Fig. 3 I/O At Task Start And Real-Time Resources Exceeded; Fig. 4 I/O Not At Task Start And Real-Time Resources Not Exceeded; Fig. 5 I/O Not At Task Start And Real-Time Resources Exceeded - Beckhoff FC3102 Documentation

FC310x as master
Fig. 3: I/O at task start and real-time resources exceeded
This case would still not be a problem, because the DP cycle was completed within the available time. If "I/O
at task start" is not selected, then the process is somewhat more critical, as is described below.
I/O not at task start
If the setting "I/O at task start" is not selected (checkbox) for the task (default for PLC task), the system
checks before the task starts whether the previous DP cycle was completed, and the inputs are copied. After
this the task is processed, and at the end of the task the outputs are copied and the DP cycle is started. In
the example, the task cycle time is 2 ms, and real-time resources are 80%:
Fig. 4: I/O not at task start and real-time resources not exceeded
In the event of "I/O not at task start" the task and the PROFIBUS have to share the bandwidth. Exceeding of
the real-time resources therefore has a much stronger effect than for "I/O at task start":
Fig. 5: I/O not at task start and real-time resources exceeded
In the case described, the DP cycle starts later, and is no longer finished within the time available before the
following task cycle begins. The effect of this is that it is seen before the task is executed that the previous
DP cycle has still not been completed. It follows that inputs are not copied before starting the task, so that
the task computes with the old inputs; after the task has been processed no outputs are copied, nor is the
DP cycle restarted, so that a DP cycle is omitted. The omission of a DP cycle can be detected with the
CycleCounter, as described in the Master Diagnostics [} 20] chapter.
FC3101 and FC3102 Version: 3.0 13