Industrial Engineer · phase 4 of 16 · noun
Compare Instruction
Instruction that evaluates relationships such as equal, not equal, greater than, or less than.
The Explain card
- Plain English
- A compare instruction tests two values against each other and passes power if the relationship holds. Equal, not equal, greater, less, and the two or equal versions.
- Example
- A rung with a greater than compare on Tank_Level against 90 feeds the high level alarm coil. When the level crosses 90 the rung goes true.
- Why it matters
- Compares are how analog values turn into decisions. An equal compare on a REAL almost never matches, and a compare against a raw count instead of a scaled value gives an alarm at the wrong place with no error anywhere.
- Hook
- Turn a number into a yes or no.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
A Compare Instruction checks the analog pressure input against the high-pressure setpoint before the PLC energizes the pump trip output.
Why these words
- compare Latin comparare, to pair or match, through Old French comparer specifies the operation named by the phrase
- instruction Latin instructio, arrangement or direction, from instruere, to build or prepare identifies the expression as a command in a PLC program
Where it came from
- Origin
- Modern English technical compound formed from compare and instruction
- Entered the language
- 20th century
- What changed
- Compare moved from general examination or likening into formal logic and computing, while instruction moved from teaching or direction to a machine-executable command.
How it is spelled
- Pattern
- compare retains a final silent e
- Pattern
- instruction retains ct before the suffix -ion
Spelled like
- prepare
- construction
- protection
Broken into chunks
-
com
- combine
- compose
-
pare
- parity
- peer
-
struct
- structure
- construct
-
ion
- operation
- protection
What it sits beside
Same subject
- analog input
- setpoint
- PLC scan
- limit instruction
Same shape
- Move Instruction
- Timer Instruction
- Counter Instruction
IEC 61131-3 comparison operators
- EQ
- NE
- GT
- GE
- LT
- LE
Where it sits in the deck
Phase 4: PLC Memory, Tags, and Instructions
The data a program moves and the instructions that move it, from a single true or false value up to a structured array.
