SubSystems are treated by other elements as if they were a single element (analogous to a complex Expression element). This has an important implication for systems that include feedback loops. In particular, if a SubSystem contains elements with state variable outputs, and these state variable outputs are referenced outside of the SubSystem, they will not be treated as state variables outside of the SubSystem.
As a result, if you try to create a feedback loop by referencing a state variable contained within the SubSystem outside of the SubSystem, GoldSim will not be able to create the loop (and will provide an error message indicating that creating such a link would result in a recursive system).
In most cases, the most appropriate way to “close” such a loop is to reference the state variable from the SubSystem in a Material Delay element (whose primary output is a state variable) located outside of the SubSystem.
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