Glossary
What is 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.
In ST it is one line: Pump := (Start OR Pump) AND NOT Stop;. The OR Pump part is the seal, and AND NOT Stop makes sure stop always wins.
On a real panel the stop button is wired normally closed so that a broken wire stops the motor, which is the fail-safe principle.
Related terms
- Interlock
- A condition that prevents a device from starting, or stops it, to protect the process or people, whatever the operator wants.
- Ladder logic (LD)
- A graphical PLC language drawn like relay circuits: rails, contacts and coils. The most widely used language in maintenance.
- SR and RS flip-flops
- Two standard blocks that hold a boolean state: SR is set-dominant, RS is reset-dominant.