# Guy and Seth on Simulink

## Accessing Block Data During a Simulation

This week I discovered that event listeners also work with bus signals!

You might be saying: "What is an event listener?"

In this post, I will describe how to register an event listener for a bus signal to display signal values on a figure. Here is the final result:

If you want to see a complete example that is really good, I recommend the File Exchange submission Simulink Signal Viewing using Event Listeners and a MATLAB UI by Phil Goddard. It has been selected as pick of the week some time ago.

Let's see how this stuff works.

Event Listener

An Event listener gives you a way to access block data during simulation. A typical use case is to display simulation data in a MATLAB GUI while a model is running. The key function you need to know about is add_exec_event_listener.

Setup a nonvirtual bus

Before setting up the event listener, we need to prepare a model. Let's create a simple test model including a bus with 2 elements:

We need to make the bus nonvirtual. For that, you can manually follow the steps outlined in Creating Nonvirtual Buses. Alternatively, here is a script that should do it automatically:

Registering the Event Listener

The event listener must be registered while the model is running using add_exec_event_listener. Because of this requirement, a good place to register the listener is in the model StartFcn callback. Here is what it looks like:

The callback function

In the image above, you can see that I register a function displayBusdata to execute after the block's Outputs method executes.

When it calls this function, the Simulink engine passes to it the Simulink.RunTimeBlock object for this block. This object allows you to access everything relevant to this block. For example, if you want to display the value of the bus elements at the MATLAB prompt, the registered function could be as simple as:

If you want to go more complex and display the values in a MATLAB figure. Here is an example where I plot the sum of the two bus elements:

I hope this inspires you to create nice MATLAB GUIs to interface with your simulations. Share your event listener stories by leaving a comment here.

### 3 Responses to “Accessing Block Data During a Simulation”

1. Sean replied on :

This seems like a good example that would work well with either a persistent variable for “h”, or an application of nested function handles. Using either approach, you could avoid running “findobj” twice for each iteration.

I would assume you would be worried about optimizing the speed of this event code.

2. wei replied on :

@Guy, Could you discuss simulation performance with event listener? My past experience is that it slows down simulation quite a bit.

Further, is there way to profile such application, and handle graphics speed-up tips?

3. Guy Rouleau replied on :

@Sean, thanks for the suggestion. I agree my code could be more optimal.

@Wei, event listeners definitely slow down simulations. The effect can be classified in the same category as MATLAB S-Functions. I do not recommend using this technique if simulation speeds is very important for you application.

I find that event listeners are useful to put a GUI on top of desktop simulations that run close to real-time and interact with it. In many cases, this type of simulation is slowed down using one of those blocks:

http://www.mathworks.com/help/aeroblks/simulationpace.html

http://www.mathworks.com/matlabcentral/fileexchange/21908

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Guy Rouleau and Seth Popinchalk are Application Engineers for MathWorks. They write here about Simulink and other MathWorks tools used in Model-Based Design.

These postings are the author's and don't necessarily represent the opinions of MathWorks.