A.3.3.TR:4.3 - Combine jointly active relations and alternative actions

Relations that must hold during the same modeled change are imposed together. For two coupled carts, both force equations and the fixed-separation condition hold together. The variables shared by those equations must keep the same meanings, reference frames and units.

Actions that provide alternative ways to take the next step are joined as alternatives. In the counter case, the next action can be an enabled read or write by either participant. Requiring every one of those actions in the same step would describe a different computation.

Check how participants interact through shared values. A local action’s condition and effect refer to the global state at the selected step. If actions occur together, define their joint effect or derive it from the governing relations; textual order alone supplies neither simultaneity nor a conflict-resolution rule.

For equation-based modeling, keep the coupled relations available while choosing a computation. A solver’s evaluation order is part of obtaining a solution. If the actual system has a communication delay, sequential update or other timing effect, represent that effect in the modeled rule. Modelica’s instantaneous event semantics is one explicit modeling convention; it requires modeled delays when those delays matter.

A.22.CGUS can help expose alternative continuations and their conditions when that is the working question. The present construction supplies the rules used to judge those alternatives. The jointly applicable equations can also determine a single continuation.

A.3.3.TR:4.3.1 - Combine continuous evolution with events

When the model mixes continuous evolution and discrete events, state the flow law in each mode, where that flow is permitted, each event’s condition and timing, and its reset relation. A reset specifies which values change and which remain continuous.

For a condition that triggers an immediate event, evolve only to its first occurrence. Stop the continuous segment, apply the event relations, and resume from the resulting state under the applicable law. Process an already-enabled immediate event before advancing time. If the reset enables another immediate event, resolve that event according to the model before resuming flow. Incompatible event relations or an unresolved sequence of instantaneous changes are reasons to return a model or computation limitation.

An event that is merely permitted has a different timing rule: retain the allowed waiting and choice. Where simultaneous events can conflict, supply their joint relation or a justified priority. The thermostat in :5.4 uses mandatory immediate switching and a reset that preserves temperature.