Sun's XDBus was a joint project with Cray and Xerox. It was an outgrowth of the already-established MBus for small-scale multiprocessor systems, extended to link entire motherboards together. It is mechanically compatible with MBus, though uses much different signaling. Most MBus processor modules have a way to automatically sense if they're connected to MBus or XDBus and reconfigure accordingly.
Systems with XDBus comprised the Sun4d sub-architecture. These were single-system-image machines with multiple system boards, which connected together over the XDBus to form a single machine. Unlike similar systems from Silicon Graphics, these systems were not NUMA - all memory in the system was equally local to each processor.
XDBus-equipped systems include the SPARCserver 1000, SPARCcenter 2000 and Cray CS6400, which were Sun's first serious attempt at mainframe or supercomputer-class gear.