Glossary · PLC and control programming
Watchdog timer
Also known as: Watchdog, Cycle time monitoring
German: Watchdog-Zeitgeber
In control systems, a watchdog timer is a supervision function that must be retriggered within a set time by the monitored program or device; if it is not, the watchdog detects a fault and initiates a defined reaction, such as stopping the controller or switching outputs off.
- PLC programming
- Functional safety
In one sentence
A watchdog timer detects a stalled or overrun program when it is not retriggered in time and triggers a defined fault reaction.
Example
The controller's cycle watchdog is set to 150 ms; when an endless loop blocks the program, the watchdog puts the controller into STOP and the outputs switch off.
How it applies
- Engineering: PLC operating systems monitor task and cycle times with watchdogs. Communication watchdogs detect lost connections to remote I/O or drives. The reaction (stop, alarm, substitute values) is configured and must match the intended Fault reaction.
- Functional safety: In safety controllers, watchdogs are part of the diagnostic measures that detect faults in the logic and processing, and contribute to the Diagnostic coverage (DC). Their contribution is established in the safety system's design, not assumed from the presence of a watchdog.
- Commissioning: Set watchdog times with margin above the measured maximum cycle time, but short enough to detect real faults quickly.
- Documentation: Record watchdog settings and the resulting behavior. Service documentation should explain what a watchdog stop looks like and how to diagnose its cause.
Watchdog vs. timeout
A Timeout limits how long one operation may take. A watchdog checks continuously that a program or device is still alive. Both detect missing activity; the watchdog supervises the whole system.