Glossary
What is Permissive?
A condition that must be true before a device is allowed to start, but which does not stop it once it is running.
For example: the pump may start only if the suction valve is open and the source tank is above its minimum. If closing that valve must stop the pump, that is a separate interlock.
Keeping permissives and interlocks apart makes "why will the pump not start?" quick to answer: show the unmet permissives on the screen.
Related terms
- Interlock
- A condition that prevents a device from starting, or stops it, to protect the process or people, whatever the operator wants.
- Seal-in (latch)
- A pattern that keeps an output running after a momentary button press, by feeding the output back into its own start condition.