The NOT Logic Operation

1. The NOT Operation

NOT is logical negation. It changes a signal to its opposite value:

Output = NOT Input

TRUE becomes FALSE; FALSE becomes TRUE.

2. The Basic NOT Rule

NOT has one input and one output and inverts the Boolean state.

3. FBD Diagram

[Input_1] ──> [NOT] ──> [Output_1]

Read from left to right: Input_1 enters NOT, and the opposite value reaches Output_1.

4. Where NOT Is Used

• prevent starting when a fault is active;
• permit motion when an obstacle sensor is not active;
• activate an alarm when a door is not closed;
• detect the absence of a part;
• express that Stop is not pressed.

5. Example: Blocking a Start During a Fault

Alarm_Active is TRUE when a fault is active. Starting requires the opposite condition: no fault.

No_Alarm = NOT Alarm_Active

When there is no fault, No_Alarm becomes TRUE and can serve as a start permissive.

6. Starting with a Fault Permissive

The motor should run when Start is pressed and no fault is active:

No_Alarm = NOT Alarm_Active
Motor_Run = Start_Button AND No_Alarm

An active fault prevents running even if Start_Button is TRUE.

7. Inversion and Signal Names

You can compute No_Alarm = NOT Alarm_Active or use a positively named permissive such as Emergency_OK. Names such as Safety_OK, Door_Closed, and Drive_Ready often reduce the number of inversions and make learning diagrams easier to read.

8. Example: Door Not Closed

Door_Closed is TRUE when the door is closed. Invert it to obtain the opposite condition:

Door_Not_Closed = NOT Door_Closed

The result can be used for a warning or start inhibit.

9. Simulator Practice

Open the lesson example and start the simulation. Its first path uses one NOT: Input 1 FALSE produces Output 1 TRUE; Input 1 TRUE produces Output 1 FALSE. The second path uses two NOT blocks, so Output 2 follows Input 2.

For another simple inversion, open the NOT example in the examples catalog. Door_Open FALSE produces Door_Closed TRUE; Door_Open TRUE produces Door_Closed FALSE. In edit mode, insert a second NOT after the first and verify that two inversions reproduce the original input.

10. Key Points

• NOT changes TRUE to FALSE and FALSE to TRUE.
• Use it to invert signals and express opposite conditions.
• When reading a diagram, account for every NOT in the signal path.
• Clear signal names can reduce unnecessary inversions.