Patent GB-1446188-A (GB)
Title: Data transmission system for a cathode-ray tube terminal non-destructive testing apparatus
IPC: G06F3/147
Filed: 1973-12-10
Abstract: 1446188 Cathode-ray tube displays INTERNATIONAL BUSINESS MACHINES CORP 10 Dec 1973 [20 Dec 1972] 57145/73 Heading H4T In a data transmission system involving twoway transmission of data between a "host" computer and a selected one of a plurality of terminal stations employing a cathode-ray tube display device operating by selective brightening of a line scanned raster, data is transmitted in "message frames" comprising lines of data corresponding to lines of the raster, some of the lines containing data display information and the remaining lines containing control information which is not displayed, information identifying the content of a line being transmitted within the horizontal fly-back time of the display. Certain ones of the control information lines may each be associated with a different group of lines containing bright-up pulses for displaying a row of characters and contain (1) coded information indicating whether or not one or more characters of the row may be changed by terminal operator action (2) coded information representing only characters displayed as a result of terminal operator action and (3) coded information by means of which a cursor may be displayed. Means are provided to transmit from a terminal station to the "host" computer only those lines of a (message) frame which contain the coded information. Each message frame transmitted by the "host" computer is preceded by a selected terminal station address occurring in the vertical raster retrace interval and at the terminal station message frames are stored in a recirculating store operating synchronously with the raster beam. Alternately the "host" transmission message frames may occur at a submultiple of the terminal display frequency and be stored in different sections of cyclic buffer stores to allow higher quality displays, i.e. interlaced displayed to be produced (e.g. for photocomposition usage). Fig. 1 shows the general arrangement of the "host" computer station (left-hand side) and addressable terminal stations 12 (right-hand side) the organization of a typical terminal being shown in Fig. 2. The system is adapted to handle interlaced lines of video and control information, four discrete types being described although this number may be extended. As shown in Fig. 4, the four lines designated Type 1 line-Type 4 line are each associated with an identification "tag" two-bit code occurring in the horizontal retrace interval HRT. This interval may also contain other "tag" bits and other information, e.g. address information allowing individual lines to be distributed to different terminals. Also, the frame itself may be preceded by a "tag" distinguishing between video frames and other information frames (e.g. audio). Type 1 lines are associated with video information, Type 2 lines convey character synch information (clock pulses), Type 3 lines convey entry separation markers and Type 4 lines convey character code information. When a Type 3 line marker pulse is present in a line character space the corresponding character space segments in Type 1 and Type 4 lines may be modified by the terminal circuit but, if such marker is absent, modification is prevented. Fig. 5 shows the sequence of lines involved in the display of a row of characters and Fig 5A shows details of a single character the Type 2 line clock pulse character synch pulse being made visible if desired to provide a point-ofentry cursor.  A terminal may be employed for student instruction the student replying to computer generated displays by operating a keyboard. The overall operation is described with reference to waveform diagrams Figs. 6-9 (none shown) and logic diagrams Figs. 10, 11, 13 and 14 (none shown).