Discrete event example does not work

[Jan Kokert]

Hi, I'm struggeling with an example from the Modelica Tutorial (​https://www.modelica.org/documents/ModelicaRationale13norev.pdf) on page 29:

block DiscreteStateSpace2 
parameter Real a, b, c, d;
parameter Real Period=1;
input Real u;
discrete output Real y;
discrete Real x, NextSampling(start=0); 

equation
when time >= pre(NextSampling) then 
  x = a*pre(x) + b*u;
  y = c*pre(x) + d*u;
  NextSampling = time + Period;
end when;
end DiscreteStateSpace2;

NextSampling will be zero for all times. Is this a bug or is the code wrong? Replacing equation with algorithm didn't change the behavior.

[Jan Kokert]

Well... Changing line 9 to
when {time >= pre(NextSampling), initial()} then
will solve the problem... Is this normal or is this a bug?

[Lennart Ochel]

This is just a rather bad model. If the when clause should be evaluated at the beginning, then it would be reasonable to add initial() to the condition. If the first evaluation of the when clause should be performed after the first period, then one could either change the when condition to time > pre(NextSampling) or, even better, change the definition of NextSampling to discrete Real x, NextSampling(start=Period, fixed=true).

[Lennart Ochel]

Or you could take the model as it is and start the simulation before time=0, e.g. at startTime = -0.1.

[Jan Kokert]

Thank you for your fast response! So it is not a bug but the model is just really bad? - I mean this is an example code from an official tutorial (well a rather old one from 1999...). If it is not a bug, you can close the ticket of course.