@Paul J - Thanks for the rant. You are right… the second type of algebraic loop are the ones that cause me the most frustration too. In some cases I find conflicting specifications are to blame. If a subsystem is marked as Atomic, Simulink must be able to compute all of the outputs at the same time. If there are dependencies on inputs, and those have a direct feed-through path back to the outputs, there is a sorting problem to be handled. Thanks for contributing to the discussion!

Dealing with algebraic loops is by far the most frustrating problem in simulink. It seems to me that there are two types of algebraic loops. The first is the mathematical type, e.g. y = 2-y as in your example. These are usually easy to understand and deal with if we don’t want the solver to solve them. The other type, however, are apparent algebraic loops found by the solver because of how it treats atomic subsystems, embedded matlab function blocks, model references, etc. This type is not really discussed all that well in the documentation. Most, but not all, of the problems I’ve had involve models with these types of blocks, and in these cases the “minimize algebraic loop occurences” hasn’t worked for me. I’ve had problems where the red highlighting of the algebraic loop stops in midstream and I can’t determine where the problem is; I had a case where an algebraic loop was reported when a gain block was set to something other than 1; and I just had a problem where an algebraic loop error was reported in a very simple model, without any atomic subsystems or embedded function blocks. In this particular case, I had an algebraic loop error when I passed a width-3 signal output from a virtual subsystem through a demux, but didn’t have an error when I passed out the 3-signals individually. In this same model, I also had an algebraic loop error when I put a gain block (with gain=1) onto a line, but didn’t have any problems w/o the gain block. This kind of stuff is very frustrating. A similar problem is with enabled subsystems where the enabling signal is dependent of the states of the subsystem. For example, consider two balls moving on a wire and we want to disable them when they collide. I’ve yet to figure out how to do this w/o putting an extra memory block into the system, outside of the enabled subsystems that govern the motion of the balls, which allows the balls to continue to move for one time step after they’ve collided. Thanks for the forum for this rant. Keep up the blog; I’ve been finding it very interesting.

Good luck.