Patent GB-1528021-A (GB)
Title: Graphical display apparatus
IPC: G09G5/42
Filed: 1975-02-27
Abstract: 1528021 Graphical display generators INTERNATIONAL COMPUTERS Ltd 24 Feb 1976 [27 Feb 1975] 8152/75 Heading H4T A graphic display system uses a CRT1 scanned by a beam-displacement raster, first storage means 2 for storing data words of a first type representing incremental beam displacement and of a second type representing differences between one raster line and its successor; means 10, 14 for establishing a raster line from words of the first type; second storage means 26 for storing a line of data; means 5, 17 responsive to receipt of a word of the second type for inhibiting supply of data derived from words of the first type into the store 26; and means, also responsive to second-type words for modifying the content of the store 26 in order to establish the succeeding raster line and for feeding said raster line to the store 26 for transfer to the CRT; and means responsive to completion of the modification of the content of store 26 for removing the inhibition. The means for modifying the content of store 26 includes a shift register 38, Fig. 5, having AND gates 41, 42, 43 connected to respective stages A, B, C of the register one input of each gate being connected to the output of the store 26, and a second input being connected via lines 45 to code detector 5 for receipt of a certain one of a set of control signals R, L, SA, SB derived from words of the second type. The output of the shift register 38 is connected by line 39 to the input of the line store 26. It is also connected to supply clocking signals to a counter 46 which is set to a reset count level by the set of control signals R, L, SA, SB, whereby the counter is clocked in synchronism with the shift register 38, the output of the counter providing over line 47 a signal to the decoder 5 for removing the inhibition. The decoder 5, Fig. 2, decodes predetermined ones ("unterminated strings") of first-type words to produce data signals representing beam displacement increments and a decrementing counter 14 is set according to this data. The output of the counter 14 is connected to the input of the line store 26 through a control gate 24 which is opened by a control signal on line 27 from the decoder 5 on receipt of predetermined other ones of first-type words ("terminated strings"). A normally-open control gate 17 is interposed between the clock 15 and the stepping-pulse input of the counter 14. This gate is closed by a signal on line 21 from the decoder 5 on receipt of a word of the second type (a "correlation" code word) whereby the counter is inhibited. Words of the first type include 8-bit unterminated strings having zero in the four most significant places, and 4-bit halfwords all containing one-bits. The words are read out of the store 2 four bits at a time, the four zeros of the full words being applied to lines 4A, the four least significant bits on lines 4B. The data on lines 4A is applied to the set side of a bi-stable 7 and also produces a pulse on line 9 which causes the store 2 to emit the last four bits of the full word. The bistable 7 emits a "one" on line 11A which memory unit 10 interprets as a series of zeros giving a beam displacement of between 25 and 250 increments. The four last bits are supplied over lines 11 B to unit 10 which sets the 8-bit counter 14 to the total number of displacement increments specified by the code words. The counter is decremented via a gate 17 which is closed during flyback by the synch pulse generator 19. The gate 17 is also closed when words of the second type are detected by logic circuits 34, 35 of the code detector 5. A logic circuit 28 detects the presence of a "terminated" string code on lines 11A, 11B and opens a gate 24 between the output of the counter 14 and the input of the store 26. Thus, only "terminated" string codes are effective to set a "one" into the store 26 at the correct position with respect to the raster lines appearing on CRT1. A zero count signal over line 33 resets the bi-stable 7 thereby to remove the indicator bit from line 11A. Words of the second type are "correlation codes" and indicate a difference between one line and its successor. Thus, code R may signify a left shift of one increment, L a right-shift of one increment, S, SS, SSS may signify that the next one, two or three transitions are to occupy the same positions in the next line. The circuit 34 connected to lines 11A, 11B is arranged to produce four signals R, SA, SB and L on lines 36 and through logic circuit 35 to close gate 17. In addition, the outputs from the circuit 34 are used as control inputs for the line modification circuit 37, Figs, 1, 5.