The subject invention is directed to a new and useful line replaceable unit (LRU) including an LRU case defining an interior compartment for housing critical function circuit card assemblies and including a back plane wall and opposed interface wall. At least one critical function circuit card assembly (CCA) is housed within the LRU case. The critical function CCA includes an input output connector mounted in proximity to the interface wall, as well as a backplane connector. The LRU includes a backplane CCA operatively connected to each backplane connector of the at least one critical function CCA. The backplane CCA is configured and adapted to provide operative communication among multiple critical function CCA's. The critical function CCA's can be mounted to the LRU case proximate the interface wall and the backplane wall to be mechanically and electrically isolated from one another except for connections provided by the backplane CCA.