EMX Form Factor Unites COM and SBC Camps

By integrating COM Express modules and a new, efficient, stacking I/O connector into a standardized shape, EMX offers advantages to both vendors and customers for new small form factor designs.

By Jonathan Miller, Diamond Systems Corp.

The Benefits of a COM-Based SBC
Until recently, the world of embedded computing consisted of two separate camps: Single board computers (SBCs), usually with some form of stackable I/O, and computer-on-modules (COMs). Each product category has its own advantages and disadvantages: SBCs offer off-the-shelf solutions which are ideal for situations involving short time to market, limited engineering capabilities and low volume; however switching from one SBC to another is frequently a major redevelopment effort. COMs offer performance scalability, increased product longevity and greater efficiency for high-volume applications, but they require the development of a custom baseboard, which is often out of the customer’s reach due to lack of time or resources.

The COM-based SBC offers a way to combine both COMs and SBCs together into a form factor that yields the best of both worlds. A COM-based SBC is a two-board solution that uses an off-the-shelf COM for its processor and a baseboard for its I/O, power supply and connectors. Usually the baseboard is the same size as the COM; however that is not a requirement.

Using a COM for the processor brings a multitude of valuable benefits to both board designers and customers: Instant access to a wide range of interchangeable CPU modules, lower development costs and faster time to market by buying instead of designing the processor circuit, protection from processor technology obsolescence, scalability of product performance, and an ideal ratio of product size to features (a two-board stacked solution occupies half the area of a single board containing the same circuitry, with typically a minimal sacrifice in height).

There is only one main missing ingredient in the above list: An industry standard for I/O expansion. The benefits of such an industry standard are equally compelling: It gives customers the flexibility to select from a wide range of interoperable off-the-shelf products to configure their systems exactly to their needs, it gives customers confidence in the longevity of the form factor (due to the presence of multiple vendors), and it gives vendors the opportunity to sell into a ready market of customers familiar with the technology.

However until now there has been no expansion module form factor that is compatible with any COM form factor. Trying to combine COMs with stackable I/O has resulted in inefficiencies in terms of cost, PCB utilization, design complexity and feature mismatch. Thus the need arises for a new form factor that can successfully bring about the merger of these two separate worlds and yield the numerous and substantial benefits for customers and vendors described above.

Introducing EMX™
EMX™ (short for EmbeddedXpress™) was developed in response to this need. EMX defines a form factor for COM-based SBCs around the most popular category of COMs – COM Express™ – while defining a new, highly efficient form factor for I/O modules. The flexible design of EMX defines two sizes of processor module, matching the COM Express Basic (125mm x 95mm) and Compact (95mm x 95mm) form factors. CPU modules can be either Basic or Compact size, while I/O modules are always Compact size and will work with all CPU modules.


Although EMX has been optimized for use with COM Express, it works equally well with the emerging Qseven form factor. A Qseven board (70 x 70mm) fits easily within the outline of the 95 x 95mm EMX Compact outline, and the Qseven expansion bus signals map well to the EMX expansion connector, as will be seen.

An EMX SBC does not have to consist of a carrier board plus a COM. EMX CPU boards can also be single board products. The EMX Basic form factor is slightly larger than PC/104 and is large enough to contain a full-featured high-performance processor circuit without violating the specification or making sacrifices in the feature set or connector scheme. The EMX Compact form factor is approximately the same size as PC/104 and provides a suitable size for many processor circuits.

The following 3 illustrations show some of the various combinations of COMs, SBCs, and I/O modules possible in the EMX form factor.

EMX Basic COM-based SBC (This is also what a Qseven carrier stack would look like)

EMX Basic SBC with expansion module


EMX Compact SBC with expansion module

Emx Basic COM-based SBC


Observe that in all cases the bottom of the “stack” consists of a metallic heat spreader. This heat spreader not only provides a uniform and convenient method of mounting the EMX subassembly to the system chassis, it also provides a highly efficient means of cooling the processor electronics. This increases the usable operating temperature range of the system as well as increasing reliability by keeping the electronics inside the box cooler than the traditional heat sink approach of an SBC. This phenomenon is illustrated by the figure below, which documents actual tests performed on a conduction-cooled SBC vs. heat sink-cooled SBC using the same processor circuit in the same physical environment.


EMX Expansion Module Scheme
Like other popular form factors, EMX uses a stacking method for I/O expansion. A primary motivation in creating EMX was to improve the efficiency of the expansion I/O method and reduce its impact on the SBC in terms of size and cost. Thus the existing stackable expansion connector options (PCIe/104™ and SUMIT™) were rejected for a variety of reasons in favor of a new, higher density connector.

First, since these form factors use an outline and a mounting hole geometry incompatible with COM Express, utilizing them for the I/O would have required the addition of 4 more mounting holes to the processor board, consuming precious PCB area while resulting in an odd mix of incompatible shapes. Thus the form factor for EMX I/O modules was defined to be the same as COM Express Compact, in order to retain the same outline and the same four mounting holes.

Second, the EMX expansion connector is smaller and significantly lower cost than these other standards, helping to reduce system cost and increase the availability of PCB area for processor and I/O circuitry. The connector is positioned further inside the board outline, freeing up more PCB coastline for I/O connectors. The result is that EMX I/O boards can accommodate more features at a lower cost than competing I/O boards of similar size.


EMX Bus Connector
The EMX bus connector was “right-sized” to include the most popular expansion buses utilized by current and planned processors and chipsets from Intel and other vendors, as well as the most popular peripheral chips. This approach ensures compatibility long into the future, without burdening the form factor with excess cost to support features that are unnecessary. The EMX bus connector contains ample reserved pins, ensuring a long lifetime for the standard by providing the capacity to incorporate new features as they become available. EMX offers all these features in a compact connector requiring only 0.38 in2 / 2.5 cm2, less than half the space required by other expansion methods.

The one notable absence is the wide-lane PCIe x4, x8 and x16 interfaces. Since these high-performance interfaces require large numbers of connector pins (driving up size and cost) but are rarely utilized in small form factor systems, the decision was made to omit them in favor of compactness and economy. However, should the need for these interfaces increase in the future, a second connector could be added to the standard to provide a high-performance option, thereby creating a scalable expansion bus.

The EMX expansion bus features compare favorably to competing stacking expansion methods as shown in the table below.


These features also map well to the expansion interfaces available in COM Express and Qseven, as shown in the table below.



The Altair is an EMX Basic format SBC based on the Intel E6xx (Tunnel Creek) processor family, ranging from 600MHz up to 1.6GHz. Altair offers processor, soldered-on memory up to 2GB, and a full suite of I/O features including Ethernet, SATA, USB, multi-protocol serial ports, and PCI Express MiniCard socket. All I/O is provided on latching connectors for increased ruggedness.
Vega EMX COM-based SBC with EMX Ethernet and serial I/O module

EMX Benefits
By integrating COM Express modules and a new, efficient stacking I/O connector into a standardized shape, EMX offers a significant range of advantages to both vendors and customers over existing small form factor embedded computers:

  • Performance scalability: A customer can select a range of COMs to use with the same baseboard, providing a range of performance, price and power levels with the exact same I/O and connector configuration.
  • Longer lifetime and easier migration: When one COM becomes obsolete, it can be easily replaced with another, while the baseboard, its connections to the customer’s application, and the mechanical system configuration remain exactly the same. Product lifetimes are greatly enhanced, while migration efforts are significantly reduced.
  • Optimization: The EMX expansion connector is optimized for size, cost, interconnectivity, PCB area and PCB coastline, plus it contains sufficient reserved pins for future upgrades. EMX therefore offers greater economy, higher functional density and longer potential lifetime than other stackable I/O formats.
  • Time to market: Since EMX offers the ability to use COM modules as the computing engine, it eliminates the need for traditional SBC vendors to design complex processor circuits, enabling them to get to market faster with a wider range of products, as well as offering performance scalability, longer lifetime and greater return on investment for their products.

This combination of benefits provides a compelling reason for embedded computing vendors and customers to utilize EMX as the form factor of choice for new small form factor product development and applications. To encourage adoption by the industry at large, EMX is available as an open standard without any royalties or license fees. More information about the new EMX standard is available at www.diamondsystems.com/emx.



Jonathan Miller is founder and president of Diamond Systems Corp. located in Mountain View, California, USA. The company has been making small form factor SBC and I/O boards for over 22 years. Prior to Diamond Systems, Mr. Miller was OEM Sales Development Manager at Logitech. Mr. Miller holds a BSEECS from MIT.