I would like to know if I connect two CPU's PM866 with a TK851 and a BC810, can I use only one CI854? In case one CPU fails the other assumes the control of the same CI854... Or I need to connect two CI854, one in each CPU?
Yes, you can use one CI854 connected to any module of BC810 pair, e.g. at upper one. If Upper CPU fails, bottom CPU takes over control and BC810 pair secures communication between bottom CPU and CI854. Note1: TK850, black CEX-bus extension cable cannot be connected. Note2: this configuration is not fail safe because one CI854 is single point of failure. For Profibus master redundancy use two CI854 modules.