PROCESSOR FUNDAMENTALS 3.4 CIRCUITS AND TRUTH TABLES
WHAT IS A TRUTH TABLE
A truth table is a method of visually displaying the outcomes of a logic gate or all of the possible outcomes of a logic circuit.
SINGLE GATE TRUTH TABLE
As can be seen on the previous page each logic can be represented with their own truth table. The first thing you should notice is that the input values are listed in a logical binary order, this ensures that you do not make any mistakes when creating the input values and helps maintain consistency so all truth tables are easy to read.
Notice the pattern when listing binary, the left hand column will always read down 1 zero then 1 one, the next column will be 2 zero's then 2 one's the third column 4 zero's then 4 ones, this pattern continues as you add column. The other reason to list the binary this way is if you convert the binary value to normal numbers then you are simply counting from Zero upwards. You can also work out the total possible input combinations by doing 'Number of inputs ^2'. For example the logic gate in the diagram above has 2 inputs so 2^2=4, there is a total of 4 possible combinations to the input values.
NOTE: MANY STUDENTS LOSE MARKS OR GET THE ANSWER WRONG BECAUSE THEY DO NOT LIST THE INPUTS USING THIS METHOD.