Patent GB-1198834-A (GB)
Title: Improvements relating to Electronic Display Systems
IPC: G09G1/10
Filed: 1967-09-21
Abstract: 1,198,834. Cathode-ray tube displays. SANDERS ASSOCIATES Inc. 21 Sept., 1967 [10 Oct., 1966], No. 43094/67. Heading H4T. In an arrangement for tracing symbols &c. at preselected positions on the screen of a cathode-ray tube under the control of instructions, specifically, digital words, those of the latter which define the preselected positions are first supplied to the positioning and tracing circuitry and when the tube beam has reached a selected position a request signal is generated by the circuitry which causes the symbol defining words to be supplied thereto. When the symbol has been traced a further request signal causes the position defining words relating to the next symbol to be supplied and the cycle then repeats. This manner of operation thus optimizes the time for tracing each symbol as opposed to a system in which a fixed time is allotted to the tracing of each symbol irrespective of the time which the tracing actually requires. The invention is described in relation to a data display in which the symbols comprise characters selected from a repertory of characters or comprise line, e.g. vector type, traces. Each instruction (digital word) contains a control byte specifying whether it relates to a character or to a line trace display. In the former case the word includes bytes specifying the X, Y co-ordinates at which the character is to be produced and a byte specifying the actual character (Alpha byte) and in the latter case the word includes bytes specifying the X, Y co-ordinates at which the line trace is to terminate (the start being at the X, Y position of a previous instruction) and a byte specifying the length of the line (Alphabyte). The instructions are contained in an addressable memory 10 (Fig. 2) which is continually updated by an input source, e.g. a radar receiver and associated computer and is also continually cycled by readout circuits to provide a flickerfree display. In initial operation or, in the event of failure of a system generated request signal, an artificial request signal is produced by the output of a counter 21 supplied with pulses from a clock 22. This request signal causes memory 10 to transfer an instruction to a first buffer register 20 via gates 18. Since there is yet no system generated request signal a further output from counter 21 (further artificial request signal) together with the output from a decoder 32 responsive to the character or line trace specifying byte conditions gates 23 so that on the occurrence of the next clock pulse (fed to the various gates via arrow-headed connections as distinct from the diamond-headed data line connections) the contents of register 20 are fed to a second buffer register 24. The further artificial request signal from counter 21 also operates to cause the transfer of another instruction from memory 10 to the, now empty, register 20. Assuming that the word in register 24 relates to a character the output of decoder 42 causes transfer of the X, Y position information via gates 34 to a register 36 and thence via digital-to-analogue converters 60, 62 and summing amplifiers 64, 64 to deflection, amplifiers 68, 70 to position the (blanked) beam of the display tube 9 at the selected position. When this has been reached a circuit 71 (detailed in Fig. 3, not shown), responsive to cessation of the voltage induced in the scanning coils, supplies a "position done" signal which, together with the output of decoder 42, causes transfer of the Alpha-byte (specifying the actual character) to a register 40 and thence to a character generator 72. The latter in response to a timing pulse from gate 43, supplies an unblanking signal via 74, 76 to unblank the tube beam during tracing of the character which is effected under the control of appropriate deflection waveforms supplied from generator 72 to the X and Y deflection circuitry. When the tracing of the character has been completed generator 72 supplies a signal to OR gate 44 which initiates the transfer of the next word from memory 10 to register 20. In the event that the character instruction in register 24 is a null or space "character", decoder 42 conditions a gate 45 so that on the occurrence of the "position-done" signal from circuit 71 gate 45 supplies a signal via OR gate 44 which causes transfer of the next word from memory 10 to register 20. When the word transferred to register 20 relates to a line trace the bytes specifying the X, Y co-ordinates of the "end" of the line are combined in a subtractor 46 with the bytes in register 24 which specify the present X, Y position of the beam, i.e. the "start" of the line trace. The output of subtractor 46 comprising #X, #Y is then fed, whilst the previous character specified by the word in register 24 is being traced, to a register 50 and thence together with the byte specifying the line length derived from register 20 via gates 52, to a line generator 78 which forms the appropriate deflection and blanking signals for application to the display tube. The output from generator 78 is such that the line is traced at a constant velocity to achieve uniform intensity regardless of line length. When the line trace has been completed generator 78 supplies a "line complete" signal to OR gate 44 to cause the transfer of the next instruction from memory 10 to register 20.