DIGITAL LOGIC - AND GATE

1.7 DIGITAL LOGIC - AND GATE

 General

The AND gate is a device which requires all inputs to be present simul­taneously before an output will be generated. It can have any number of inputs. The symbol for an AND gate is shown below. The input at A and B must be 1 for the output at C to be a 1. The notation used to express a logic statement is called Boolean algebra. The Boolean statement for an AND gate is C = A-B and is read "C equals A AND B".

C = AB is another way of writing the AND function.

 Function

The function of an AND gate is similar to switches in series. Both A and B must have an input to energize both relays to get an output at C.

 Truth Table

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

  Example

The example below shows a simple safety device utilizing an AND gate which requires both switches to be closed to operate the motor.

 Timing Diagram

The timing diagram shows that only when both switches are closed will the motor operate. Even though the foot switch was closed sooner than the hand switch, the motor did not operate until both switches were closed simultaneously.

Figure 1.3: Digital logic - AND gate