EMX Backgrounder

This EmbeddedXpress (EMX) backgrounder introduces the EMX concept and briefly describes its features, benefits, and fundamental characteristics.

The need for EmbeddedXpress

Until EmbeddedXpress (EMX), the small form factor embedded computing market has consisted of two distinct types of products:

  • Single board computers (SBCs) with stackable I/O
  • Computer-on-modules (COMs)

Each type has its own particular advantages and disadvantages, often presenting a dilemma to customers. Single board computers present a “complete” solution, requiring no additional hardware and no customer hardware development. In addition they often provide a means to enable customers to customize them by adding off-the-shelf expansion modules using a stacking system.

Popular SBC form factors however suffer from being either too small for the latest processor designs (e.g. PC/104) or too large for many rugged or portable applications (e.g. EPIC or EBX). Popular expansion systems such as PC/104 and its variants further utilize I/O connectors that are large and expensive, present manufacturing difficulties, utilize too much PCB area, and occupy too much PCB coastline needed for I/O connectors.

COMs offer a convenient way to incorporate CPU technology into a product without having to design the complex CPU. Since COMs are extremely popular in high volume applications, they are generally extremely cost competitive. Furthermore they are the format of choice for companies designing embedded CPUs with the newest processors.

The main disadvantage of COMs is that they are not a complete solution: a baseboard, or carrier board, is required to provide power and connection to the I/O on the board. Additionally, COMs by definition do not offer any expansion connector for adding off-the-shelf I/O. All I/O is incorporated onto the carrier board (although it is possible to accommodate stackable or slot-based I/O on the carrier board). Because COM carrier boards are designed by customers for their own applications, until EMX there were no industry standards for carrier boards, and few off-the-shelf products were available for customers who didn’t want to design their own COM baseboards.

The solution

EMX defines a compact embedded computer form factor that combines COMs with stackable I/O expansion. This approach offers the advantages of SBCs and COMs, without the disadvantages of either. EMX boards are off-the-shelf products requiring no customer hardware development. They offer easy expansion with stackable I/O modules. They also provide an easy way to utilize the wide selection of COMs available from multiple vendors worldwide, resulting in performance scalability, protection from CPU obsolescence, and earlier access to the latest processors and chipsets.

The flexible design of EMX defines two sizes of processor module (shown below), each of which is compatible with all EMX expansion modules. The two sizes of EMX processor modules match the COM Express Basic and Compact form factors, in order to integrate those two popular families of COMs into the EMX architecture.

EMX Compact form factor
(125 x 95 mm)

EMX Basic form factor
(95 x 95mm)

(click images to enlarge)


The EMX Basic form factor is larger than PC/104 and is large enough to contain a full-featured 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 format for I/O expansion modules. EMX Compact can also be used for processor modules.

The EMX I/O expansion connector (shown below) is smaller and less expensive than that of other standards, helping to reduce cost and increase the availability of PCB area for processor and I/O circuitry. The connector includes all the most popular expansion buses utilized by current and planned processors, chipsets, and peripheral chips, ensuring compatibility long into the future. Furthermore it contains ample reserved pins, ensuring a long lifetime for the standard by providing the capacity to incorporate new features as they become available and desirable.

EMX’s high density, self-stacking bus


Benefits of EMX

EMX offers a significant range of advantages to both vendors and customers over existing form factors:

  • Because EMX utilizes COMs and also provides a complete SBC-like solution, it appeals to a wider range of high- and low-volume customers than either COMs or SBCs alone. Furthermore it enables COM module vendors to reach traditional SBC customers by offering a complete solution.
  • Since EMX offers the ability to use COMs 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 providing longer lifetime and greater return on investment for their products. Customers enjoy earlier access to new CPU technology, longer product lifetimes, wider choice of processors and better protection from CPU obsolescence and performance scalability.
  • The size of EMX Basic is larger than PC/104. Designers of EMX SBCs can fit more processor circuitry on the board without resorting to shortcuts or form factor extensions. Customers can be assured of standard size and shape boards that will fit into their systems without worrying about non-standard board shapes and sizes.
  • The size of EMX Compact is roughly equivalent to PC/104, providing sufficient room for popular I/O circuits.
  • The expansion connector is optimized for size, cost, interconnectivity, PCB area, and PCB coastline, plus it contains sufficient reserved pins for future upgrades.

The combination of all these benefits provides a compelling reason for embedded computing vendors and customers to adopt EMX as the form factor of choice for new small form factor product development.

EMX features

The table below summarizes the key features of EMX.

