Using Simulink

Avoiding Invalid Loops

Simulink allows you to connect the output of a block directly or indirectly (i.e., via other blocks) to its input, thereby, creating a loop. Loops can be very useful. For example, you can use loops to solve differential equations diagramatically (see Modeling a Simple Continuous System) or model feedback control systems. However, it is also possible to create loops that cannot be simulated. Common types of invalid loops include:

• Loops that create invalid function-call connections or an attempt to modify the input/output arguments of a function call
• Self-triggering subsystems and loops containing non-latched triggered subsystems
• Loops containing action subsystems

The Subsystem Examples block library in the Ports & Subsystems library contains models that illustrates examples of valid and invalid loops involving triggered and function-call subsystems. Examples of invalid loops include the following models:

• simulink/Ports&Subsystems/sl_subsys_semantics/Triggered subsystem/sl_subsys_trigerr1
• simulink/Ports&Subsystems/sl_subsys_semantics/Triggered subsystem/sl_subsys_trigerr2
• simulink/Ports&Subsystems/sl_subsys_semantics/Function-call systems/sl_subsys_fcncallerr3

You might find it useful to study these examples to avoid creating invalid loops in your own models.

Detecting Invalid Loops

To detect whether your model contains invalid loops, select Update diagram from the model's Edit menu. If the model contains invalid loops, Simulink highlights the loops

and displays an error message in the Simulink Diagnostic Viewer.

Modeling a Simple Continuous System

Tips for Building Models