The new SUMIT specification is managed by the Small Form Factor Special Interest Group (SFF-SIG). SFF-SIG is a newly formed, non-profit industry group that develops, promotes, and supports small form factor circuit board specifications and related technologies. The group embraces the latest technologies, but also has a philosophy of maintaining legacy compatibility and enabling smooth transitions to next-generation interfaces.

A SIG Is Born

Figure 1: SUMIT connectors require little board space but offer easy routing and I/O expansion flexibility.

The Small Form Factor Special Interest Group ( is an international organization devoted to identifying, creating, and promoting standards that help electronics system and device manufacturers and integrators move to small form factor technologies as building blocks in their products. Benefits of small form factor products include smaller size, reduced power consumption (eco-friendly, "green" products), and greater reliability compared to larger legacy products. Additionally, the SFF-SIG seeks to enable practical, mainstream, real-world applications rather than extremely high-performance and high-power dissipation systems. The SFF-SIG is concerned about all components of the system-level solution, not just the processor section. The rationale is that it doesn't help much to shrink the board if the packaging, cabling and thermal issues are not addressed at the same time.

The SFF-SIG has formed three working groups to address different product categories. The SBC Working Group is discussing new small form factor single board computers. The Modules Working Group is developing a specification for a new small computer-on-module (COM) form factor. The Stackables Working Group is examining approaches to adapt new high-speed serial technologies into legacy systems in a smooth manner that preserves investments in I/O, cabling, and enclosure designs.

From the Stackables Working Group came the SUMIT™ Specification. The word 'SUMIT' is an acronym that stands for Stackable Unified Module Interconnect Technology. It is pronounced "sum it". The specification takes a unique approach to separating the connector technology from a particular board size. The purpose of this 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 pinout details, therefore, are left to those respective specifications.

Upward Mobility

SUMIT is an electromechanical connectorization specification that enables all common serial and legacy chipset expansion busses for next generation products. 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 (0.025" pitch) connectors with center ground blades for impedance, EMI, and DC ground return purposes. Each connector is optional depending on the target applications of a particular SBC.

The following interface signals are included in SUMIT:

  • Two x1 channels of PCI Express (including clock and card present)
  • One x4 channel of PCI Express (including clock and card present)
  • Three USB 2.0 channels
  • Two SPI/uWire channels
  • LPC interface with Serial IRQ support
  • System Management Bus (SMBus)/ I2C with SMB Alert
  • Reset and Wake On LAN (WAKE)

A Gradual Migration

The SUMIT Type A connector contains one PCI Express x1 ("by one") lane, three USB ports with a global over-current signal, Low Pin Count (LPC) bus for expansion serial ports and other legacy I/O, SPI / uWire ("Microwire"), and a general-purpose I2C bus that is typically connected to SMBus for x86 chipsets.

Figure 2: SUMIT-ISM three-board stack of modules

The multitude of low speed buses will enable a smooth transition away from the long-standing ISA Bus for much of the embedded market that, for example, uses simple I/O for driving relays or lowrate data acquisition. The SUMIT Type B connector adds another PCI Express x1 lane plus a PCIe x4 ("by four") lane, primarily for storage/RAID, networking, video output or frame grabbers, highspeed acquisition, and scientific applications. There are three valid configurations known as SUMIT-A, SUMIT-B, and for both connectors, SUMIT-AB. The total number of pins is 104 for a SUMITAB configuration.

A Samtec QFS/QMS double row, highspeed, 15.24mm Q-strip connector pair is used. The slightly larger QFS connector measures only 22.35 mm x 8.13 mm (0.880 x 0.320-inches), which means that it requires very little real estate on a board or module. It can currently support up to Generation 2 PCI Express data rates of 5 Gbps with a stack of 4 boards. This data rate has been tested and verified by the Samtec Signal Integrity group.

