Patent GB-811325-A (GB)
Title: Improvements relating to electrical digital-analogue interconversion circuits
Inventor: REDDING ROBERT JAMES
IPC: G01R17/08
Filed: 1956-01-05
Abstract: 811,325. Digital/analogue interconverters. EVER SHED & VIGNOLES Ltd. Dec. 21, 1956 [Jan. 5, 1956], No. 448/56. Class 40 (1). In a conversion circuit current is supplied from a constant voltage source through a series resistance to a variable resistance divided into steps according to a digital code and to a parallel circuit, the current in which represents the analogue value, the current through the variable resistance being maintained constant at a datum value by adjusting either the current in the parallel circuit or the magnitude of the variable resistance according to whether the conversion is from a digital to an analogue representation or vice versa. A digital to analogue conversion circuit, Fig. 1, has a supply transformer with two secondary windings 13, 14. The output from winding 13 is rectified by bridge 15 and stabilized by neon tube 16 and series resistor 17. The D.C. voltage is applied across a triode 20 through resistor 18 and an ammeter 21. The stepped resistance 19 is connected between resistor 18 and resistor 17 with a fixed resistor 25 and a variable resistor 26 connected in series. The effective magnitude of the resistance 19 is adjusted in accordance with binary signals received serially on lines L1 and L2, relays 51 being energized thereby, to cause corresponding sections of the resistance 19 to be short-circuited. The current in the parallel triode circuit is adjusted to keep the current in resistance 19 constant, so that the current flowing in the triode circuit as indicated by meter 21 is an analogue representation of the binary signal received. The current in the triode is adjusted by a current detector 30, the coil 34 of which carries the current in resistance 19. Arm 31 balancing between coils 34 and 35 causes the grid of the triode and condenser 45 to be connected to positive or negative contacts 39, 38. If the current in coil 34 increases beyond the set maximum the arm 31 is drawn into contact with the positive contact so that current passes in the triode circuit to increase the voltage drop across resistor 18 and a consequent reduction in the current in the resistance 19. The coil 35 is supplied from the transformer winding 14 and accordingly helps to cancel errors due to variations in supply voltage. The steps of the resistance 19 are weighted according to the binary scale, the input signal on line L1 being distributed to relays which short circuit an amount of the resistance proportional to the value of the input signal. The ammeter 21 is calibrated to give a direct reading corresponding to the binary signal received. The relays 51 can alternatively be energized by binary signals received on corresponding parallel lines, no distributer then being required. In a circuit, Fig. 3, for making the opposite conversion an analogue value is applied in the form of a mechanical couple P on the axle of a contact arm 60 which is arranged to adjust the current through triode 20 to a value representing the analogue value applied. The arm floats between a current coil 62 and a magnet 63 making contact with a negative contact 66 to reduce the current and a positive contact 65 to increase it. The current in stepped resistance 19 is maintained constant as before by a current detector coil 34 controlling a contact arm 31, the effect of which in this case is to insert additional steps of resistance until the current is restored to the datum value. For this purpose arm 31 controls a chain 70 of relay networks through a pair of relays FLE, FLO, each of which is connected to de-energize itself and pick up the other so that they are picked up alternately in an automatically repeating cycle. A contact FLO2 of relay FLO controls relay TS which is accordingly picked up periodically. A first contact TS1 of this relay controls the selection of the steps of resistance 19 and a second contact TS2 transmits corresponding pulses to line L1. When contacts TS1 close they complete a circuit to a network of relays and relay contacts, the purpose of which is to short circuit each successive resistance step and allow the current detector to determine whether the current is too large or too small. If it is too large the short-circuit is removed while if it is too small it is retained, the appropriate positive or negative pulse being transmitted to line L1 immediately before the next resistance step is connected in circuit. The output signal consists of a series of positive or negative pulses according to whether or not the corresponding resistance is short-circuited or not. Where the analogue signal varies the digitalizing process may be repeated at will by pressing a button switch 75. This removes all short circuits and the rebalancing operation is repeated as before. The switch 75 may be operated by a cam to provide digital signals at definite intervals. Specifications 589,423 and, 799,864 are referred to.