RTC Magazine

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).