With respect to the working mechanism of networked control systems with time division scheduling based on
clock synchronization, the causes for data packet dropouts in ethernet for plant automation(EPA) based networked control systems are analyzed. A model of EPA systems with data packet dropouts according to different requirements of real time performance is established. The influences that the communication cycle, the length of time slot and the precision of clock synchronization exert on the stability of systems are given while the distribution of clock deviation jitter is a Markov chain. Methods to improve the real time performance of EPA systems are proposed. Finally, the stability of the EPA system is analyzed.