Running subsystem one time only

I am running my simulation model what it does it runs all block again and again. let say I want to run block A one time and block b runs till simulation time. how can I handle this ? because block B uses block A values therefore I want to run block A only one time and block B till the end of simulation. Now what happen, it runs throughout all the blocks. In attached file it generate the input signal again and again which I dont want. it should generate one time. I attached my model please have a look and help in this regard.Thanks