NOT Gate
The NOT gate is used to indicate that the output occurs only when the input event does not occur. There is only one input to a NOT gate. The output is always opposite of the input gate or event.
Summary of Logic:
The output is the opposite of the input gate or event.
A truth table for a NOT gate follows. The Boolean equation for a NOT gate is T = ~A.
A | Output |
|---|---|
T | F |
F | T |
Example
A traffic light system is installed at the crossing of two mono-directional roads. Assume that the light acts properly and is red for road 1 and green for road 2. The following figure depicts the scenario.
Given this scenario, the following events can occur:
◦ A = Car A fails to stop
◦ B = Car B fails to stop
◦ C = Car C fails to continue
An accident can occur in two ways:
◦ Car A acts properly and stops (~A) AND Car B fails to stop (B).
◦ Car A fails to stop (A) AND Car C continues to move towards area 2 in the crossing (~C).
The fault tree for this example has two NOT gates.
