Using the recently-introduced SUMIT (Stackable Unified Module Interconnect Technology) expansion interface, the Express104 SBC stackable module form factor specification from the Small Form Factor Special Interest Group provides the basis for stackable, I/O-centric, multi-board systems. Processor architecture and chipset independent, the specification describes a 90 mm × 96 mm board with two 52-pin, high-speed connectors capable of supporting PCI Express and USB as well as other popular moderate speed interfaces for I/O expansion. A single 52-pin SUMIT-A connector supports one x1 PCI Express lane, three USB 2.0 interfaces, an LPC Bus, and SPI/uWire, SMBus/I²C Bus, and ExpressCard signals. A second identical 52-pin SUMIT-B connector supports one additional x1 PCI Express lane and one x4 PCI Express lane plus additional power, ground, and control signals. Even though Express104 modules can be constructed with only SUMIT connectors, a special configuration has been defined to support expansion with PC/104 modules.
Small Form Factor Special Interest Group