Glossary · Automation fundamentals, platforms and components
Watchdog
Also known as: Watchdog timer
German: Watchdog
In automation and embedded systems, a watchdog is a timer-based monitoring function that must be retriggered regularly by the monitored program, processor or communication partner. If the retrigger does not arrive within the set time, the watchdog triggers a defined reaction, such as a controller stop, reset or switch to a safe state.
- Automation components
In one sentence
A watchdog is a timer that must be retriggered regularly; if not, it triggers a stop, reset or safe state, detecting hung programs or lost links.
Example
The PLC's cycle watchdog is set to 150 ms; an endless loop in a new function block exceeds it and the CPU goes to STOP with a diagnostic entry.
How it applies
- Engineering: Watchdogs monitor processor execution, PLC Scan cycle times and communication connections. Independent hardware watchdogs detect faults that software checks would miss.
- Safety: In safety-related systems, watchdogs are part of the diagnostic measures; the reaction must lead to a Safe state. Watchdog time settings in safety protocols affect the response time of safety functions.
- Documentation: Document watchdog times and reactions for each monitored element, and describe in the service manual how to find the cause after a watchdog trip and how to restart. Avoid setting watchdog times so generously that faults remain undetected.
Watchdog vs. heartbeat
The Sign of life (heartbeat) is the regular signal. The watchdog is the monitoring function that checks it. Together they detect a stopped partner.