Error using main_maglev_SFC_OBS (line 153) Compilation of model 'simu_maglev_SFC_OBS' failed while trying to resolve underspecified signal dimensions.

Getting an error where the function block thinks all my parameters are 1x1 but they are not.

Error using main_maglev_SFC_OBS (line 153)

Compilation of model '

simu_maglev_SFC_OBS' failed while trying to

resolve underspecified signal dimensions.

Suggested Actions:

• Enable 'warning' or 'error' diagnostics for the list of underspecified signal dimensions. -

Open

Caused by:

Error using

main_maglev_SFC_OBS (line 153)

Invalid setting for input port dimensions of

'

simu_maglev_SFC_OBS/Demux'. The dimensions are being set to 1. This

is not valid because the total number of input and output elements

are not the same

Error using

main_maglev_SFC_OBS (line 153)

Error in port widths or dimensions. '

Output Port 2' of

'

simu_maglev_SFC_OBS/MATLAB Function' is a one dimensional vector

with 1 elements.

I feel like I have tried everything.

Answer 1

[Abhipsa]

Hello @Daniel,

This error usually indicates that Simulink cannot infer the signal dimensions inside the MATLAB Function block, so one or more parameters are being treated as scalars (1 × 1).

By default, the MATLAB Function block inherits the size of every variable unless you explicitly specify it.

From the documentation, https://www.mathworks.com/help/releases/R2025b/simulink/ug/sizing-function-arguments.html :

• “The default size is –1, which means that the variable inherits its size based on the value of the Scope property.
• If the Scope is Parameter, the variable inherits its size from the associated MATLAB or Simulink parameter.”
• If that associated parameter is a scalar, the inherited size becomes 1 × 1.

When a downstream block (such as Demux) expects a wider vector, the model compilation fails with an underspecified signal-dimension error.

To resolve this issue,you can use the troubleshooting steps:

1. Declare parameter sizes explicitly

• Open the MATLAB Function block → Symbols pane > Edit Data.
• For each parameter, set
• Scope = Parameter
• Size = the intended dimension (for example [3 3], [3 1], [1 3]).

2.use Simulink.Parameter objects like:

A = Simulink.Parameter(zeros(3,3));
A.Value = yourA;      % keeps 3×3 size fixed

Then reference A in the MATLAB Function block.

3. Verify compiled sizesTurn on Display > Signals & Ports > Signal Dimensions and update the diagram (Ctrl + D).

You can also query:

get_param('model/MATLAB Function','CompiledPortDimensions')

Once each parameter’s size is defined, the Demux and all downstream blocks will compile successfully.