Patent GB-1388753-A (GB)
Title: Circuit arrangement for a line raster video display instrument  controlled by digital codes
IPC: G09G1/14
Filed: 1972-03-27
Abstract: 1388753 Cathode-ray tube displays LICENTIA PATENT-VERWALTUNGS GmbH 27 March 1972 [26 March 1971] 14195/72 Heading H4T In a cathode-ray tube display arrangement in which a character generator 3 in response to character codes drived from a recirculating shift register 1 via a delivery register 2 and control register 5 supplies, in parallel to a shift register 9, the blank = O, unblank = L pattern which, when shifted out serially to an OR gate 6, provides the beam unblanking pattern for successive lines (determined by a line counter 7) of a raster to produce a required character, the latter is produced within a "window" area (which provides for inter-character and/or interline spacing) in which, instead of characters, or in addition to characters, horizontal and vertical line traces may be produced by means of special codes (in register 1) to which the character generator 3 does not respond. In the former case, i.e. instead of characters, the codes are "recognized" by detectors 16, 23 respectively responding to code BHL = "begin" and code EHL = "end" horizontal line and by detector 11 responding to code VL = vertical line, whilst in the latter case, i.e. in addition to characters, the code of the latter is extended by one bit (position 13 in register 5) which is "recognized" by "detector" (= AND gate) 15. In the general operation, code data is transferred from register 2 to register 5 by a character pulse TZ(1) and if character data, which comprises five bits, is processed by character generator 3 and passed, on the occurrence of the next character pulse TZ(2), (which also loads the next code into register 5) to the shift register 9. Each pulse TZ starts a train of shift pulses TS (which determine the image spot times) which shift the contents of register 9 serially into OR gate 6 the output of which provides the unblank pulses for the tube beam. During this shifting the pattern O = blank and L = unblank is also shifted towards the output and when the pattern in register 9 = OLLLLLLL a "detector" 10 responds and clears the register and stops the pulses TS. In this mode of operation, if non-character data is included (i.e. for tracing lines) the additional bit in position 13 of register 5, if an L = unblank (O = blank being ineffective) is entered via gate 15 into position 7 in register 9 and when this emerges via OR gate 6 it results in the production of an image spot which is repeated in every line of the "window" (line counter 7 being connected to character generator 3 by means not shown) so that a vertical line is produced at the right hand side of the window. (beside the character, if character data was also present). If non-character, only, is present, the "detectors" 11 or 16 and 23 respond, 11 responding to the vertical line code VL and 16 and 23 to "begin horizontal line" = BHL and "end horizontal line" = EHL respectively. Considering the operation for vertical line tracing the output of 11 is supplied on the occurrence of pulse TZ (2) via AND gate 12 to register position (3) which it changes over (from "O") to L ( = unblank) so that on the third shift pulse TS (the train being started by the TZ(2) pulse) an unblank pulse is supplied to the display tube to produce an image spot this action being repeated for every line of the "window" so that a vertical line extending over the whole height of the "window" is produced. This line may be produced with double thickness by connecting the output of AND gate 12 to two adjacent positions, e.g. (3) and (4) and a double line may be produced by connecting the output of 12 to two spaced positions such as (3) and (5). For tracing horizontal lines "detector" 16 responds to the "begin horizontal line" code BHL, entered into control register 5 by pulse TZ(1), causes bi-stable flip-flop 17 to be set by the next pulse TZ(2) which causes a bi-stable flip-flop 18 to be set by the next pulse TZ(3). The set output of 18 is then supplied to AND gate 19 which receives a second input from a line counter "detector" 20 which produces an output when the state of line counter 7 corresponds to a line (e.g. line 21) located in zones below those occupied by characters and receives a third input from a "detector" 21 which responds at image spot period 3 to the pattern OOOOOOLL (a "L" = unblank bit for a vertical line if present having being shifted out to OR gate 6). The resulting output from gate 19 then sets flip-flop 22 which produces an unblanking pulse for display control via the second input of OR gate 16. This remains effective until flip-flop 22 is reset which may be done in response to the detection by "detector" 23 of an "and horizontal line" EHL code which forms one input to an AND gate 24 having a second input from line counter detector 20 (responsive to line 21) and a third input from a "detector" 25 also responsive to the pattern OOOOOOLL in register 9, gate 24 providing an input to OR gate 26. This cycle of operation then continues for each line to which detector 20 is responsive and a succession of horizontal lines is thus produced. Since the flip-flops 17 and 18 introduce a delay of two character window periods plus the three image dot periods due to shift register 9 the horizontal underlining occur under characters the data for which is subsequently entered into character generator 3 since the latter required one character period (TZ pulse period) before supplying an output. Instead of resetting flip-flop 22 by the code EHL AND gate 27 may be supplied with a line end pulse ZE together with a "L" pulse. If resetting is not required an "O" pulse replaces the "L" pulse. Oblique lines may be produced by displacing in steps the connection of AND gate 12 to the different digit positions of register 9 by means of gates controlled by the line counter 7.