MBus was a system interconnect bus used by Sun Microsystems on several of their late-model 32-bit SPARC workstations and servers, most notably the SPARCstation 10 and SPARCstation 20 plus many clones made by Axil, Tatung, Solbourne and others. A close relative of MBus, called XBus or XDBus was used on the SPARCserver 1000 and SPARCcenter 2000 - this was mechanically identical but used very different signaling.
The MBus was primarily used as a way to attach CPU modules to the system. Unlike many CPU buses, it allowed differently-clocked CPU modules to be installed, though figuring out exactly which mismatched modules would work was basically voodoo. It was clocked at several speeds, ranging from 33MHz to 70MHz, though only 33, 36, 40 and 50MHz were ever used in official Sun systems, and speeds much over 50MHz were never considered stable. CPU modules without cache required the MBus to run at the same clock speed as the CPU itself, which limited cacheless modules to around 50MHz.
A large number of MBus modules were available, ranging from the highly non-performant early modules produced by Cypress Semiconductor all the way up to dual-CPU 200MHz HyperSPARC modules. Many of the faster modules require a 50MHz MBus for optimal performance.