For efficiently handling data transactions between various system units (CPUs, I/O units and main memory units) in a multi-processor system, the system units are linked via a plurality of expandable crossbar modules, each providing a set of interconnections or well-defined mappings between the sets of input and output nodes, with each output being defined in terms of only one input. In addition to the nodes provided at the input and output sections, each crossbar module is also provided with discrete input and output expansion portions through which the module may be linked to other identically configured crossbar modules when additional nodes are to be integrated into the system. The expansion ports allow serial linking of crossbar modules so as to establish a connection between source and destination nodes which are spread across different crossbar modules. The serially-linked expansion ports realize direct mapping between all system nodes in the form of a two-stage network; the basic crossbar design remains the same and there is no need for the problematic redesign of crossbar modules for accommodating added data transfer nodes. The modular crossbar design is particularly adapted for use with different system configurations having different numbers of communication nodes and permits a system to be upgraded by adding communication nodes by using additional identically configured crossbar modules.