Understanding Truth Tables

Truth tables help understand the behaviour of logic gates.

• They show how the input(s) of a logic gate relate to its output(s).
• The gate input(s) are shown in the left column(s) of the table with all the different possible input combinations. This is normally done by making the inputs count up in binary.
• The gate output(s) are shown in the right hand side column.

Truth Table Diagram

Drawing Truth Tables for Combined Gates

Truth tables can also help understand the behaviour of combinations of logic gates linked together.

• They are drawn in the same way as before but this time adding more columns in between the input and output columns.
• Each of the new columns show the outputs of stages in the logic circuit.

The following example shows the steps for drawing up a truth table for a combined gate configuration.

```Example
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Exercise

See if you can draw the truth table for the following logic circuit and find the output expression. What gate does the expression represent? When you have done this, you check with the answers.

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Richard Bigwood 2005