BECKHOFF DK9222-0211-0021 - Manual (Page 2)

I/O
XFC
Application Note DK9222-0211-0021
in a simple manner in a central place in the machine controller without requiring external black boxes. XFC technology is
completely compatible to existing solutions in all areas and can be used simultaneously on the same hardware and software
platform as other control processes such as PLC and motion control, for example.
Validity of process data
Each PLC-based controller works cyclically: it communicates with its environment by receiving input data, processing these on
the basis of the calculation specification (’program’) and then outputting them to the field as output data. Ideally, the fieldbus
cycle can be executed so quickly that refreshed input data are already present at the beginning of the next program cycle.
Depending on the requirements, these cycle times vary between 50 µs to over 100 ms, and are dependent on the fieldbus used
and the performance of the controller. Independent of the PLC cycle time, the cycle time of the fieldbus must also be taken into
account: Not every fieldbus technology available on the market permits synchronous operation of the PLC and fieldbus with
low cycle times, so that a further temporal inaccuracy can also arise here.
cycle time
task task
synchronous operation of PLC and fieldbus
I/O update per fieldbus
Fig. 1 Synchronous operation
Due to this cyclic orientation, however, an event that occurs in reality at a certain, singular point in time can only be assigned
temporally by the PLC in a certain pattern, i.e. the length of the cycle time. If the controller works, for example, with a cycle
time of 10 ms, the edge change at the read-in input actually took place ’sometime’ within the preceding 10 ms before the I/O
update. A more exact assignment than the cycle time resolution of 10 ms is not possible.
PLC
point
real-time
cycle time used = maximum resolution of an event
time
time
Fig. 2 Temporal relation
New Automation Technology Beckhoff 2
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