Despite the efforts devoted to increase the dependability of highly-reliable distributed fieldbus systems by means of simplex stars and replicated stars/buses, literature lacks of appropriate analyses that quantify the system reliability these topologies yield. In previous work, we proposed models to adequately quantify the system reliability benefits of simplex buses and simplex/replicated stars. However, a model for replicated buses is an open issue that needs to be addressed, as they normally include less components than stars and, thus, can be more reliable and cost-effective. To fill this gap, this paper presents a model that makes it possible to appropriately quantify the reliability that a highly-reliable distributed system can achieve when using a replicated bus.