The semi-Markov performance model of a bus protocol (IMAP—improved token bus multi-access protocol), suitable for embedded networks, is presented. IMAP is an improvement over the token bus scheme, and has been proposed by Sood et al. (1986). The semi-Markov model is developed by considering the normal and interrupt modes of operation of IMAP. The performance of IMAP is compared to that of the token bus scheme.