Abstract: The realtime performance of distributed computer system (DCS) is guaranteed by realtime capability of fieldbus, that is why authors focus on how utilizing fieldbus to meet realtime requirement of tasks in DCS. In last paper, authors do global analyzing realtime capability of fieldbus on many aspects. In this paper, the authors primarily analyze realtime capability of FF and WorldFIP from their MAC protocol in detail and propose the improved methods for the found deficiency in the MAC protocol. FF and WorldFIP both explicitly distinguish periodic and aperiodic messages and support them with centralized token and distributed token. For effectively utilizing the two kinds of token, the authors investigate concerned problems deeply, such as construction and size of schedule table,schedule algorithm and algorithm schedubility, setting of parameter of e do PT token operation period. Further, the authors proposes general performance evaluation architecture for computers network through realtime quality of service, after investigating the actual temporal requirement of industry applications.