Logic Expressions
- A logic expression is a way of expressing a logic gate or logic circuit as an equation
- The output appears on the left of the equals sign with the inputs and logic gates on the right
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NOT |
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Z=NOT A |
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AND |
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Z=A AND B |
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OR |
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Z=A OR B |
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NAND |
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Z=A NAND B |
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NOR |
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Z=A NOR B |
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XOR |
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Z=A XOR B |
- Logic circuits containing multiple gates can also be expressed as logic expressions/statements
An example logic circuit containing two inputs
- The logic circuit above can be expressed as the logic expression Q= NOT(A OR B)
An example logic circuit containing two inputs
- The logic circuit above can be expressed as the logic expression Q= (NOT A) AND B
An example logic circuit containing three inputs
- The logic circuit above can be expressed as the logic expression P = ((NOT A) OR B) NAND C
An example logic circuit containing three inputs
- This logic circuit above can be expressed as X = NOT (A NAND B) OR (B NOR C)
Exam Tip
- You may be required to write a logic expression/statement from a truth table or a logic circuit. You may also have to do the opposite - draw a logic circuit and complete a truth table for a logic expression
Worked example
Consider the logic statement: X = (((A AND B) OR (C AND NOT B)) XOR NOT C)
a. Draw a logic circuit to represent the given logic statement.
[6]
One mark per correct logic gate, with the correct input