1.3.4. Watchdog Handling

All IEC-tasks of an application can be supervised by the runtime system to avoid cycle time overruns. The watchdog function can be enabled and configured in the task configuration of the PLC application. The task will be terminated with error status (“Exception”), when the currently set watchdog “Time” gets exceeded, whereby the currently set “Sensitivity” is included. Two cases are possible:

1. Contiguous time overruns; the following is true:

   Sensitivity	Exception in cycle …
   0,1	1
   2	2
   …	…
   n	N

2. Single time overrun: Exception if the cycle time for the current cycle is greater than (Time * Sensitivity). Example: Time=t#10ms, Sensitivity=5 -> Exception as soon as the task (once) runs longer than 50ms. This serves to detect endless loops in the first cycle.

If the watchdog expires, the outputs are reset to its default values, if in the device dialog of the PLC application the entry “IO update while in stop” and “set outputs to default values” is specified.

By use of the library function in CmpIecTask.library a watchdog may be suspended for particular PLC cycles; this may be helpful for cycles requiring an extra amount of time due to initialization processes.

Having declared an appropriate variable of type RTS_IEC_HANDLE to handle the task

hIecTask : RTS_IEC_HANDLE;

deactivating and subsequent reactivating of the watchdog can be realized with help of the interface functions:

hIecTask := IecTaskGetCurrent(0);
IecTaskDisableWatchdog(hIecTask);
... // Code that is protected against watchdog
IecTaskEnableWatchdog(hIecTask);

1.3.4.1. Monitoring

The monitoring of the IEC variables is always done task-inconsistently. This is done because here no influence of the IEC tasks on the runtime can be guaranteed.

But in the future there will be an option to read IEC variables task-consistently.