use modulo for periodic step function

Hi!

I want to create a periodic step function to control the input concentration of a contaminant in an aquifer. I want the concentration to be 1 for 3 months and 0 for the rest 9 months.

I have created a rect function with upper limit=0, lower limit=3*24*30*3600 (months in sec), and a rise time=0.5*24*30*3600 (1/2 month for smoothing).

Then I use as a dirichlet boundary condition that C=1 * rect1(mod(t,12*24*30*3600)).

This should work, no? Is there a way to plot the input function before runing the model? (the model is very big with many meshes and it takes a long time to solve).

When I try to run it I get an error at time=1 year (12*24*30*3600). It says that solver can not converge to a solution. Any idea why I get this? I thought that it could be the sharp step increase.. however it worked ok at t=3months where there was a step decrease... I have also used a 1/2 month smoothing which should be ok.. I have set the solver to take "free" time steps while solving it, so I was hoping that it will take smaller time steps around the concentration changes.

thank you so much!