Patent GB-1474756-A (GB)
Title: Display apparatus
IPC: G09G5/34
Filed: 1974-08-07
Abstract: 1474756 Cathode-ray tube displays TELETYPE CORP 7 Aug 1974 [13 Aug 1974] 34721/74 Heading H4T In a cathode-ray tube character display system in which a memory, which stores character display data, has a capacity exceeding the display capacity of the tube screen, address means selects for display a segment of the stored data and means responsive to the address means causes a visual indication representative of the selected segment to be displayed. Thus, when the display is "scrolled" vertically over the screen (upwardly or downwardly) the operator is provided with a visual indicator related to the particular portion of the total stored data being displayed. The described embodiment is concerned with a display system in which the tube screen is capable of displaying only a total of twenty-four rows of characters but in which the memory stores display data for seventy-two rows of characters. The first, twenty fifth and forty ninth rows which are the first rows of three respective segments each of twenty four rows are therefore visually identified on the display. The characters are produced in a 7 Î 9 dot matrix form within a character space of 9 x 14 dot position as shown in Fig. 1 in respect of the character E by means of the circuitry of Fig. 1 and Fig. 2. In general this circuitry operates to intensity modulate the tube beam under the control of the output 63 of a Dot Generator 28 at positions determined by a Parallel To Serial Shift Register 62 responsive to the output of a Font Decoder 56 receiving character data from a Random Access Memory 20 the actual character line being displayed being determined by a Memory Address Row Counter 66 operator controlled via a Scroll Counter 78. The horizontal trace period occupies the time (as determined by dot generator 28) of display of eighty characters and a twenty character interval is allowed for horizontal re-trace. During the retrace period a ninetynine count AND gate 40 responsive to the count nine outputs of two serial connected divide by ten circuits in a Scan Character Counter 32 supplies an output on line 108 (equivalent to a beam position at the far left-hand side of the screen and beyond the normal display area) which output conditions AND gates 102, 104 and 106 to supply outputs on the simultaneous occurrence of certain scan line (from Scan Line Decoder 110) and character row numbers. The row numbers are derived from a Row Decoder 88 which supplies row one and row twenty five outputs to respective OR gates 96, 98 and row forty nine output to both OR gates. Thus, during the simultaneous occurrence of a row 1 address output, a scan line 6 output and the ninety nine output, AND gate 106 supplies an output to OR gate 118 which via diodes 120, 122 causes bright-up pulses at line 6 and columns 6 and 7 at which position the scan is non-linear so that the dots illuminated by the pulses merge to produce a single elongated dot as shown at 10, Fig. 1, which dot is indicative of the first segment stored in the memory 20. In a similar manner dots 12, 12 indicate the second segment and dots 14, 14, 14, indicate the third segment are produced.