Watchdog - IFM Electronic ecomot mobile R 360 Serie Starterset-Handbuch

What you should know

Watchdog

A PLC monitors itself if the cycle is correctly processed. The case that a
program cycle is not complete is dangerous. Then no inputs are read. A change
of the state on the input, e.g. "limit switch reached" cannot be processed. The
status of the output cannot change, e.g. the drive keeps on running although the
limit switch has been reached.
The watchdog function exists to avoid such states. If the end of the cycle has
not been reached within a certain time which is started with each new cycle the
watchdog becomes active. The controller is stopped and the outputs are
disconnected. This time does, of course, depend on a maximum possible cycle
time. In ecomat systems it is approx. 40 ms.
What can cause such an event?
1. Hardware error
Causes
If a processor fails, for example, the controller must pass into a safe state.
2. Program error
We will describe this second point here in more detail. Those who already have
some programming experience, for example on the PC, are especially
vulnerable to such errors. In a PC program a window can be created with the
text "Enter value" and an input field.Then the keyboard is enquired about if a
value has been entered. If so, the value will be processed. If not, the keyboard
will be enquired about again until the input has been made. It does not matter if
the user has a coffee break, a lunch break or does something else during that
time.
There must not be any closed loop in a PLC program; only the program cycle
itself may be such a loop. A possible cause of an unintended closed loop may
be a backward jump. Since this is dangerous, a warning is issued during
compilation of the program.
In the worst case it is possible that the communication between the PC and the
controller is impaired. Log-in is then no longer possible. In this case the
downloader tool ( page 3-36) will help.
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