Let's say you have two Modelica models A and B.
model A
output Real x (start=1,fixed=true);
equation
der(x) = 2*x;
end A;
model B
input Real x;
Real y;
function myExternalFunction
input Real x;
output Real y;
external "C" annotation(Library="myExternalLib");
end myExternalFunction;
equation
y = myExternalFunction(x);
end B;
where B is using some external function myExternalFunction from myExternalLib.dll. This could be a function using your trained network to predict some y from some value x.
External functions in Modelica are standardized, see the Modelica specification 3.4, Section 12.9.
Now you can of course connect these models in a new Modelica model
model connectedAB
A a;
B b;
equation
a.x = b.x;
end connectedAB;
Now to FMUs. The Functional Mock-up Interface is a standard that defines a container to exchange dynamic models.
There are different tools available, some can export a dynamic model as FMU (e.g. OpenModelica, Dymola,...) and some can import FMUs to simulate them (e.g. PyFMI, OMSimulator). Of course some tools can do both.
So models A and B could be exported as FMUs.
However it's not intended to export a single external function like myExternalFunction.
So in OpenModelica you can export A.fmu and B.fmu, create a new strongly connected SSP Model and add these FMUs as submodels. Under the hood OMEdit will use OMSimulator to simulate SSP/FMUs.
Even while it is not intended to do so you could probably still export your external function as a FMU. Just create a tiny model that will only call the external function (more or less like I did in B). But you are not getting any benefits from doing that.
