Patent GB-1212110-A (GB)
Title: Display device
IPC: G06F3/02
Filed: 1968-11-29
Abstract: 1,212,110. Cathode-ray tube displays. INTERNATIONAL BUSINESS MACHINES CORP. 29 Nov., 1968 [15 Jan., 1968 (2)], No. 56681/68. Heading H4T. In an arrangement for tracking the movement of a light-pen across the screen of a cathode-ray tube by constraining the beam to follow the movement of the pen a visible pattern is " painted " by the beam which has a first configuration whilst undetected by the pen and a second, different, configuration when detection by the pen occurs. The invention is described in connection with a system in which the X, Y co-ordinates of successive points on the path traced by the light-pen are fed to a computer from which they are subsequently derived and utilized to reproduce the actual path as a visible trace on the screen, the overall system being as shown in Fig. 1. Fig. 3 shows the visible pattern, termed a " tracking pattern " which has a first configuration comprising a single square and a second configuration comprising this latter square and three outer concentric squares. The pattern is traced in a counter clockwise sense commencing and finishing at the origin 0 1  for the innermost square and then successively for the other squares commencing and finishing at the lower left-hand corner until the outermost square has been traced. The relative dimensions, shown in raster units, ensure that the light pen (10 raster units in diameter) cannot " miss " a segment of the pattern as it is moved across it. In operation, the system, which is initially quiescent, is brought into use by switch means actuated by the operator, specifically switch means incorporated in the light pen and actuated when it is pressed against the tube screen. This results in what is termed MODE 2 operation in which the beam commences to trace the entire pattern of four squares commencing at the origin 0 1  the absolute X, Y co-ordinates of which are supplied by the computer but which may be predetermined by the operator. The operator then moves the light-pen (e.g. towards the pattern) and if it does not detect any of the pattern before its tracing is completed the system changes to what is termed MODE 1 operation in which the inner square only is repetitively traced commencing at the origin 0 1 . If during this MODE the light pen detects portion of the pattern the system reverts back to MODE 2 operation and commences to trace the entire pattern of four squares from a new origin located at the position in the pattern at which detection occurred and if, before this pattern is completed, detection by the light-pen occurs, the system remains in MODE 2 and again commences to trace the entire pattern from another origin located at the position of the last detection. Also, at each detection, signals representative of the X, Y co-ordinates of the new origins are successively supplied to the computer from which they may be subsequently derived and utilized to produce a visible trace. In a second embodiment the " tracking pattern " is produced by bright-up of a line scanned raster and initially comprises a square as shown in block 1, Fig. 11. In operation, the light-pen is initially positioned at the centre of this square which, if no detection occurs (lightpen stationary) is repetitively produced. If a detection occurs the pattern is modified by the omission of the vertical sides (block 3) and if a further detection occurs it indicates that the light-pen is moving up or down whereas if no further detection occurs it indicates that the light-pen is moving either towards the right or towards the left. In the former case the pattern is modified to display only the upper horizontal side (block 4) whilst, in the latter case, only the right-hand vertical side is produced (block 7). The motion of the light-pen is then resolved in accordance with whether or not further detections occur (blocks 5 to 9) and the necessary information for changing the position of the initial pattern thereby deduced and employed to update the pattern position means. Details of the circuit arrangements for carrying out both embodiments are described with reference to block circuit diagrams (not shown).