       TABLE 1______________________________________Control signal statusCRT 1 CRT 1   CRT 2   CRT 2G.    C.      G.      C.ACT   ACT     ACT     ACT   Display status______________________________________0     0       0       0     CRT 1  No display                       CRT 2  No display1     0       0       0     CRT 1  GRAP.                       CRT 2  No display0     1       0       0     CRT 1  CHR.                       CRT 2  No display1     1       0       0     CRT 1  GRAP. + CHR.                       CRT 2  No display0     0       1       0     CRT 1  No display                       CRT 2  GRAP.1     0       1       0     CRT 1  GRAP.                       CRT 2  GRAP.0     1       1       0     CRT 1  CHR.                       CRT 2  GRAP.1     1       1       0     CRT 1  GRAP. + CHR.                       CRT 2  GRAP.0     0       0       1     CRT 1  No display                       CRT 2  CHR.1     0       0       1     CRT 1  GRAP.                       CRT 2  CHR.0     1       0       1     CRT 1  CHR.                       CRT 2  CHR.1     1       0       1     CRT 1  GRAP. + CHR.                       CRT 2  CHR.0     0       1       1     CRT 1  No display                       CRT 2  GRAP. + CHR.1     0       1       1     CRT 1  GRAP.                       CRT 2  GRAP. + CHR.0     1       1       1     CRT 1  CHR.                       CRT 2  GRAP. + CHR.1     1       1       1     CRT 1  GRAP. + CHR.                       CRT 2  GRAP. + CHR.______________________________________ 
    
     Below is a simplified example where the control status is &#34;1101&#34; and CRT1 and CRT2 are color CRT&#39;s having the three fundamental colors of red(R), green(G), and blue(B). FIG. 2 (a) illustrates graphic data comprising a circle graph on a CRT display unit, FIG. 2 (b) is character data comprising graph notations; FIG. 2(c) a combination of the two. The indications (B), (R), (G) and (Y) in the circle graph in FIGS. 2a and 2c represent that the segments are colored blue, red, green, and yellow, respectively. 
     FIG. 3 is a circuit diagram of the connection between the control register 6 and the display selection circuit 7. In FIG. 3, the display selection and combination circuit 7 consists of two signal mixing circuits MXl and MX2, the outputs of which are connected to the CRT display unit CRT1 and CRT2 through the video signal converting circuits 8A and 8B, respectively. The signals CRT1.G.ACT and CRT1.C.ACT and the signals CRT2.G.ACT and CRT2.C.ACT from the control register 6 are supplied to the input terminals of the signal mixing circuits MX1 and MX2, respectively. Three image signals G. BLUE, G. RED, and G. GREEN for the graphics output from the graphic image memory 3 and three image signals C. BLUE, C. RED, and C. GREEN for the characters output from the character image memory 4 are also supplied to the input terminals of the circuits MX1 and MX2. 
     FIG. 4 is a detailed circuit diagram of an example of the signal mixing circuits MX1 and MX2. In FIG. 4, gates G1, G5, and G9 represent AND gates; gates G2, G6, and GlO NAND gates; gates G3, G7, and G11 inverters; and gates G4, G8, and G12 OR gates. Gates G13 through G24 in the signal mixing circuit MX2 use the same symbols as the above. 
     Since the control status is &#34;1101&#34;, CRT1.G.ACT, CRT1.C.ACT and CRT2.C.ACT are &#34;1&#34; and CRT2.G.ACT is &#34;0&#34;. In this case, C. BLUE applied to the NAND gate G2 in the circuit MX1 is inverted by CRT1.C.ACT being equal to &#34;1&#34;, inverted again at the inverter G3, and applied to one of the input terminals of the OR gate G4. In addition, since CRT1.G.ACT is &#34;1&#34;, G. BLUE applied to the AND gate G1 is output to another input terminal of the OR gate G4 when the output of the NAND gate G2 is not a logical &#34;1&#34;, i.e., C. BLUE is not &#34;1&#34;. Therefore, the signal CRT1 BLUE at the output of the OR gate G4 is the combination of the above-mentioned G. BLUE and C. BLUE. When G. BLUE and C. BLUE are both &#34;1&#34;, CRT1 BLUE represents C. BLUE rather than G. BLUE by operation of gates G1, G2 and G3. In other words, the direct path of C. BLUE through the inverter G3 acts as a priority selection of the character display which prevents deletion of character information by the graphic display when the graphic display interferes with the character display portions. CRT1 RED and CRT1 GREEN are output by the circuit MX1 after being formed in the same manner as CRT1 BLUE. 
     On the other hand, since CRT2.G.ACT is &#34;0&#34; and CRT2.C.ACT is &#34;1&#34;, G. RED, G. BLUE, and G. GREEN are inhibited at the AND gates G13, G17 and G21, while C. RED, C. BLUE and C. GREEN are output through OR the OR gates G16, G20 and G24 to form CRT2 RED, CRT2 BLUE, and CRT2 GREEN, respectively. 
     As mentioned above with reference to the drawings, when the control status is &#34;1101&#34;, the combined display illustrated in FIG. 2(c) is presented on the color CRT1, and the character display illustrated in FIG. 2(b) is presented on the color CRT2. The combined display in FIG. 2(c) is a mixture of the graphic display in FIG. 2(a) and the character display shown in FIG. 2(b). but the character display dots overlay the graphic display dots where both exist in the same position on the CRT. 
     As can be easily understood, various combinations of displays on the CRT1 and CRT2 are possible, as indicated in Table 1. 
     In the above explanation, the CRT display units CRT1 and CRT2 were both color CRTs. They may, however, be both monochrome CRT display units or any combination of the same, as listed in Table 2. 
     
                       TABLE 2______________________________________CRT 1               CRT 2______________________________________Color               ColorColor               MonochromeMonochrome          ColorMonochrome          Monochrome______________________________________ 
    
     When the CRT display units CRT1 and CRT2 are both monochrome, simpler signal mixing circuits as illustrated in FIG. 5 can be used. In FIG. 5, the symbols which are the same as those used in FIG. 4 designate the same type of gates as in FIG. 4. 
     In the embodiment set forth with reference to FIG. 1, a graphic image memory and character image memory were used. The memories may, however, be both graphic image memories or both character image memories, as listed in Table 3. 
     
                       TABLE 3______________________________________First image memory Second image memory______________________________________Graphic            CharacterGraphic            GraphicCharacter          GraphicCharacter          Character______________________________________ 
    
     According to the signal mixing circuits illustrated in FIGS. 4 and 5, the character display is given priority over the graphic display by means of the priority selection element, i.e., the inverter G3, when the displays interfere with each other on a display unit. The graphic display, however, may be given priority over the character display by a minor change in the above circuits. 
     According to the present invention, when a person displays information, such as a distribution of sales, to a third person, such as a manager, he requests a display used for selecting the control command on CRT2 by using a keyboard, then requests a graphic display on CRT1 through the display on CRT2. Accordingly, the third person can easily read the desired information without the control data used by the initiator of the display. In addition, the program execution can be displayed on CRT1 and the program list displayed on CRT2. The CRT display units CRT1 and CRT2 can be arranged in any position, for example, can be arranged near each other or apart from each other. 
     The display system according to the present invention is not limited to CRT display units, but can be applied to any display devices, such as liquid crystal devices and plasma display devices.