Change is a concept that is well known in the computer industry. Designers and manufacturers alike have embraced this trend in an ongoing race to increase computing power while reducing size, weight, and power and cost (SWaPaC). With the latest round of ultra-low power processors from Intel, VIA, AMD and DMP, stackable computer systems need to adapt as well, especially with respect to I/O, while striking a balance between new and legacy technology.

Figure 1. SUMIT connectors and signals

The best-known stackable computer architecture centers on the popular PC/104 form factor. It is an industry work-horse known for its rugged design and Figure 1. SUMIT connectors and signals small size. These self-stacking modules can serve as either a mezzanine with a single card or multiple cards for even greater I/O requirements. As it approaches twenty years of service with continuing growth and design wins, it’s in the process of a third technology refresh to accommodate the latest generation of high-integration, x86-compatible processors that support high-speed PCI Express, SATA, and USB serial I/O buses to enable an even more diverse range of embedded applications.

Originally, PC/104 was defined as a small 90 x 96mm module with unique mounting holes and the ISA bus implemented in a 104-pin connector capable of stacking multiple boards without the need of a card cage or enclosure. Later, when PCI-based processor and chipsets migrated from the desktop to the embedded space, PC/104-Plus was defined as a technology refresh to take advantage of the higher bandwidth I/O capability. This “second-generation” implementation defined the location and pin-out of a 120-pin stackable connector for PCI signals placed on the opposite side of the module from the PC/104 connector. Even though many different SBC (Single Board Computer) platforms support the PC/104-Plus implementation, the majority o fI/O expansion modules only have a PC/104 connector.

Today the parallel PCI bus is being replaced by the high-speed serial PCI Express both in chipset silicon and on SBC and I/O boards. For PC/104-size stackable systems, a third generation architecture was required for the addition of the new signals plus a stackable connector system that could handle the bandwidth of these signals. Specifically, the Generation 2 of PCI Express requires 5 GT/sec which is well beyond the capacity of either of the previous connectors. But how and where do you add another connector on a 90mm x 96mm module? How are legacy issues resolved to allow current users to migrate to faster processors while keeping their existing I/O modules? Should either or both of the 104-pin and 120-pin connectors be replaced?

To solve these dilemmas, the SFF-SIG (Small Form Factor Special Interest Group) developed two complementary design solutions. The first is a high-speed stackable interconnect standard called SUMIT and the second is a 90 x 96mm industry-standard module (ISM) board outline specification.

SUMIT Defined

Figure 2. SUMIT-ISM defines the Legacy Type 1 and Legacy Type 2 options for designers wishing to use either PC/104 or PCI-104 modules in their system.

The SFF-SIG specification took a unique approach by separating the connector technology from a particular board size. The purpose of their specification was to define a unified expansion interface that could be supported by many standard and custom SBC form factors in a flexible and compact manner to meet a very broad range of application requirements. Form factor details are distinct from the SUMIT connector and pin-out details, which are left to their respective specifications.

The word “SUMIT” is an acronym that stands for Stackable Unified Module Interconnect Technology and is pronounced “sum it”. SUMIT is an electro-mechanical connectorization specification that enables common serial and legacy chipset expansion buses for next generation products. It is a board-to-board interface whose signals include Gen 2 PCI Express, USB, SPI, I2C/SMBus, and LPC. It is a stackable, I/O-centric, serial

expansion approach that is independent of any particular board form factor. SUMIT defines two 52-pin, high-density, high-speed connectors and their signal assignments. The standard configuration includes both the SUMIT A and B connector, but the B connection is optional, depending on the configuration and signals used by the SBC or I/O board.

Industry Standard Module Defined

ISM defines a pure board outline without regard to bus expansion for modular, stackable systems. This is an umbrella concept to provide coherence to the many different boards that are available in a 90mm x 96mm footprint which is, coincidently, the same size as the PC/104 form factor. The key feature is that an ISM board maintains the same module physical dimensions and mounting holes while allowing expansion connectors of various size, locations, and I/O signals. This is especially important for legacy packaging and mounting issues in existing OEM equipment.