Key EMX features

  • Defines two form factors:
    • ETX Basic — 95 x 125 mm
    • ETX Compact — 95 x 95 mm
  • Low-cost, high-density 100-pin expansion bus:
    • Includes four PCI Express x1 lanes, two USB 2.0 ports, LPC, and SATA
    • 12 pins reserved for future upgrades
    • Supports up to four EMX expansion modules
    • 14mm standard board-to-board distance; 8mm option for host board
  • Processor module may be a single board computer or a carrier module with a COM Express module mounted underneath
  • Supports efficient conduction cooling to chassis
  • 5VDC main power; 3.3VDC and 5VDC power for expansion modules
  • Open standard, freely usable with no licenses or royalties


EMX stack configurations

The EMX form factor always consists of a processor module in the bottom position, with optional expansion modules added on top.

Bottom to top: Heat spreader; COM Express module;
EMX carrier board; EMX expansion module

(click image to enlarge)


EMX defines two physical formats for the processor module: EMX Basic and EMX Compact. As shown below, a processor module may consist of either a COM-plus-baseboard sandwich or an all-in-one SBC.

EMX carrier-plus-COM sandwich EMX all-in-one SBC
(click images to enlarge)


The two EMX form factors differ in the following ways:

  • EMX Basic — 95 x 125 mm — This EMX form factor matches the size of COM Express Basic modules. Its four corner mounting holes are identical to those of COM Express. Two additional mounting holes are added to match the hole pattern of COM Express Compact (one of these is also standard in COM Express Basic). EMX Basic is intended for SBCs and COM Express carriers, with one notable exception: carriers (adapters) for adding PC/104-family I/O modules to EMX stacks.
  • EMX Compact — 95 x 95 mm — This alternate EMX form factor matches the size and mounting holes of COM Express Compact. It is intended for use as SBCs, carriers for COM Express Compact modules, and I/O expansion modules.

Due to the physical compatibility between COM Express Basic and COM Express Compact modules, along with the fact that EMX Basic carrier modules have mounting holes matching those of both sizes ot COM Express modules (Basic and Compact), an EMX Basic COM carrier board can be used with either size COM Express module. However, the typical use is to match the size of the EMX carrier board to that of the COM Express module.

EMX processor modules may use either the EMX Basic or EMX Compact form factor. EMX expansion modules use the EMX Compact form factor, with the exception that EMX carriers for PC/104-family expansion modules use the EMX Basic form-factor.

COM Express carriers may be designed in either Basic or Compact size. Basic carriers may be physically and electrically compatible with both Basic and Compact COM Express modules. Compact size carriers may only be used with Compact COM Express modules. EMX uses a board-to-board stacking height of 14mm. Component height restrictions and other board-to-board spacing details are provided in the complete EMX Specification.

An EMX expansion module is always imlemented in the EMX Compact size, unless it is an adapter for another form factor that requires a larger board. One such non-standard expansion board is currenty defined: an EMX Basic form factor carrier board for adding PC/104-family expansion modules to EMX stacks.

Four possible EMX stack configurations, which depend on the combination of processor and expansion module form factors, are illustrated below.

EMX Basic SBC + EMX Compact expansion module

EMX Compact SBC + EMX Compact expansion module

EMX Basic COM Express carrier + EMX Compact expansion module

EMX Compact COM Express carrier + EMX Compact expansion module


Bus signals and considerations

EMX uses a pair of high density (0.635mm / 0.025” pitch) surface mount connectors for board-to-board connection for the purpose of adding expansion modules. Processor modules use only the top side connector, since they stack only upward. Expansion modules use both top and bottom bus connectors. The connectors have 100 pins, offering the most popular expansion bus interfaces plus enough reserved pins for future addition of new signals as may be desired by the user community.

The table below summarizes the signal assignment on the EMX bus connector.

Function Quantity
PCI Express 4 x1 links
USB 2.0 2 ports
SATA 1 port
LPC 1 bus, 2 independent clocks
GPIO 2, for CPU-to-board communications
SMBus 1 link
+5V DC 7 pins, 11.9A max steady state
+3.3V DC 8 pins, 13.6A max steady state
+5 DC standby 2 pins, 3.4A max steady state
Ground 15 pins
Reserved 12 pins


There is no minimum feature requirement specified for the EMX expansion bus, except that EMX processor modules and baseboards must provide at least +5V and +3.3V to the EMX expansion connector. Implementation of any and all other features is at the discretion of the EMX processor module vendor. Each processor module’s product literature should state the features available, including the expansion power available to modules plugged into the bus.

EMX I/O modules may use any number of PCIe links from 0 to 4. Links are always used starting with position 1. If no links are used, all links are passed directly up from the bottom connector to the top connector. If one or more links are used, the remaining links are shifted down according to the number of links used, so that position 1 on the top connector always has a link available (unless all links have been used).

Further details regarding the bus connector, its signals, and other considerations are provided in the complete EMX Specification.

Join the EmbeddedXpress Initiative!

EmbeddedXpress (EMX) is an open standard, freely usable with no licenses or royalties. For further details regarding the EMX standard, request a copy of the EMX Specification. For more information on the EMX Initiative, please contact us here.

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