Industrial Engineer · phase 2 of 16
DO / Discrete Output
On or off output used to command a relay, solenoid, starter, lamp, or other load.
The Explain card
- Plain English
- A discrete output is the controller saying on or off to the world. The card closes a contact or switches a transistor, and a relay, valve or lamp responds.
- Example
- The run bit for a conveyor lands on a discrete output that drives the coil of a motor starter. The starter carries the motor current, never the card.
- Why it matters
- Outputs are where a program bug becomes a physical event. Know each card's contact rating and what its channels do on a fault, because a stuck output can keep a valve open with nobody asking it to.
- Hook
- The bit that can actually hurt someone.
Word knowledge
How the word is built, where it came from, and what it sits beside in memory.
In a sentence
The PLC's DO energizes the interposing relay after the ladder-logic permissives become true.
Letter by letter
- D Discrete distinguishes two-state signals from continuously varying signals
- O Output marks the signal direction as leaving the controller
Where it came from
- Origin
- English controls-engineering initialism formed from discrete output
- Entered the language
- 20th century
- What changed
- The established engineering words were compressed into a label for drawings, terminal schedules, module types, and PLC tags.
How it is spelled
- Capitalisation
- Written DO in uppercase, including mid-sentence; Discrete Output is commonly title-cased in labels and headings.
- Plural and possessive
- The plural is DOs, the singular possessive is DO's, and the plural possessive is DOs'.
- Out loud
- Spelled out letter by letter.
Same family
- DI
- AI
- AO
Broken into chunks
-
discrete
- discretely
- discreteness
-
output
- input
- outputting
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.
