Glossary · PLC and control programming
Latch
Also known as: Seal-in circuit, Self-holding circuit, Latching
German: Speicherfunktion
In control engineering, a latch is a logic function that keeps an output in its state after the triggering signal disappears, until a separate reset condition occurs; in PLC code it is realized with a seal-in rung, set/reset coils or an SR/RS function block.
- PLC programming
In one sentence
A latch keeps an output on after the triggering signal ends, until a reset condition occurs, for example with a seal-in rung.
Example
A short press on the start button latches the conveyor motor on; it stays on until the stop button or a fault breaks the latch.
How it applies
- Engineering: Latches turn momentary commands into lasting states: start/stop circuits, alarm memories, fault states that need acknowledgment. The classic seal-in circuit uses the output's own contact in parallel with the start button.
- Priority: When set and reset are active at the same time, the design must define which wins. Stop and fault conditions usually take priority over start (reset-dominant).
- Safety: After a power failure or a stop, a latch must not restart hazardous motion by itself. Restart behavior is a safety topic, see Restart prevention and Manual reset. Whether a latch should be retentive therefore needs a deliberate decision.
- Documentation: Describe latched states in the operating manual in terms of what the user sees and does: which alarms stay active until acknowledged, what must be pressed to restart.
Latch vs. set-reset flip-flop
A latch is the function: holding a state. A Set-reset flip-flop is one way to implement it, with explicit set and reset inputs and a defined dominance. A seal-in rung is another.