Industrial Engineer · phase 1 of 16 · noun

Watchdog Timer

Diagnostic timer that faults or resets the controller when program execution exceeds an allowed time or stops responding.

The Explain card

Plain English
The watchdog is a timer the controller resets every scan. If a scan takes too long or the program stops responding, the timer runs out and the controller faults rather than running blind.
Example
A loop in structured text never exits. The scan runs past its limit, the watchdog trips, and the outputs go to their safe state instead of holding the last command forever.
Why it matters
A controller that hangs with outputs frozen on is a machine nobody is driving. The watchdog turns that into a visible fault with a timestamp in the log.
Hook
The dog barks when the program stops walking it.

Word knowledge

How the word is built, where it came from, and what it sits beside in memory.

In a sentence

On the CompactLogix, a FOR loop in structured text never completed, so the continuous task's watchdog timer expired and the CPU posted a major fault.

Why these words
  • watchdog English compound of Old English wæcce, a vigil, and docga, dog marks the timer as an overseer, so the phrase is about keeping watch, not timing a process step
  • timer Old English tima, a stretch of time, plus the instrument suffix -er supplies the head noun, turning that overseer into a clock the program must retrigger
Where it came from
Origin
English computing compound from the closed compound watchdog (Old English wæcce, a vigil, plus docga, dog) and timer (Old English tima, a stretch of time, plus the instrument suffix -er)
Entered the language
1970s
What changed
Watchdog first named a dog kept to guard a house, then a person or committee that oversees a trust. Watch in the compound is the vigil, not the pocket watch. Timer first named a person who times a race, then a device that measures an interval. Microprocessor manuals of the 1970s joined the two words for a hardware counter firmware had to retrigger, and PLC catalogs kept the same pairing on the processor.
How it is spelled
Pattern
tch spells /tʃ/ after a short vowel, so watch sits with catch and match
Pattern
drop silent e before the suffix -er, so time becomes timer as late becomes later
Pattern
watchdog is a closed compound, written as one word like underdog and hotdog
Breaks the pattern
the a in watch is rounded to /ɑ/, unlike the /æ/ of catch and match, because w rounded it
Breaks the pattern
the noun phrase stays two words; English never writes watchdogtimer

Spelled like

  • catch
  • match
  • later
  • underdog
  • hotdog
Broken into chunks
  • watch
    • watchman
    • watchful
    • overwatch
  • tch
    • catch
    • match
    • batch
  • dog
    • underdog
    • hotdog
    • doghouse
  • time
    • overtime
    • timeline
    • untimely
  • er
    • later
    • baker
    • rider
What it sits beside

Same subject

Same shape

  • watchdog group
  • watchdog agency
  • egg timer
  • kitchen timer

Open timer compounds

  • egg timer
  • kitchen timer
  • oven timer

Watchdog compounds

  • watchdog group
  • watchdog agency
  • watchdog circuit

Controller diagnostics

Where it sits in the deck

Phase 1: PLC Controller Hardware

The box itself, from processor and rack to the modules that plug into it, because nothing else in the deck lands until you know what a PLC physically is.

Neighbours in this phase