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2.2. Function of the monitoring concept
While the program is running the monitoring relay is under the
complete control of the software user. A parallel contact from
the safety circuit for example can be evaluated as an input and
the monitoring relay can be switched off. For further safety the
appropriate national regulations must be applied.
If a µcontroller error occurs while the program is running the
watchdog signal switches the relay off so that important parts of
the plant can be protected.
When creating the program the programmer has to make
sure the program is left in a safe state (so that automatic
operation is reset) in the case of an internal (e.g. watchdog)
or external error (e.g safety circuit). For this purpose the
outputs in question have to be switched off by software.
If an output to be monitored is permanently switched and the
contact of the monitoring relay is welded it is not possible to
switch off this output. However, since the relay is always
switched load-free in normal operation, the contact wear is very
low.
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