Patent GB-1252485-A (GB)
IPC: G06F3/048
Filed: 1969-05-05
Abstract: 1,252,485. Cathode-ray tube displays. RCA CORPORATION. 5 May, 1969 [6 May, 1968], No. 22772/69. Heading H4T. In a display system in which successive coded character signals received from a computer are utilized to control the production of a graphic display on a cathode-ray tube with which a light-pen is co-operatively associated, an output signal from the light-pen, when it senses a selected part of the display, is converted into coded data indicative of the position of the coded character corresponding to such selected part in the sequence of coded character signals which contains it and such coded data is transmitted to the computer. The invention is contrasted with known systems in which the signal from the light-pen is employed to send to the computer an " interrupt " signal together with signals indicative of the co-ordinates of the selected part and is stated to be of particular advantage when used in conjunction with a remote, time-shared, computer connected by a transmission channel of narrow bandwidth, (e.g. an ordinary telephone line), since it may not be convenient for the computer to grant priority to one user over another and even if it does, it may be at a step in its programme beyond that corresponding to the received light-pen position because of the transmission time of the narrow bandwidth channel. Also, when such a channel is employed the displayed cannot be continually refreshed by the computer at a rate sufficiently high to prevent flicker and to overcome this difficulty a storage-type display tube is employed. As shown in Fig. 1, coded character signals indicative of graphic information are supplied by a time-shared computer 10 in parallel form to a converter 12 in which they are translated into serial form and sent, as audio tones, over a telephone line 14 to a remote user location. The received tones are supplied via a translator 16 to control circuits 18 in which they are converted to parallel form coded character signals and utilized to control the production of the graphic display on a storage type cathode-ray tube 20. The coded character signals, each consisting of seven bits (plus a parity bit) comprise instruction characters, indicating e.g. start-of-message, erasure of the display etc. and operation characters, indicating the length and direction of the graphic traces. When a message is received the start signal derived via decoder 22 resets two 6-stage counters 24 and 26 and resets register 3 and as each operation character is received counter 26 is advanced by a count of " one ". If the graphic figure being traced requires more than 64 computer commands counter 24 commences to count when counter 26 is full (i.e. at a count of 64). Each received operation character controls the production of the display via the blocks 18 and 20 (as described in Specification 1,187,556) so that the state of the counters 24, 26 at any instant is indicative of the element of the display being traced at that instant. When light-pen operation is required the operator depresses a key on keyboard 70 which conditions a switch 28 to pass the output of the counters to a parallel-to-serial converter 30 and when the operator positions the light-pen 34 adjacent a selected element the resulting pulse produced thereby opens gate 32 which passes the output from converter 30 to the translator 16. The latter then converts the input signals to a form suitable for transmission over line 74 to the converter 12 which translates them into parallel 7-bit words for application to a temporary buffer stage in the computer 12. When this stage is examined on the time-shared basis the computer interprets the words as a " pointer " to a location in its memory which stores the operation information required to trace the selected element forming part of a particular programme e.g. " Drawing a Cube " (Fig. 2, not shown) and the operator by means of a keyboard 70 then instructs the computer to effect any required changes, modifications etc. in the selected element. The modified programme is then returned to the same memory location and the computer may then be instructed to produce the graphic trace in its amended form. A " track " mode of operation in which an " octagon " figure drawn under the control of the computer is moved across the display screen in accordance with movement of the light-pen is also described (Fig. 2, not shown).