Stadnards Update

SUMIT-ISM—Uniting Legacy and Advanced I/O in a Stackable Form Factor

A new standard brings together the popular form factor begun by PC/104 with advances in processor-based COM modules to include IDE, PCI and newer high-speed serial I/O technologies that can be combined in a stack.


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In early 2008, the Small Form Factor Special Interest Group (SFF-SIG) introduced a ground-breaking new interconnect technology called Stackable Unified Module Interconnect Technology (SUMIT). SUMIT is a stacking, I/O-centric expansion approach that includes both high- and low-speed bus signals. Two high-speed 52-pin connectors offer a wide range of serial peripheral interfaces from low to high end including USB, SPI, LPC, I2C/SMBus and PCI Express. Power is also made available on both connectors with a large centrally located ground connection. 

The SUMIT interface is defined around two connectors, each having unique signaling characteristics. The SUMIT-A connector, which can be used alone, includes four Universal Serial Bus 2.0 (USB) ports, a low pin count bus (LPC), System Management Bus (SMB/ I2C), Serial Peripheral Interface (SPI), and a 1x PCI Express (PCIe) link that can support an ExpressCard. The SUMIT-B connector is an optional connector that adds another x1 PCIe and either a x4 PCIe link or four more x1 PCIe links.

In October of this year, the SFF-SIG membership approved another new standard in support of SUMIT. Called “SUMIT-ISM” (Industry Standard Module), these standard modules are defined by the SFF-SIG as having “a 90 mm x 96 mm board outline plus mounting holes, without regard to bus expansion.” SUMIT-ISM takes a new approach by employing backward-compatible dimensions, I/O zones, and mounting holes to enable a flexible stacking design format. This “ISM” size will be familiar to many as the PC/104 form factor. 

Since the SUMIT specification itself defines only a board-to-board interface (connectors and pin definition), the ISM specification was necessary to define how SUMIT is implemented on ISM modules. The SUMIT-ISM standard opens the door for new and more flexible combinations of I/O capabilities, bringing in high-speed serial interfaces while still supporting legacy low-speed peripherals such as ISA (PC/104) or PCI (PCI/104). The SUMIT-ISM specification incorporates an existing standard industry practice, “I/O zones,” into the specification itself, now defining these zones as part of the SUMIT-ISM form factor.

The SUMIT-ISM Standard Supports Flexibility

Embedded developers of long-life industrial, medical, defense and other applications are looking for a small footprint, compact dimensions, and the ability to use the multitude of existing expansion modules in use today. Systems must be backward-compatible with legacy devices in the installed base, while still supporting a hardware environment that accommodates the new high-speed devices.

The SUMIT-ISM standard defines the SUMIT A and B connectors and two legacy stack types, one using slotted mounting holes. This enables SUMIT-ISM modules to be designed to support the SUMIT connectors as well as legacy ISA (PC/104 form factor) or PCI (PCI-104) interfaces. By rotating the ISM module 180 degrees as necessary, the module can fit either legacy bus type, while still maintaining the SUMIT interface. This is important because many embedded designs begin with the required I/O, not the CPU. Since the I/O drives the bus requirements, then the CPU requirements, it makes sense for the designer to start with the most unique I/O and then work backwards to a standard CPU. 

As embedded designers seek higher-performance systems and need to incorporate growing numbers and types of I/O into legacy designs, SUMIT-ISM supports this approach by allowing for a flexible modular stack that can accommodate a variety of new high- and low-speed serial buses while maintaining compatibility with either the legacy ISA or the PCI bus.

The PC/104 and PCI-104 buses have gained worldwide adoption as industry standard system expansion interfaces for small, modular systems. SUMIT-ISM was conceived to support the ISM concept, which provides compatibility with many different boards that are available in the 90 mm x 96 mm industry standard footprint. SUMIT-ISM defines “Legacy Type 1” and “Legacy Type 2” options for designers wishing to incorporate PC/104 or PCI-104 modules and enclosures into their system. A Legacy Type 1 board supports the legacy 104-pin PC/104 connector in addition to the SUMIT-A and/or SUMIT-B connectors. A Legacy Type 2 board supports the legacy 120-pin PCI-104 connector in addition to the SUMIT-A and/or SUMIT-B connectors (note the Legacy Type 2 mounting slots due to the PC/104 offset mounting holes). In this way the SUMIT-ISM specification can maintain legacy applications while supporting newer, high-speed serial buses (Figure 2).

Figure 2
Legacy Type 1 and Legacy Type 2 options.

The SUMIT-ISM specification supports:
• Expansion interfaces decoupled from form factors
• Defined connector expansion areas
• Flexible expansion buses and I/O interfaces
• Top- and bottom-side component height restrictions
• Previously undefined and unnamed bus combinations

The stacking order for the SUMIT-ISM modules is also a significant feature of the specification. The stack is assembled in one direction only, which is “up” from the processor board (SBC), defined as the bottom board in the stack. This one direction stacking requirement is a result of the point-to-point nature of USB and PCIe interfaces. I/O cards “consume” a USB or PCIe resource and shift the remaining links for use by I/O modules above them. If a Legacy Type 1 or 2 parallel bus is incorporated on the CPU, the legacy bus, being a non-point-to-point parallel bus, may stack in either direction.

Due to I/O cards consuming and lane shifting of the serial bus resources, some attention must be paid to the actual module stacking order. The resources provided by the base CPU board must also be taken into account. A full set of resources is not a requirement on a SUMIT connector. A label containing a resource table is defined in the SUMIT specification to convey this interoperability information (Figure 3).

Figure 3
The SUMIT resource label shows a column for each connector. If a resource is not supported by that connector, the box is grey. A dash mark indicates that a supported resource is not implemented in this particular module while a check mark indicates that the resource is supported.

As with the above definitions for Legacy Type 1 and Type 2, the incorporation of the SUMIT interface basically replaces either the 120-pin PCI or the 104-pin PC/104 connector with a 104-pin SUMIT connector pair. None of the dimensions, I/O zones, or mounting hole locations on “stackable” form factor boards (EPIC, EBX or PC/104) need to change, other than the use of slotted mounting holes when SUMIT Type 2 is the primary expansion format. The use of SUMIT-ISM results in greater I/O interface options and higher potential bandwidth, while requiring far less real estate for the connectors. Keeping the same form factors, and updating only the interfaces, lets form factor standards and interface standards evolve separately. 

When a new form factor becomes available, it can support all of the required interfaces. This helps minimize revisions to the specifications and increases the platform’s longevity. Plus, the signals provided by the new SUMIT connectors align well with new low-power chipsets, for example the Intel Atom and VIA Nano microprocessor families. To date, three SFF-SIG member companies, VersaLogic, WinSystems Inc. and ADLINK/Ampro, have all recently announced CPU and/or I/O products designed using the SUMIT or SUMIT-ISM specifications from the SFF-SIG.

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