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Toward a Unified Framework for Embedded Modules

Some rather minor modifications to the original PC/104 form factor may not only smooth the path from legacy ISA through PCI to PCI Express data paths in the same system, but also provide a way forward when future interconnect technologies appear.

JONATHAN MILLER AND RICK LEHRBAUM, DIAMOND SYSTEMS

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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.

Introducing the “UEMF”

One interesting “feature” of the original PC/104 specification is its non-symmetrical mounting hole pattern. Consequently, SUMIT-ISM’s mounting hole pattern (as currently defined) would not be compatible with a version that simply substitutes PCI-104’s 120-pin PCI bus connector for PC/104’s 104-pin ISA bus connector, since on a PC/104-Plus module footprint—which defines the location of these two connectors—the PCI connector is located at the top, while the ISA is located at the bottom.

Figure 2 provides a comparison of the mounting hole locations on SUMIT-ISM Type 1 and SUMIT-ISM Type 2 modules, assuming the legacy buses (PC/104 ISA and PCI-104 PCI, respectively) are located as defined by the PC/104-Plus specification.

From the pair of drawings in Figure 2, it can be seen that modules built with only SUMIT A and/or B connectors of these two types could not be combined in a stack, because when either module is rotated by 180 degrees, the mounting holes will not align.

To alleviate this incompatibility, and thereby allow SUMIT-ISM Type 1 and Type 2 modules built with only SUMIT A and/or B connectors to be used interchangeably and combined together, a universal hole pattern is proposed for both versions—or at least for Type 2, so that it can accommodate Type 1 modules. This hole pattern, along with the 3.775 x 3.550 inch module footprint, are collectively dubbed the “Unified Embedded Module Framework” (UEMF). SUMIT-ISM Type 2 implemented on the UEMF is illustrated in Figure 3. A dimensioned mechanical drawing appears in Figure 4.

Implications of the UEMF

By virtue of its symmetrical mounting capability, the UEMF provides much more flexibility than the original, non-symmetrical PC/104 form factor. For example, a SUMIT-ESM Type 1 module could stack atop a SUMIT-ESM Type 2 module (illustrated in Figure 3), providing the Type 1 module’s 104-pin ISA bus pass-through connector is removed.

It is now possible to envision an extremely long lifecycle and broad range of applications extending well beyond the current transition from parallel to serial buses and interfaces.

As already demonstrated in this proposal, the UEMF potentially enables SUMIT to team up with a choice of 104-pin ISA bus (SUMIT-ISM Type 1) or 120-pin PCI (SUMIT-ISM Type 2), for maximum flexibility of legacy support. Similarly, UEMF could provide this same flexibility to the PC/104 Consortium’s new stackable PCIe bus (PCIe/104) standard.

More importantly, UEMF enables a technology roadmap beyond the current transition, for there will surely come a time—perhaps five years from now—when today’s next- generation bus (PCIe) will have become tomorrow’s legacy bus, and the stackable SBC market will once again be wondering which of several legacy buses is the most important to retain. Who knows—in two years we may use the UEMF to add PCIe/104 to SUMIT A/B, to keep up with the latest Intel embedded processor’s increased number of available PCI Express lanes. Or, perhaps UEMF will be harnessed to introduce some sort of fiber-optic system expansion bus.

The drawings in Figure 5 illustrate the flexibility of the UEMF and suggest some possibilities. And last, but not least, the SFF-SIG’s SUMIT A/B bus combined with the PC/104 Consortium’s PCIe/104 PCI Express bus is shown in Figure 6.

Issues Concerning Slotted Mounting Holes

Must all UEMF module formats adopt the UEMF’s slotted mounting hole pattern? The short answer is no, although it makes sense for newly designed modules of all types.

Each bus used on PC/104-style modules was originally situated at either the “top” or “bottom” edge of the original PC/104 footprint, thus associating it with a specific mounting hole pattern. With that in mind, here is a simple rule: Each bus on a UEMF module or UEMF stack host board must be accompanied by its originally defined reference hole pattern (all four corners).

Thus, dual-bus UEMF modules, having both buses in their “standard” locations, are free to use legacy PC/104-style (round) mounting holes. Modules such as the SUMIT-ISM Type 2 format proposed in this document, with SUMIT at the “bottom” instead of at the “top” (as defined by SUMIT-ISM Type 1), would require slotted holes in order to accommodate both buses’ reference hole patterns.

Enclosure mounting holes can—and probably should—retain the original asymmetrical PC/104 hole pattern, at least for now. However, it may be desirable to migrate all UEMF modules to the slotted pattern over the next five years, and after ten years to the symmetrical hole pattern that corresponds to the outer foci of the currently proposed slotted pattern.

There have been some concerns and suggestions that the slotted mounting holes may compromise resistance to shock and vibration. We believe that the four corner-mounting screws will be fully capable of holding stacked boards rigidly in place. We plan to perform a number of tests to verify that the approach is sufficiently robust to survive the environments where PC/104-style modules are typically used.

It should be noted that the four 0.125 diameter mounting holes on PC/104-style modules are already larger than the diameter of the screws, so transverse forces, which are already present in PC/104-based systems, have not been an issue to date. PC/104-style modules are typically secured via 4-40 mounting hardware with 0.112 diameter threads. Regarding the requirement for the slotted holes, SUMIT-ESM Type 2 could be usable with PCI-104 modules regardless of whether the Type 2 modules’ holes are slotted or normal.

The advantage of the UEMF is that it lets PC/104-sized modules with a wide variety of buses be combined together in stacks, without forcing module vendors to modify legacy modules or build each module two ways (other than leaving off unused bus connectors). Thanks to the UEMF approach, stackable ISA, PCI and PCI Express buses (from the PC/104 Consortium), the stackable SUMIT PCI Express bus (from the SFF-SIG), the stackable USB bus (from the StackableUSB association), and future stackable buses yet to be defined can all coexist synergistically.

Diamond Systems
Mountain View, CA.
(650) 810-2500.
[www.diamondsystems.com].