In April of 2008, the Small Form Factor Special Interest Group (SFF-SIG) unveiled a new modular standard for stackable expansion of single board computers. The SFF-SIG’s new standard, known as the Stackable Unified Module Interconnect Technology (SUMIT), implements a PC/104-like self-stacking bus based on the latest serial bus and interface technologies.

SUMIT defines the electrical and mechanical characteristics of a stackable expansion bus for single board computers and expansion modules. SUMIT defines two identical 52-pin connectors (“A” and “B”), which together carry up to 6 lanes of PCI Express, four USB 2.0 ports, ExpressCard support, a low pin count (LPC) parallel bus, SPI.uWire and SMBUS/I2C serial buses, and various power and ground lines. The signals carried on SUMIT’s A and B connectors, according to the soon-to-be-released SUMIT Version 1.3 specification, appear in Table 1.

The SUMIT specification defines three configuration alternatives based on having one (connector A), the other (connector B), or both the A and B SUMIT connectors present on the host single board computer (SBC). They are designated as follows:

• Configuration A — consisting of the SUMIT A connector, only

• Configuration B — consisting of both the SUMIT A and B connectors

• Configuration C — consisting of the SUMIT B connector, only

Combining SUMIT with PC/104

It should be emphasized that the SUMIT specification is independent of any particular form factor. That noted, a separate SFF-SIG draft specification defines a SUMIT-based stackable module format that offers backward compatibility with the PC/104 Consortium’s PC/104 module standard. Initially dubbed “Express104,” the standard is currently known as “SUMIT-ISM” where “ISM” standards for ”industry standard module.”

Basically, SUMIT-ISM features a PC/104-compatible board outline and mounting hole pattern along with SUMIT’s pair of 52-pin connectors (A and B) near the top edge. In addition, the PC/104’s 104-pin connector array is near the bottom edge just as it is in legacy PC/104 modules. The result is a compact, self-stacking, embedded computer module format that supports expansion via SUMIT’s PCI Express and other bus signals, along with PC/104’s ISA bus.

SUMIT-ISM thus enables the use of “legacy” PC/104 (ISA) expansion modules within SUMIT-based module stacks—a convenient way to include both PCI Express and ISA in a compact, rugged module stack. Additionally, SBCs can be designed with SUMIT-ISM bus connectors, allowing expansion with a choice of modules based on either the SUMIT bus configurations, a PC/104 (ISA) bus, or a combination of both.

Figure 1 shows the basic connector, board outline and mounting hole layout of the SFF-SIG’s preliminary SUMIT-ISM module format. For the purposes of this discussion, this version will be designated “SUMIT-ISM Type 1.”

While SUMIT-ISM builds a bridge from yesterday’s PC/104 module standard to tomorrow’s PCI Express requirements, it does not directly support modules that interface via the PC/104 Consortium’s 120-pin stackable PCI bus (also known as “PCI-104”). Use of PCI-104 modules in a SUMIT-ISM stack is possible, but it requires the addition of a dedicated PCIe-to-PCI adapter card to create the necessary 120-pin PCI bus.

Under the assumption that many system developers may prefer to incorporate today’s widely available, high-performance PCI-104 modules and PC/104-Plus modules (with their 104-pin ISA stackthrough connectors removed) in new designs that utilize SUMIT, we are proposing a SUMIT-ISM Type 2 module, which substitutes a PCI-104 (PCI) bus option for SUMIT-ISM’s PC/104 (ISA) bus option.

Put simply, SUMIT-ISM Type 2 builds a bridge from today’s PCI-104 (PCI) module standard to tomorrow’s PCI Express requirements. PC/104-sized (or larger) single board computers built with SUMIT-ISM Type 2 can thus be expanded via SUMIT (PCIe etc.) modules, legacy PCI-104 (PCI) modules, or both. Additionally, an adapter module could provide compatibility with legacy PC/104 (ISA) modules.