6. The OR logic gate¶

The OR logic gate has two or more inputs and one output. Its symbol is the following:

Symbol of the two-input OR logic gate.¶

The output has a logic high value (1) if any of its inputs has a logic high value (1). That is, if input A or input B is high, the output will be high. Hence the name OR in English.

The logical OR function is represented by a sum, so that the output of the gate will be the logical sum of the inputs:

$Out = A + B$

If both inputs are zero, the output will be zero, but if either input is one, the output will be one.

The truth table of the logical OR function is as follows:

Tabla de verdad de la puerta lógica OR de dos entradas.

Truth table of the two-input OR logic gate.¶

The NOR logic gate¶

The NOR logic gate has two or more inputs and one output. Its symbol is the following:

Symbol of the two-input NOR logic gate.¶

The output will be the same as that of an OR gate, but inverted. That is, the output will only be worth one when all the inputs are worth zero.

The NOR logic function is represented by a negated sum, so that the output of the gate will be the logical sum of the inputs that is finally inverted:

$Out = \overline{ A + B }$

The truth table of the NOR logic function is as follows:

Tabla de verdad de la puerta lógica NOR de dos entradas.

Truth table of two-input NOR logic gate.¶

Simulation¶

In the following simulation we can see the operation of the OR logic gate and, below, the operation of the NOR logic gate.

Exercises¶

Draw the OR logic gate symbol, its logic function, and its truth table.
Draw the NOR logic gate symbol, its logic function, and its truth table.
Check with the simulator that the truth table of the OR function and that the truth table of the NOR function are correct.
In the simulator, add an inverting gate to the output of the OR gate and check that its response is the same as that of the NOR gate.
Draw a three-input OR logic gate and its truth table.