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5.4.
Remarks for solutions
We pointed out above:
There is hardly ever one and the optimum solution.
Furthermore it is better at the beginning to find solutions on one's own instead
of adopting ready-made patterns.
Therefore we will give only some hints as suggestions.
Example 1
The second network defines the state of the output. The first one can be left
(page 4-3)
out.
It would serve the obviously intended purpose to combine the two networks and
to link the results of the AND operation with an OR operator. Here you can see
again that an S (set) function is of advantage. If you had an S before the output
of networks 1 and 2 there could be no conflicts. You need, however, more
networks for resetting.
Task 1 (page 4-
To begin with, simple logic functions are sufficient. Using SET/RESET is
4)
somewhat more complex. This could only become necessary for a more precise
or changed definition of the task. An example in this context:
Should be no problem (remember the priority!)
Task 2 (page4-
4)
Task 3 (page 4-
It is confusing to program very many links in one network. It is also confusing to
5)
have one network for each link. The point is find a happy medium. In the
example network 2 does not look too confusing. What can you find in network 1
(remember useful priorities!)?
Figure 138: Example 1
We should briefly comment on the timer function. The preset value PT (preset
time) is fixed here. It cannot be changed while the program is running. The only
possibility is to change the source program. This may be useful in some cases.
If, however, it should be possible to change the value, e.g. by the engine driver
via an input device, this value has to be declared to be a symbolic variable
which can be changed by other POUs. The actual value ET (elapsed time) does
not have to be assigned to a variable in each case. In Figure 138 above you
can see when this may be useful, e.g. for displaying this value.
R
Appendix
5-13
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