You can 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 diagrammatically (see Model a 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 (see Using Function-Call Subsystems for a description of function-call subsystems)

Self-triggering subsystems and loops containing non-latched triggered subsystems (see Using Triggered Subsystems in the Using Simulink

^{®}documentation for a description of triggered subsystems and Inport in the Simulink reference documentation for a description of latched input)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`

(`sl_subsys_trigerr1`

)`simulink/Ports&Subsystems/sl_subsys_semantics/Triggered subsystem/sl_subsys_trigerr2`

(`sl_subsys_trigerr2`

)`simulink/Ports&Subsystems/sl_subsys_semantics/Function-call systems/sl_subsys_fcncallerr3`

(`sl_subsys_fcncallerr3`

)

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

To detect whether your model contains invalid loops, select **Update
Diagram** from the model's **Simulation** menu.
If the model contains invalid loops, the invalid loops are highlighted.
This is illustrated in the following model (open),

and displays an error message in the Diagnostic Viewer.

If there are two Model files with the same name (e.g. `mylibrary.slx`

)
on the MATLAB^{®} path, the one higher on the path is loaded, and
the one lower on the path is said to be "shadowed".

To help avoid problems with shadowed files, turn on the Simulink preference **Do
not load models that are shadowed on the MATLAB path**. See Do not load models that are shadowed on the MATLAB path.

The rules Simulink software uses to find Model files are similar to those used by MATLAB software. See What Is the MATLAB Search Path? (MATLAB) in the MATLAB documentation. However, there is an important difference between how Simulink block diagrams and MATLAB functions are handled: a loaded block diagram takes precedence over any unloaded ones, regardless of its position on the MATLAB path. This is done for performance reasons, as part of the Simulink software's incremental loading methodology.

The precedence of a loaded block diagram over any others can have important implications, particularly since a block diagram can be loaded without the corresponding Simulink window being visible.

When using libraries and referenced models, you can load a block diagram without showing its window. If the MATLAB path or the current MATLAB folder changes while block diagrams are in memory, these block diagrams can interfere with the use of other files of the same name.

For example, open a model with a library called `mylib`

,
change to another folder, and then open another model with a library
also called `mylib`

. When you run the first model,
it uses the library associated with the second model.

This can lead to problems including:

Simulation errors

"Unresolved Link" icons on blocks that are library links

Wrong results

To help avoid problems with shadowed files, you can turn on
the Simulink preference **Do not load models that are
shadowed on the MATLAB path**. See Do not load models that are shadowed on the MATLAB path.

When updating a block diagram, the Simulink software checks the position of its file on the MATLAB path and will issue a warning if it detects that another file of the same name exists and is higher on the MATLAB path. The warning reads:

The file containing block diagram 'mylibrary' is shadowed by a file of the same name higher on the MATLAB path.

`mylibrary.slx`

is
being used. To see which file called `mylibrary.slx`

is
loaded into memory, enter:which mylibrary C:\work\Model1\mylibrary.slx

`mylibrary`

which
are on the MATLAB path, including MATLAB scripts, enter:which -all mylibrary C:\work\Model1\mylibrary.slx C:\work\Model2\mylibrary.slx % Shadowed

`mylibrary`

and
let the Simulink software load the file which is highest on the MATLAB path,
enter:close_system('mylibrary')

Here are some model-building hints you might find useful:

In general, more memory will increase performance.

More complex models often benefit from adding the hierarchy of subsystems to the model. Grouping blocks simplifies the top level of the model and can make it easier to read and understand the model. For more information, see Create a Subsystem. The Model Browser provides useful information about complex models (see

**Simulink Editor**).Well organized and documented models are easier to read and understand. Signal labels and model annotations can help describe what is happening in a model. For more information, see Signal Names and Labels and Describe Models Using Notes and Annotations.

If several of your models use the same blocks, you can save these blocks for easy reuse. For example, you can save a collection of blocks in a custom library. Then, when you build new models, you can copy these blocks from the library.

Generally, when building a model, design it first on paper, then build it using the computer. Then, when you start putting the blocks together into a model, add the blocks to the model window before adding the lines that connect them. This way, you can reduce how often you need to open block libraries.

- Model a Continuous System
- Best-Form Mathematical Models
- Model a Simple Equation
- Model Differential Algebraic Equations