Industrial Engineer · phase 2 of 16
DI / Discrete Input
On or off input from a field device such as a switch, sensor, or pushbutton.
The Explain card
- Plain English
- A discrete input reads one thing that is either on or off. A switch is closed or open, a sensor sees the part or does not, and the card turns that into a single bit.
- Example
- The guard door switch feeds a discrete input. When the door opens the bit drops and the logic stops the machine.
- Why it matters
- Most of the decisions a machine makes come from these bits, so a wrong input type, a wrong voltage or a chattering contact corrupts everything downstream. Read the module's indicator light before you read the tag.
- Hook
- Yes or no, and the machine believes whichever one it sees.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
The PLC reads the guard-door limit switch on DI 3 before allowing the conveyor motor starter to energize.
Letter by letter
- D Discrete identifies the signal class
- I Input marks the direction into the controller
Where it came from
- Origin
- English engineering initialism formed from Discrete Input; discrete ultimately comes from Latin discretus, meaning separated or distinct
- Entered the language
- 20th century
- What changed
- The older sense of discrete as distinct was adopted in control terminology and shortened to DI on drawings, modules, and software tags.
How it is spelled
- Capitalisation
- Written DI in uppercase, including mid-sentence; the expansion is usually written discrete input in running text
- Plural and possessive
- Plural DIs; singular possessive DI's; plural possessive DIs'
- Out loud
- Spelled out letter by letter.
Same family
- DO
- AI
- AO
Broken into chunks
-
discrete
- discreteness
- discretely
-
input
- output
- input/output
What it sits beside
Where it sits in the deck
Phase 2: PLC I/O and Field Signals
Where the controller meets the machine: discrete and analog signals, the wiring conventions behind them, and the field devices that send them.
