Patent GB-1313858-A (GB)
Title: Position resolving apparatus
IPC: G06F3/038
Filed: 1969-10-09
Abstract: 1313858 Cathode-ray tube circuits MARCONI CO 28 July 1970 [9 Oct 1969] 49574/69 Heading H4T In an arrangement for resolving the coordinates of a position in a display produced by a direct viewing storage tube the position is selected by light-pen means which, via " touch "  or manually operated switch means changes the normal operating condition of the tube by causing the writing gun to produce an unmodulated beam which passes through the storage mesh electrode without affecting previously stored charges and scans the display screen with a line raster of known instantaneous co-ordinates which in combination with an output signal from the light pen enable the required coordinates to be determined. Referring to Fig. 1, the direct viewing storage tube DVST, with switches SW1-SW4 in the positions shown, produces a display on screen T1 controlled by a computer ND supplying X, Y deflection signals to the deflection plates XT4, YT4 and by a source NS supplying appropriate bright-up modulation signals to the beam control electrode T6, the tube cathode T5 being maintained at a suitable negative potential supplied at NB via a short circuited resistor R so that the writing beam energy is sufficient to traverse the secondary electron collector mesh T3 and produce a stored charge image on mesh T2 for production on screen T1 by means of the flood gun T4. When it is required to resolve the co-ordinates of a particular position on the display the lens S1 of a light-pen LPS is placed over such position and by operation of manual switch means OS (or by a known " touch " arrangement) a pulse on line OL1 actuates a relay control means RC which causes switches SW1-SW4 to change-over from the positions shown to positions in which the deflecting plates are supplied from the outputs of respective D/A converters X2, Y2, the beam brightup modulation is replaced by a fixed negative bias - NB1 and the short circuit across resistor R is removed so that the writing beam now has sufficient energy to traverse both meshes T3, T2 (without affecting the stored charge image) and produce a line scanned raster on screen T1 determined by the D/A converters X2, Y2. The D/A converter X2 produces a triangular deflection voltage waveform so that successive line scans are effected in opposite directions (see Fig. 2) under the control of an up-down counter X1 which counts the output of an oscillator 3 (the frequency of which is high relative to line frequency) supplied via bi-stable gate 1 when the latter is opened by the output of a pulse generator PG actuated by the pulse on line OL1. The direction in which counter X1 counts is reversed after each full count by a control means XR and each operation of the latter produces a single count in an up-counter Y1 which counts up to a predetermined number-the desired number of line scans in a copmlete field. The output of counter X1, in addition to being supplied to D/A converter X2, is fed to a pair of registers X3, X4 and the output of counter Y1 is similarly fed to registers Y3, Y4. When the light pen is first activated, " first-hit " (FH, Fig. 2), it generates a pulse on line OL which closes bi-stable gate 2 (previously opened by the pulse on line OL1 via pulse generator PG) and gate 2 then supplies a pulse to registers X3, Y3 the counts in which are held (i.e. the X and Y co-ordinates of the first-hit). The pulse produced by gate 2 also conditions a " D " type bi-stable 4 which, in response to successive outputs from reverser XR, controls a gate 5 such that it passes only those pulses on line OL produced by " hits " during line scans in the opposite direction to that which produced the " first-hit ". Each such pulse is fed to the register X4, Y4 to increment the counts already set-up (i.e. the co-ordinates of the " first-hit ") and this action continues until the " last-hit " LH (Fig. 2) in an even numbered line occurs. The outputs of registers X3 and X4 are fed to a circuit X5 and the outputs of registers Y3 and Y4 are fed to a similar circuit Y5 which, in response to a pulse produced by the " up " counter Y1 when full, at the end of a field scan (and which also activates switch relay control BC to restore normal operation) operate as combination adders and divisors by 2 and produce outputs equal to the mean of the two input counts. These outputs are then a measure of the X and Y coordinates of the approximate centre of the effective cross-section of the light-pen (dotted circle in Fig. 2) and are therefore a close approximation of the co-ordinates of the desired position. Instead of employing the arrangement described a sufficient approximation of such co-ordinates may be obtained by adding to the co-ordinates of the " first-hit ", fixed values of X and Y determined by the known relationship of the " first-hit " to the centre of the light-pen. The light-pen LPS comprises a lens S1, a light-sensitive field effect transistor S2, an amplifier threshold clipper S3 and a pulse forming monostable circuit S4.