A redundant device unit includes a first device connected with a field device via a first link and a second link in series. The first device, initially active in use, when active sends diagnostic information indicating whether there is a fault in the first link and/or the second link. A second device connected with the field device via the second link and a third link in series, initially passive in use, receives the first diagnostic information from the first device. When the first diagnostic information indicates that there is a fault in the first link and/or the second link, the first device switches to passive and the second device switches to active and generates second diagnostic information indicating whether there is a fault in the second link and/or the third link. A determination regarding the fault is made based on the first and second diagnostic information.