Abstract: This paper presents modeling and scheduling for real time tasks with time constraints and precedence constraints,i.e.,Function Blocks tasks and communication tasks,in a single segment FF system.Firstly,Function Blocks tasks and communication tasks are regarded as same tasks,and only considering precedence constraints,we propose a compact mode to build model of tasks,in which each job is completed as earlier as possible.Secondly,considering communication overtime,a extended compact mode scheduling algorithm based on job rate monotonic (JRM) is presented to schedule all real time tasks in order to meet their time constraints and precedence constraints.Finally,an example of application shows the process of scheduling of all real time tasks.