Form Factors Facilitate Fit, Function, Features

The term "form factor" in the context of industrial circuit boards, single-board computers (SBCs), and embedded control cards refers to the shape and physical size of these products. However, exact terminology is elusive in practice. Other terms are too often interchangeably used to convey the notion of a board-level product form factor.




  • Single-board computers (SBCs)

  • Embedded control

  • Industrial board form factors

  • Long-term product availability

  • Standard board formats

The term "form factor" in the context of industrial circuit boards, single-board computers (SBCs), and embedded control cards refers to the shape and physical size of these products. However, exact terminology is elusive in practice. Other terms are too often interchangeably used to convey the notion of a board-level product form factor. These include architecture, bus, footprint, format, module, platform, protocol, specification, standard, and so on.

Industrial board form factors tend to have a relatively small aspect ratio. Experts say that staying close to a square format is best to reduce shock and vibration effects on the board, and also helps with obtaining a firm mounting. Beyond specifying size, choice of form factor offers other attributes. It sets the basic fit and function of boards within their application space. Mechanical layout, connectors, and input/output (I/O) zones also are typically associated with board form factors. Embedded controls demand special consideration for space, power, and reliability in their form factors.

Whatever you call them, use of standard form factors streamlines the work of developers. It also gives machine builders and their customers some assurance of long-term availability of products based on common sizes and attributes.

An umbrella analogy

One way to categorize form factors is to visualize an umbrella, representing a computer board standard that encompasses items such as mechanical dimensions, I/O capability, expansion with other boards, mounting, and certain electrical characteristics, according to Robert Burckle, vice president of WinSystems Inc. "Then, form factor represents the size of the umbrella," says Burckle. "Some specifications are more detailed than others as they spell out where power, I/O points, and expansion connectors should be placed along with keep-out areas and other mechanical issues."

Connector and I/O placement, even mounting holes, often are considered part of form factor. "Such form factor specifications make sure that different manufacturers' board products will be consistant so that enclosures, cabling, and other system integration burdens will be lessened," Burckle says. "However, electrical specifications, operation, processor, and other chips used do not fall into the definition of 'form factor.'"

Common industral computer board standards have specific strengths, explains Burckle. "For example, VME and CompactPCI are expandable via a bus and backplane system. PC/104, EPIC, and EBX have self-stackable I/O modules, which eliminate the need for a rack and backplane."

Derrick Lavado, product manager, Embedded Systems-Kontron America, agrees that common attributes of form factors for embedded and/or industrial system boards include I/O point location, expansion slot placement, component height, and mounting hole location. "All these considerations are essential to help a system engineer design a flexible and familiar configuration that reduces time-to-market while providing an upgrade path as technology shifts further along the path of Moore's law," he says.

Besides physical dimensions, Lavado regards I/O zones as one of the most notable form factor attributes. For example, I/O connector location is specified for standard interconnects (LAN, serial parallel, USB, and PS/2) on PC/104, EBX, and other form factors. "Specific zones are provisioned for I/O headers so that additional features may be broken out from the board," explains Lavado. He also mentions component height restrictions associated with some form factors, such as PC/104, which uses a stackable expansion concept. Because I/O modules are stacked on top of each other starting with the CPU module, a minimum clearance restriction is needed for stacking modules and also to allow for proper cooling throughout the system.

Popularity = longevity

"The most popular form factors tend to be ones that offer high performance CPU/chipset combinations in a very small footprint," Lavado continues. He includes PC/104 in this category. "PC/104 modules offer extremely flexible mounting options for applications with compact enclosure requirements. All I/O is cabled out from the board, making remote I/O location flexible with the design," he says. Today, typical PC/104 modules include powerful CPUs and processors (Intel Pentium M and Celeron M, for example) that are said to offer great performance and excellent low power characteristics.

Embedded Control

Dimensions of EPIC form factor are set to obtain a board area midway between PC/104’s stackable format and EBX’s SBC format.

At National Instruments Corp., form factor refers to physical dimensions and basic characteristics of the board. However, features such as guide rails and connectors also could be involved, suggests Tim Fountain, NI's hardware product strategy manager for Instrument Control. Placement of interface connectors becomes important for convenient I/O access.

