IFM Electronic ecomot mobile R 360 Serie Starterset-Handbuch Seite 142

Features of a PLC
At that very moment somebody flips the selector switch. If the state of the input
were enquired about again, the condition for the next program step would also
be fulfilled: the output "move to the right" would also be switched with a possibly
spectacular result.
It may also come to conflicts which cannot be seen and prevented during
program creation.
Unambiguous Thus the meaning of cyclical program processing is for example to ensure
signal states unambiguous signal states during program processing. The program works
with the process data image.
This is also of importance for the outputs. It is e.g. possible to switch an output
to TRUE and then again to FALSE 10 times during program processing. This
cannot result in "chattering" of the output, i.e. constantly changing its state. The
state is clearly defined. During program processing only the state of one bit in
the process data image changes. The last state is switched, i.e. provided to the
physical outputs. The only objection to be raised is that the program structure is
unclear.
Priority This may seem quite theoretical but it does have some consequences for
practical use. The priority of instructions can be determined in the program by
their sequence!
If, for example, there is an instruction in the program:
then it is usually placed behind the instruction for switch-on. Usually the
conditions for switch-on and switch-off should not be fulfilled at the same time.
If, however, due to a malfunction this does occur, then switch-off takes priority
(compare pages 4-2 and 4-3).
Thanks to the cyclical program processing, reliability is provided due to
unambiguous signal states. However, other consequences have to be put up
with.
1-6
if limit switch reached, then stop drive