A PLC watchdog timer monitors whether the controller continues executing its program. The controller software must provide regular updates, often called “kicks,” within a configured timeout. If an update does not arrive before the timeout expires, the watchdog treats the PLC as unresponsive and initiates corrective action.
Watchdog Timer Operating Sequence
- The watchdog starts a predefined timeout interval.
- The PLC program provides an update while the CPU executes normally.
- Each valid update confirms continued execution and restarts the monitoring interval.
- If no update arrives before the interval expires, the watchdog detects a fault condition and triggers corrective action, such as resetting the PLC to a known state.
Faults the Watchdog Helps Detect
| Condition | Watchdog indication | Corrective response described by the evidence |
|---|---|---|
| Software hang or bug | Expected update stops | Trigger a controller reset |
| Hardware failure affecting execution | CPU no longer provides updates | Return the PLC to a known state |
| Communication error affecting operation | Required update does not arrive | Initiate corrective action |
The watchdog detects missed execution updates; it does not identify which of these conditions caused the failure. Diagnose the underlying software, hardware, or communication problem after recovery.
Configure the Timeout for the Application
PLC manufacturers integrate watchdog timers into their hardware, while users configure the timeout according to application requirements. The evidence provides no universal timeout value. Select the period only after determining how long valid PLC execution may take under the application’s expected operating conditions.
A timeout that expires during valid execution can cause unnecessary resets. A timeout that is too long delays detection of a stalled controller. Record the selected interval and the execution condition used to justify it.
Test Watchdog Recovery
Use proper programming practices and test the watchdog regularly. Verify that normal execution supplies updates before the configured timeout, then confirm that withholding an update causes the intended corrective action and returns the PLC to the defined known state. In a safety-critical application, do not treat watchdog operation alone as proof of safety performance; verify the complete safety design against its applicable requirements.
FAQ
What causes a PLC watchdog timer to trip?
It trips when the watchdog does not receive the expected software update within the predefined timeout interval. Possible underlying causes include a software hang, hardware failure, or communication error.
What happens when a PLC watchdog timer expires?
The watchdog assumes the PLC is not operating correctly and initiates corrective action. The evidence specifically identifies an automatic reset that returns the controller to a known state.
How do I choose a PLC watchdog timeout?
Base the timeout on the application’s valid execution requirements, then test normal operation and missed-update recovery. No universal timeout value is supported by the supplied evidence.
