Wow thanks everybody for the helpfull tips.
I'll be a bit more clear on what I have to do:
I have to generate a square wave to modulate the pH of a system. The pH must varie from 7.4 to 8, and the periode is 6hrs. I would like to generate a square wave in a Function Bloc Diagram by only using Elements (no LADDER). So is there a block that would generate a TRUE(for 3hrs) or FALSE(for 3hrs) automatically (because it has to run for a week, so I can't turn it on or off manually)...?