MATLAB: How to set the sample time of a subsystem

multitasksimulinkSimulink Real-Timesubsystem timestepxpc

I have a model for xPC in which I have different masked subsystems. As they do different tasks, they need to run with different timesteps (some do fast acquisition, others need longer time to run and do communication or other slow tasks).  Subsystems trigger each other (start each other - obviously the faster one is in control). 
   How can I define the sampling time of a subsystem (and all components inside)?  
   I'm using fixed step solver. What timestep do I need to set for it? - the shortest one that I use in any subsystem?
Thank you for any suggestion.
P.S.: I am aware of this: http://www.mathworks.com/matlabcentral/answers/96560
-any single core solution out there?

Best Answer

Try a zoh block just before your subsystem and set sample time of your subsystem blocks to inherited
Add
f your subsystem doesn't contain a continuous block, right click on your subsystem, click on block parameter(subsystem) then check treat as atomic unit, and set your sample time
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