Scope displays unexpected puls generator output

Question

I am having troubles understanding the output of my scope in this simple simulink model:

I am using a fixed step solver (tried with ode3 and ode8). Pulse type of the puls generator is set to Sample based and I varied the Period and Pulse Width.

First I set the simulation time to 10 and set the puls generator to Period = 10 and Puls width = 5. The output of the scope is as expected:

But when I tried with simulation time 10,000 and the puls generator with Period = 1,000 and Puls width = 500 it seems my scope is wrong:

Why is the first falling edge at 5,500? I used the Autoscale button every time.

Using sim time 100,000 and Period = 10,000 and Puls width = 5,000 I don't even get a single falling edge:

Even with longer simulation time there seems to be a single rising edge at the end of the scope window.

What am I doing wrong? Is the scope not suitable for such long simulation times using fixed step solver? Or is it not "safe" to use the Autoscale button?

Answer 1

All of the plots you show are correct. Simulink is fine with long simulation times. It is "safe" to use the Autoscale button.

By default a scope is set to only display the last 5000 simulation time steps. Since your model is taking a step size of 1s (this is based on using the default step size of the Pulse Generator, which is 1s), in your second plot you are only seeing points from t=5000 to t=10000 (so the first down step in that time period is at 5500), and in your third plot you are only seeing points from t=95000 to t=100000 (which is a period in which the value of the pulse is low/zero).

To see all simulation times, open the Scope block's parameters (by clicking the button with a picture of a cog on it), go to the History tab, and deselect the Limit data points to last: check box.

Then rerun your simulation and press the autoscale button. You'll then see what (I think) you are expecting.