3. More about logic tables
• To produce a truth table from a logic diagram you need to
work out the outputs for every possible combination of
inputs.
– If a logic diagram has only 2 inputs then there will only be 4 input
combinations (00, 01, 10 and 11).
– If there are 3 inputs then there will be 8 possible combinations.
– 4 inputs would give 16 combinations.
• Therefore n inputs gives 2n outputs
4. Now try making a TRUTH TABLE for this diagram…
This is called a half-adder circuit.
Can you work out why?
5. Half-adder Circuit
• Are designed to add 2 binary numbers with a
carry (C) if the sum (S) is greater than 1.
• So this can be expressed as a logic statement:
S = (A OR B) AND (NOT (A AND B))
C = (A AND B)
