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7.6. Dynamic Semantics for the Cyclic Execution
It has to be acknowledged that not for all objects with outputs those cannot be calculated in
parallel for the dynamic semantics of a gMainFunctionBlock and gSubFunctionBlock graph.
Furthermore, it is meaningful if first the outputs are calculated, which then are used as inputs
for other objects. In a simple chain or rather an open loop [30], the first element can easily be
identified at the beginning of the open loop. Figure 7.13 shows a simple example for an open
loop. In this example, in every execution step k the outputs are calculated in the following
Sensor Controller Actuator
Figure 7.13.: Simple example of an open loop
order:
1. Sensor
2. Controller
3. Actuator
Also, the definition for closed loops is possible if they still have a “start” element, which normally
only have outputs as in Figure 7.14.
Sensor + Controller
-
Actuator
Figure 7.14.: Simple example of an closed loop
Similar to the open loop example the execution order is accordingly:
1. Sensor
2. Sum (“+”)
3. Controller
4. Actuator
When the Sum is calculated, the new output of the Controller is not yet available. Hence,
for such feedback flows the last output value is taken. In other words, the negative input
of the Sum is in calculation k the output of the Actuator in calculation 14 k − 1. This also
demonstrates that it is necessary that all data flow elements must have initial output values if
they have outputs to avoid undefined calculation results for k = 0.
In rare cases, it also may occur that a closed loop does not have a beginning or ending
element. For example, Figure 7.14 without the Sensor. Then the starting element would be
once chosen arbitrarily.
14 Although in the openETCS meta model actuators do not have outputs, here it is used for simplification.
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