ES353929A1 - Improvements in or relating to logic devices - Google Patents

Abstract

An inverting D.C. amplifier 1a, Fig. 3, has two Zener diodes 3a, 4a oppositely connected in series from output 9a to input 8a, whereby both positive and negative output voltages (Es, Fig. 2, not shown) are limited to narrow zones (11a, 11b) on the voltage output-current input characteristic, and has an odd number of inputs, for example three, 12a, 13a, 14a. The amplifier may perform NOR, NAND, and majority logic functions, according to whether the input voltage at 13a is fixed positive +U, fixed negative - U, or a free signal, respectively. Two such amplifiers Ia, 1b are connected to give a bi-stable circuit, which, as shown with + U at 13a and + U at 13b performs the function S=(C+S).#D, where D is an inhibit input. Faults in the amplifier, in the Zener feedback, in a preceding stage or in an interstage coupling are detected by departures of the output voltage from the narrow zones 11a, 11b. For example, the output voltage may reach the supply voltage (A) or the amplifier saturation voltage (V), or may have a value between the two zones, and such departures are detected by connecting at 9a a voltmeter, a moving spot galvanometer with a photo-cell having shaded areas corresponding to zones 11a, 11b and controlling a relay, or a numerical calculator. Alternatively, the voltage between two successive amplifiers is measured, a fault being indicated by this voltage being greater than the width of a zone (2#). Zero voltage means a loss of one input (there then being an even number of inputs left) in the condition where the remaining inputs are halfpositive and half negative. If this condition does not prevail, the loss of one input may be tested for, by applying a known voltage to an extra, test, input when the other inputs are known, so that zero output is expected if no fault is present. In an application, the input signals are received from an input circuit consisting (Fig. 5, not shown) of a similarly voltage limited amplifier (1c) deriving its input from a voltage source (19, 20) which may be a thermocouple, or a liquid-level measuring float-actuated potentiometer or a piezo-electric crystal, the circuit thus indicating when a given level of one of the non- electric variables is reached. The threshold is set by an adjustable voltage source (21).