Patent GB-1390829-A (GB)
Title: Character representation systems
IPC: G09G1/10
Filed: 1972-12-22
Abstract: 1390829 Cathode-ray tube displays SIEMENS AG 22 Dec 1972 [29 Feb 1972] 59553/72 Heading H4T In a character display system a fixed word store, when addressed by input commands, supplies character segment data, specifying particular segments to be drawn, to digital-toanalogue converters controlling X, Y coordinate deflections of a writing element, e.g. the beam of a cathode-ray display tube, a storage cell stores a particular bit the binary value of which depends on control data stored in the fixed word store and address commands for the latter are formed from words supplied by a character input device together with the particular bit, the arrangement being that when the particular bit has a first value characters of a first group (e.g. alphanumeric), are displayed whereas when it has a second value characters of a second group (e.g. mathematical curves &c), are displayed. Fig. 1 shows the general scheme in which data, specifying characters to be displayed, derived from an input source 11 (e.g. a computer), which may be remotely located is supplied via an image repetition store 12 to a character generator 13 from which deflection quantities are derived and supplied to the deflection coils of a cathode-ray display tube 19 via respective D/A converters 15X, 15Y and respective amplifier 16X, 16Y. Additionally a beam intensity control output is supplied to the tube via a control amplifier 15. Fig. 2 shows the arrangement of the character generator 13 which includes an address selection stage 21 receiving data in the seven bit ASCII code I-VII from the image repetition store 12 and via switch position 35a supplies these bits together with bits VIII, IX and X, as a ten bit word I-X to an address counter 22 and thence to a fixed word store 23. The latter which is organized as shown in Fig. 3, and contains 1024 words each of 12 bits comprises; a storage zone 23c containing the addresses of alpha-numeric character segment data and control data, a total of 128 words; a storage zone 23d containing the addresses of segment data of graphic characters and control data, a total of 128 words; a storage zone 23e containing segment data for alphanumeric characters (3 Î 128 words), and a storage zone 23f containing segment data for graphic characters and control data (3 x 128 words). In operation, when alpha-numeric characters are to be displayed the ten bit word I-X, with the bits I-VII indicative of the desired character in the ASCII code and the bits VIII=0, (the "particular bit"), IX=0 and X=0 (see line 2, Fig. 3), addresses storage zone 23c which outputs a twelve bit word XI-XXII specifying the address of segment data and control data to a register 24 which, in response to the control data bits XXI-XXII, operates a switch control stage 26 to cause the bits XI-XX to be transferred via contacts 25c and 35b to the address counter 22 and thus to address storage zone 23e. The latter then outputs a second twelve-bit word of which the bits XI-XVIII specify the first segment, in terms of X-Y deflection of the desired character, the bit XIX, the beam blankunblank requirements (the bit XX a function not specified and not relevant to the present invention), whilst the bits XXI and XXII which specify control data cause switch control stage to produce a bit XXIV which causes the address counter 22 to be advanced by "one" on the occurrence of the next timing pulse. The segment bits XI-XVIII are then effective via a control stage 28 and the D/A converters 15X, 15Y to produce the required beam deflection and the bit XIX is operative via a control stage 30 and amplifier 14 to control the beam blank-unblank requirements in accordance with the b output from a decoder 32. When the first segment has been traced control stage 28 emits a "1" from output d and assuming the character consists of more than one segment (otherwise address counter 22 would not have been advanced by "1"), the control bits XXI = 0 and XXII = 0 and the new address in counter 22 addresses the store 23 which provides segment and control data for the second segment of the character. The operation continues in this manner until the last segment has been traced this being signalled XXI =0, XXII = 1 and XXIII = 1 at which point the control bits XXI and XXII cause the switch positions 25a and 35a to be set in preparation for new input data from the image repetition store 12. When it is desired to display graphic characters (i.e. mathematical curves and equivalent traces), a distinctive seven-bit input word together with bits VIII = 0, XI = 0 and X = 0 is fed via address counter 22 into the store 23 which then emits a twelve-bit word XI-XXII having bits XXI = 1 and XXII = 1. The latter bits then set switch position 25d so that bits XI-XV pass to a decoder 32 supplying the bit XXV="1" to a trigger stage 33 which changes bit VIII from "0" to "1". This results in a new address word (line 3, Fig. 3), which now addresses storage zone 23d from which the address of the first segment of the graphic character is read out and stored in register 24. The control bits XXI = 1 and XXII = 0 in the latter address word set switch positions 25c and 35b which causes the bits XI-XX of this word to be transferred via address counter 22 to zone 23f in the store 23. The latter then outputs the data of the first segment of the graphic character and the operation continues in the same manner as for alpha-numeric character display. When the latter type of display is again required a distinctive address word I-X is input and addresses storage zone 23d from which a 12-bit output control word XI-XXII is derived, the bits XXI=1 and XXII=1 setting switch position 25d to cause bits XI-XV to produce via decoder 32 the bit XXV=0 which changes trigger 32 back to its "0" state so that bit VIII is again "0", the value required for alphanumeric display. By giving the bits IX and X a value other than "0", i.e. a, "1", further storage zones of the store assigned to a further class of characters may be addressed.