As newer technology and faster high-speed serial buses have been developed and become standard in use by the commercial PC, it was necessary for the SUMIT-ISM definition to create a bridge from the past to the future. Adding SUMIT connectors and their location on an ISM form factor creates SUMIT-ISM. Through the SUMIT interface, SUMIT-ISM incorporates the PCI Express and USB serial buses that were developed for desktop and mobile environments while leveraging them for use into even more embedded applications. It also allows a designer to preserve the popular 90mm x 96mm size envelope without restricting them to the older legacy I/O expansion technology.

Figure 3. Both PC/104 and SUMIT-ISM cards provide I/O expansion to an EBX form factor SBC from WinSystems.

The SUMIT-ISM specification includes two legacy stack types for modules with PC/104 and PC-104 connectors. It uses slotted mounting holes on ISM modules to provide symmetry not found with PC/104. These options allow the continued use of their connectors in the existing locations along with the established physical dimensions and mounting holes, as well as the new high-speed serial signals through the SUMIT connectors.

The SUMIT-ISM legacy Type 1 board supports the 104-pin PC/104 connector and the Type 2 board supports the 120-pin PCI-104 connector in addition to the SUMIT A or AB connectors. Because the ISM mounting hole pattern is the same as the asymmetrical PC/104 module, a Type 2 module form factor was defined with a slotted hole pattern. This enables SUMIT-ISM modules to support either the PC/104 ISA bus or the PCI-104 PCI bus by simply allowing the module to be rotated 180 degrees as necessary to fit the legacy type required. However, the SUMIT connector location and interface is the same for both. This allows I/O manufacturers to create a single design that will stack on SUMIT-enabled SBCs with different PC/104-type expansion configurations that support 90 x 96 mm I/O boards that are marketed as either PC/104 or PCI-104 compatible.

The strength of PC/104 is its ecosystem of I/O expansion modules that stack “piggy back” on top of each other on a variety of SBCs of differing size and form factors including PC/104, EPIC, and EBX. The main reason that SUMIT-ISM with the PC/104 connector was defined and subsequently adopted by the SFF-SIG is that they did not want to abandon existing customers that have designed or purchased PC/104 modules for years. The reason is that system designers preferred its simple interface and ease of use which is required for legacy systems compatibility. From 60% to 80% of existing customer stacks include PC/104 (ISA) bus cards. Using this form factor size and mounting holes, allows a designer to upgrade their system using many of their same I/O boards and sometimes the same enclosure with new 3rd generation processor boards.

New Architecture, New Applications

With support of the latest generation of low-power, high-speed processors with PCI Express, even more applications can be served by small form factor boards. For example, the processors’ performance enables designs that need multiple video input data streams and high speed A/D. This opens up applications for security, automated inspection of production lines, and data acquisition from various analog and digital sources. Further, these processors are optimized for both wired and wireless M2M networking so that better and perhaps redundant connections can be made with Ethernet, WiFi, and WLANs. Not only is it important for a system to be able to interface with various machines and sensors, it needs to seamlessly pass the data on to other machines for reporting, analysis, and control. Finally, the new high-performance processor silicon will support extended temperature applications so that thermal problems are less of an issue. This means new applications in the mil/aero, medical, industrial, transportation, smart grid, and pipeline areas can be realized. It strikes a balance with performance and reliability and is cost-effective for applications requiring relays, digital I/O, low-to-moderate speed analog I/O, etc.

Through the SUMIT interface, a SUMIT-ISM solution offers a compact, stackable, I/O-centric, multi-board solution for small-form-factor embedded systems. SUMIT-ISM is a straightforward method to implement the 3rd generation processor technology for 90 x 96mm small form factor, stackable systems. It allows powerful processors to be matched with high, medium, and low speed I/O for both legacy and legacy-free systems. This reduces cost and bulk while increasing mounting and packaging options for small-form-factor embedded systems. To get the SUMIT, ISM, or SUMIT-ISM specifications, go to 

This article was written by Robert A. Burckle, Vice president, WinSystems, Inc. (Arlington, TX). For more information, contact Mr. Burckle at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit .