The AND Logic Operation

1. The AND Operation

AND is the logical conjunction operation. Use it when an output should turn on only if all required conditions are satisfied at the same time.

Output = Input_1 AND Input_2

The output is active if Input_1 is active and Input_2 is also active.

2. Where AND Is Used

AND is common in industrial automation, especially for operating permissives:

• start a motor only when Start is pressed and no fault is active;
• permit motion only when the door is closed and the drive is ready;
• open a valve only when pressure is available and the operator requests it;
• run a conveyor only when operation is permitted and it is not full;
• send a robot command only when the line is ready and there is no error.

3. The Basic AND Rule

The AND output is TRUE only when every input is TRUE. If any input is FALSE, the result is FALSE.

4. FBD Diagram

AND normally has several inputs and one output:

[Input_1] ─────┐
               ├── [AND] ──> [Output_1]
[Input_2] ─────┘

Read from left to right: input conditions enter AND, and its result is passed to the output.

5. Example: Starting a Motor

The motor should start only when Start is pressed and Safety_OK permits operation.

Motor_Run = Start_Button AND Safety_OK

Pressing Start is insufficient while Safety_OK remains FALSE.

6. Several Required Conditions

A practical diagram often has more than two conditions. A conveyor might require all four:

Conveyor_Run = Start_Button AND Emergency_OK AND Door_Closed AND Drive_Ready

If any permissive becomes FALSE, Conveyor_Run also becomes FALSE.

7. AND in Interlocks

AND makes it explicit that equipment operation depends on a set of conditions. This helps readers understand the control logic.

Use Door_Closed as a permissive so the condition reads naturally: the motor runs when the door is closed.

8. Simulator Practice

The lesson's example demonstrates the four-condition conveyor command from section 6.

• 1. Open the example and start the simulation.
• 2. Set Emergency_OK, Door_Closed, and Drive_Ready to TRUE, leaving Start_Button FALSE.
• 3. Check that Conveyor_Run remains FALSE.
• 4. Set Start_Button to TRUE and check that Conveyor_Run becomes TRUE.
• 5. Set any one input to FALSE and check that Conveyor_Run turns off.

For a two-input exercise, open the AND example in the examples catalog. Set Button_A TRUE while Button_B is FALSE: Lamp_On remains FALSE. Set both TRUE: Lamp_On becomes TRUE.

9. Key Points

• AND produces TRUE only when all inputs are TRUE.
• Use AND for permissives, interlocks, and start conditions.
• Clear names such as Safety_OK, Door_Closed, and Drive_Ready make diagrams easier to read.