COMBINATIONAL LOGIC CIRCUIT PROBER

ABSTRACT The objective of this report is to design and implement a logic circuit prober to display truth tables of a three input combinational logic circuit. The truth table is to be as “1” and “0” on an ordinary 60 MHz oscilloscope. This report meets this objective by using Lissajous Patterns to plot a “0” or a “1” on the oscilloscope screen. To plot a “0” on the oscilloscope screen, two sinusoidal signals in quadrature are supplied to the two inputs of the oscilloscope with the scope set to X-Y mode. To plot a “1” on the oscilloscope, only the signal to the Y input is allowed to reach the oscilloscope screen. To display all the 32 patterns required to obtain a three input truth table, two staircase waveforms are employed. The staircase waveforms, one eight step and the other four step, are added to the two sinusoidal signals to shift the patterns along the X and Y directions to produce all the 32 patterns. Suitability of the design as a logic prober is demonstrated by a comparison of the truth tables for a simple full adder, a simple majority logic circuit and a simple even parity generator generated by the Combinational Logic Circuit Prober with the theory.

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