Disclosed is a modularly expandable switch-based planar apparatus for inserting multiple bus-based processor cards and/or expansion cards and interconnecting the said cards via a multi-stage switch network which resides on the invention planar. The switching network is built into the planar. The cards require no modification or change of any kind, since the connection to the planar is made as if the planar contained the standard MicroChannel interconnection. However, the disclosed planar implements bus converter units to convert the standard bus interface provided by the cards to the switch network interface, so that functions provided by the cards can communicate in parallel over the switch network.