DIGITAL LOGIC - OR GATE

1.8 DIGITAL LOGIC - OR GATE

The OR gate is a device which requires either input to be present for an output to be generated. It can have any number of inputs. The sym­bol for an OR gate is shown below. The input at either A or B or both must be a 1 for the output at C to be a 1. The notation used to express this logic statement is C = A+B, and is read "C equals A OR B".

  Function

The function of an OR gate is similar to switches in parallel. Either A or B must have an input to energize either relay to get an output at C.

 Truth Table

The various combinations of inputs for a two input OR gate and the resulting outputs are shown in the truth table.

Example

The example below shows a high pressure light which is lit when either the lamp test switch is closed or the pressure closes its switch. Note the high pressure light is also lit even when both the lamp test switch is closed simultaneously with the pressure sensor switch closed.

Timing Diagram

The timing diagram shows that when either switch is closed, the high pressure light is lit and when both switches are closed, the light is also lit.