There are three main types of signals in any typical physical modeling block diagram: 1. Standard Simulink signals: Ports that take in/ouput these signals have open arrowheads;
2. Physical signals: Ports that take in/ouput these signals have closed arrowheads;
3. Domain-specific signals. Ports that take in/ouput these signals have round circles;
The under-the-hood representation for these signals is different. You cannot inter-connect any two of these signals or even physical signals from different domains.
Use the Sensors and Sources blocks in the respective Domain library to convert the Domain Signals to Physical Signals and vice-versa. Then use the 'Physical to Simulink' or 'Simulink to Physical' blocks in the 'Utilities' library of Simscape. You can then interface the two domains ( Simscape and SimPowerSystems in your case ) using the Simulink signals.
Look at some demo models for detailed reference.
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