How to output text to Command Window if signal changes from 0 to 1 in Simulink

I want to output text to the command line in Simulink when a signal turns from 0 to 1.

For example, in my model, signal A is zero unless a condition is exceeded in the model, to where signal A is then turned to one. At that point, I want to write "Signal A is 1. Data is invalid" to the command window. I'm currently trying to use a MATLAB function block:

function a = fcn (u)
if u == 1
    disp('Signal A is 1. Data is invalid')
    a = 1;
else
end

This doesn't seem to work. any suggestions?

Answer 1

[Fangjun Jiang]

You might want to do that, but MATLAB and Simulink are quite different.

I don't think you can do that because disp() or fprintf(), etc are not supported.

https://www.mathworks.com/help/slci/ref/matlab-function-block-support.html

MATLAB System block can output free style text, but to the Simulink Diagnostic Window, not to the MATLAB Command Window.

https://www.mathworks.com/help/simulink/slref/matlabsystem.html

[Walter Roberson]

That link is specific to Simulink Code Inspector, not a link about what is generally supported by MATLAB Function Block.

[Paul]

Output from a disp command in a Matlab Function block is sent to the Diagnostic Viewer (at least when running in Normal mode, I didn't try any other mode).

[Fangjun Jiang]

@Walter Roberson, you are correct. I could not find a link that shows which functions are supported or not supported by the MATLAB Function block.

[Fangjun Jiang]

@Paul, I tried in R2022b. Neither disp() nor fprintf() messages are shown in Diagnostic Viewer. The functions seem to be ignored.

I think it has value to enable the message to be shown somewhere. It could help debugging although there are other ways to do it. I remmeber Stateflow allows to do that.

[Paul]

I'm quite surprised that disp doesn't work that way in 2022b. My vague recollection is that disp has worked that way for quite some time; I only checked it in 2024a to verify my memory.

This doc page talks about using only supported functions in a Matlab Function block.

"Because the MATLAB Function block relies on code generation technology to compile and simulate, C/C++ code generation limitations for MATLAB Coder also apply to MATLAB Function blocks. As a result, you can only use MATLAB language features that are [NOT] supported for C/C++ code generation in MATLAB Function block code if you declare them as extrinsic."

I added what I think is the appropriate correction.

Or maybe they meant to say: "As a result, you can only use MATLAB language features that are supported for C/C++ code generation in MATLAB Function block code [UNLESS] you declare them as extrinsic."

But I still think that second statement isn't 100% correct as there are some functions, like plot() for example IIRC, that will work even if not declared extrinsic.

That page has a link to this doc page from which the full list of functions that support code generatation is accessible.

[Fangjun Jiang]

@Paul, @Ian, the disp() messages are shown in Simulink Diagnostic window, verified in R2022b, R2024b and assumed others too.

Previously, my experiments in R2022b didn't show the message. That is because the "if u == 1" logic. It's better to change it to "if u >= 1"