Abstract: A system backplane has a pair of dual purpose connectors and a system bus controller. A shared resource card has a pair of connectors and a shared resource, such as memory. In an entry level system, the connectors on the shared resource card mate with the dual purpose connectors on the system backplane, connecting a system bus from the system bus controller to the shared resource. There are no unused expansion slots on the backplane. To form an advanced level system, an expansion backplane having mating connectors is inserted between the system backplane and the shared resource card. The expansion backplane allows the system bus to pass through from the system backplane to the shared resource card. The expansion backplane has an expansion bus controller, an expansion bus, and expansion slots. The expansion bus is connected to the expansion bus controller and to the expansion slots. The expansion bus is also connected to the shared resource on the shared resource card.