The paper presents a decoupled scheduling approach for distributed hard real-time embedded automotive systems while considering schedulability and extensibility. Here the extensibility is defined as the adaptivity of the schedules to changes resulting from later upgrades. The idea is to use the time-slicing technique to assign transmission timewindows for messages on the bus so that the precedence relations between different processors are enforced by those time-window assignments, and the scheduling problems for each processor and the bus are decoupled. Thus, the existing uni-processor scheduling methods can be applied. The most innovative aspect of the approach in this paper is that the system level consideration is provided in determining those time-windows for the extensibility of the schedules and the schedulability on each individual processor.