Cost is a key element in any design. SUMIT is defined in such a way so that only a single, one-bank connector can be used. This saves PCB board space plus the cost of an additional connector. By using two smaller, separate connectors instead of one large connector, an expansion or add-in board built with only a single connector can plug directly into other processor or expansion cards populated with both connectors, further reducing overall system cost. It is also possible for just the second connector to be used for applications only needing one PCIe x1 and/or one PCIe x4 lane.

SUMIT is designed for applications needing high- and low-speed I/O expansion. It serves any number of markets such as industrial control, medical, MIL/COTS, transportation, communications and homeland security. Board designs are already underway with product introductions based on both the new Pico-ITXe specification and the Industry Standard Module.


As a first step in unifying the small form factor market, the SFF-SIG has introduced the Industry Standard Module (ISM). It is a 90mm x 96mm size board that supports an embedded processor plus one or more stackable I/O modules in a very small space. It is a pure board outline without regard to bus expansion. This is an umbrella concept to provide coherence to the many different boards that are available in this footprint. Examples include PC/104, PC/104-Plus, PCI-104, PCIe/104, PCI/104-Express, USB-104 and StackableUSB. The key point is that an ISM module maintains the same module physical dimensions and mounting holes while allowing expansion connectors of various size, location and I/O signals. This is important for packaging and mounting issues in OEM equipment, so that system companies can preserve their NRE investments. Also, ISM can even be used within computer-on-module (COM) architectures, not just for stackable SBCs.

Figure 3: An EPIC form factor SBC uses SUMIT-ISM I/O module to tailor the I/O to application-specific requirements.

An ISM board combined with SUMIT expansion is called SUMIT-ISM™ (Figure 2). SUMIT-ISM enables the construction of small computer systems that are powerful, easy-to-use, cost-effective, and scalable for a variety of different embedded applications. Even though SUMIT-ISM modules can be constructed with just SUMIT connectors, a special configuration has been defined to support expansion with existing PC/104 modules. Ampro (now ADLINK Technology), over 15 years ago, designed and developed PC/104-based modules incorporating the ISA bus architecture. It became very popular due to its easy of use, low profile (horizontal rather than vertical installation), simple design, and variety of I/O boards from vendors worldwide. However, as newer technology and faster high-speed serial buses have been developed and become standard in use by the commercial PC, it was necessary to create a bridge from the past to future.

The philosophy behind SUMIT-ISM preserves legacy investments while adding an upward performance migration path, paving the way for small, rugged, scalable and reliable computer systems for the long haul. SUMIT-ISM expansion maintains legacy support for the vast number of 90mm x 96mm stacking expansion I/O modules and enclosures. This is done by allowing the continued use of the legacy PC/104 (ISA bus) connector in its existing location, and re-using established physical dimensions and mounting holes.

Industry Standard Modules can be stacked together for a small system solution or used as I/O expansion modules for a larger host single board computer. Consequently, SUMIT-ISM™ can also serve as stackable, I/O expansion modules for EBX, EPIC, and other customsized boards (Figure 3). Similar to PC/104, none of the dimensions, I/O zones, or mounting holes of an EPIC or EBX board change, only the replacement of the old 120-pin PCI-104 connector with a 104-pin SUMIT™-AB connector pair. This results in more I/O interface options and higher bandwidth while requiring less real estate for the connectors. Plus, it aligns well with the new ultra-low-power chipsets from the Intel Atom™ and VIA Nano™ microprocessor families.


Embedded system designers are asking for simple, modular ways to implement emerging low power, high-speed processors plus their various I/O requirements without sacrificing packaging and legacy issues. Systems conforming to SUMIT-ISM can then maintain legacy support for the vast number of existing stacking expansion I/O modules and enclosures available worldwide, yet keep up with the modern chipsets on a tiny board. Couple this with its modularity and ruggedness, it makes an excellent solution for demanding industrial applications in cost- and size-sensitive projects.

This article was written by Robert A. Burckle, Vice President, WinSystems (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 here .