GB1186152A - Improvements relating to Visual Display Devices. - Google Patents

Abstract

1,186,152. Transistor switching circuit; gas filled tube bi-stable circuits. G. R. HOFFMAN. 23 Dec., 1968 [13 Jan., 1968], No. 2029/68. Headings H3P and H3T. [Also in Division G5] A control apparatus for a gaseous discharge tube display device of the rectangular matrix type comprises two control matrices (one shown in Fig. 3), for addressing the row and column conductors respectively of the display matrix (Figs. 1, 2, not shown). Each control matrix respectively supplies fixed " sustain " voltages V ht (of opposite polarity), and also switching pulses ŒV t . It comprises pulse transformers T 0 -T 1023 whose primary windings are connected between row and column lines whose respective potentials are controlled by separate switching devices TY 0 -TY 31 and TX 1 -TX 31 ; these are in their turn controlled by actuating means (e.g. a computer, not shown), to supply the requisite switching pulses to a selected tube. As described, the display matrix comprises an array of 1024 x 1024 gas tubes, operated in bi-stable fashion. Yet there are only 128 signal input lines: each tube is actuated by a unique selection of 4 such lines. The duration of the input signal on one such line determines whether the tube is ignited or extinguished. General layout.-The circuit shown in Fig. 3 selects one of the column conductors x 0 -x 1023 of the display matrix. A complementary circuit (not shown) selects one of the row conductors (Y 0 -Y 1023 ). As shown, by choosing in the control matrix a row line, e.g. Y 0 , and a column line, e.g. X 1 , Fig. 3, a unique column conductor of the display matrix, i.e. x 1 , is selected. Thus to select any one gas tube, a total of four lines are chosen, in contrast to the conventional arrangement where only two conductors are selected. Memory system.-The gas tubes work at a voltage V ht = 250; extinguish at (V ht - V t ) = 150; and ignite at (V ht + V t ) = 350. In other words, each tube is bi-stable, being switched by pulses ŒV t . In one method, a waveform Y, Figs. 5, is supplied to a selected row conductor. To ignite the tube (or leave it unaffected, if already lit), waveform X, Fig. 5a, is supplied to the selected column conductor. Between times t 2 and t 3 the net tube voltage is = 350, so the tube ignites. To extinguish a tube, waveform X, Fig. 5b, replaces waveform X. Extinction occurs between times t 1 and t 2 , and the ensuing 300 volt p.d. is insufficient to re-ignite it. The circuit of Fig. 3 not only selects the desired column conductor, but also applies the appropriate " ignite " (X) or " extinguish " (X) pulse to it. Selecting a column conductor; switching a tube (Fig. 3).-Pulse transformers T 0 -T 1023 are connected at the crossovers of row lines Y 0 -Y 31 and column lines X 0 - 31 . The " X " lines are connected to a constant current source C through column transistors TX 0 -TX 31 to potential - V t through catching diodes DX 0 -DX 31 ; and to potential - ¢Vt through resistors RX 0 -RX 31 . The " Y " lines are connected to row transistors TY 0 -TY 31 whose emitters are at potential - ¢V t . Transistor Z is a current sink. At the start, the diodes "DX" and "DT" are reverse biased by a voltage ¢V t . If a control signal is applied to (say) input IY 0 , this switches transistor TY 0 and unbiases the diodes DT 0 to DT 31 . Similarly, a signal on (say) input IX, to switch driver transistor TI, unbiases diodes DT 1 , DT 33 ... DT 993 . A linearly increasing current flows in the primary of transformer T 1 , the corresponding secondary voltage being shown at a, Fig. 5a. If, as soon as the current reaches a maximum, the signal on input IX, is removed (Fig. 4b, not shown), the current decays linearly to zero and the resulting voltage is shown at b, Fig. 5a. This provides waveform X (of which waveform Y, generated in the other control matrix, is merely an inverse). To get waveform X, the signal on IX, is not removed as soon as the current reaches a maximum (Fig. 4a, not shown). Therefore the output voltage only drops to c, Fig. 5b. Thus to ignite the selected tube a short voltage pulse is applied to IX 1 , and a longer pulse is applied to extinguish it; these pulses may be applied by a computer. Modifications.-In another form, Fig. 6 (not shown), the annoying tube " ringing " effect is reduced by connecting a clamping transistor (TC) to each of the lines X 0 -X 31 . When switched on it prevents the output waveform from going negative by more than - ¢V ht , i.e. it inhibits waveform X. To allow waveform X, transistor (TC) is switched off during an appropriate part of the cycle.