Industrial Engineer · phase 3 of 16 · noun
Latch / Unlatch
Instructions that set or clear a stored Boolean state until an opposing instruction changes it.
The Explain card
- Plain English
- A latch sets a bit and leaves it set, and an unlatch clears it. Neither one cares what happens on the next scan, so the bit holds until the opposite instruction fires.
- Example
- A fault bit is latched when a motor overload trips. It stays on through the alarm display until the operator presses reset, which fires the unlatch.
- Why it matters
- Latched bits remember, which is what you want for alarms and what you do not want for outputs that should follow a condition. A latched output that nobody unlatches is a motor that keeps running after the logic said stop.
- Hook
- It stays put until something tells it otherwise.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
The technician placed the Latch instruction on the conveyor motor command rung and paired it with an Unlatch instruction on the stop rung.
How it is built
- latch base a catch or fastening
Where it came from
- Origin
- Middle English lacchen, from Old English læccan, meaning to seize or grasp
- Entered the language
- 14th century
- What changed
- The sense moved from grasping to a door fastener, then to holding mechanisms and control terminology.
How it is spelled
- Pattern
- The spelling ends in -tch after a single short vowel.
Spelled like
- catch
- hatch
- match
- patch
Broken into chunks
-
latch
- latched
- latching
- unlatch
What it sits beside
Same subject
- seal-in circuit
- set coil
- reset coil
- Boolean tag
Same shape
- batch
- catch
- hatch
- match
- patch
opposing PLC instruction pairs
- Latch and Unlatch
- Set and Reset
Where it sits in the deck
Phase 3: PLC Program Organization and Execution
How the logic is arranged and how the scan actually runs it, which is the difference between code that works and code that works every time.
