SQL & PL/SQL

Hi I am wondering how I can make a dbms_scheduler job/program incompatible with itself?

What I want to achieve is that I do not want the scheduler start the next instance of a job if the previous one has not finished.
I have a schedule to run a job every 5 minutes, but I do not want the scheduler to start the next job instance before the previous one
has finished (to make the job incompatible with itself).

I am running on 11gR2

Thanks
Henrik Brokvist