NI considers long-term availability and support of board products crucial to their application in systems, and on that basis relates form factor to the board's overall architecture and standard. "We prefer to work with specific board standards that offer the widest support and specify the form factor within the standard," says Todd Walter, group manager, Industrial Measurement and Control. Industrial systems enjoy longevity and their complement of boards needs to be replicated for a long time. Moreover, NI takes a higher, platform-level view of form factors, selecting ones that satisfy customer demands such as cooling capacity, low power consumption, vibration resistance, and multiple sourcing, explains Walter.

The company works with various board form factors, including CompactPCI, PCI Express, and other PCI derivatives. National Instruments originated an extension of PCI dedicated to instrumentation, which is called the PXI format. It defines and adds timing, triggering, and synchronization signals to enable PXI boards for test and measurement applications.

WinSystems concurs about the importance of standards and long-term product availability issues. A key factor is to base designs on recognized standards with multiple vendors building boards to ensure longevity of supply. "Ten to 15-year life cycle is not unusual for industrial products and systems," adds Burckle. A standards-based form factor also implies increased interoperability of components and greater access to software tools.

Size, applications matter

Ray Alderman, executive director of VITA (VMEbus International Trade Organization), believes that applications dictate the choice of board form factors. VITA, a non-profit vendor/ user organization, promotes VMEbus-related standards and open computing technology.

Size of a board determines the number and types of connectors that can be incorporated. If the form factor is very small it limits the number of I/O points possible. However, in simpler applications—say, a machine controller for injection molding—a small form factor can do well because not many I/O points are involved, explains Alderman. As applications become more event-driven, I/O point requirements increase, accompanied by the need for sophisticated interfaces at high-end applications; for example, in machine vision or motion control. Larger form factors accommodate these needs within the overall space constraints of a particular application.

Numerous board form factors exist, with more being created over time. The following are commonly used in industrial or embedded control applications and have the backing of consortiums or associations.


PC-based Control

EPIC offers three I/O zones (1A, 1B, 2, and 3) for implementing numerous general-purpose and application-specific I/O devices. PC/104 expansion modules provide further I/O options. Metric equivalents of inch dimensions are in

This industrial bus format, based on the standard PCI (Peripheral Component Interconnect) electrical specification, has Eurocard packaging and comes in two basic sizes: 3U (100 × 160 mm—see diagram online) with single 220-pin connector and 6U (233 x 160 mm) that supports up to three additional high-density, 2-mm pin-and-socket connectors. (See more about Eurocard below). CompactPCI boards are inserted from the front of the chassis, allowing I/O points to break out to the front or rear. Vertical orientation of boards in the chassis makes for good cooling; shock and vibration resistance is said to be excellent.

CompactPCI is designed for rugged applications—industrial automation, real-time machine control/data acquisition, instrumentation, and military systems, among others. It is administered by the PCI Industrial Computers Manufacturer's Group (PICMG), a consortium of more than 350 industrial computer product vendors. This makes for long-term manufacturer's support.


With dimensions of 5.75 x 8.0 in. (146 x 203 mm), Embedded Board, eXpandable (EBX) offers a form factor large enough to implement a single-board computer (SBC) and its operating system. At the same time, an EBX-compliant board is small enough for deeply embedded applications with inherent space constraints.

EBX typically accommodates a CPU, memory, mass storage interfaces, display controller, serial/parallel ports, and other system elements required from an embedded SBC. This form factor also provides flexible system expansion using industry standard modules. It supports stackable PC/104, PC/104- Plus , and PCI-104 specifications, which opens access to a large number of standard expansion modules and suppliers. EBX defines recommended zones for I/O options as well as precise location of connectors and mounting holes. EBX form factor is divided into 10 zones for standardizing placement of various interfaces and components. Each zone (A though J) has an associated maximum allowable component height that ranges from 0.5 to 1.5 in. These locations serve as guidelines to smooth interoperability among multi-vendor products and packaging. The PC/104 Embedded Consortium manages EBX and its latest release is version 2.0.


Embedded Platform for Industrial Computing (EPIC) was developed to fit where an EBX form factor is larger than needed or a PC/104 is too small to accommodate the necessary I/O connectors and cabling (see "Size comparison" diagram). The format focuses on providing more space for faster processors and more I/O points needed for complex functions in industrial and other applications. EPIC handles larger processors with larger heat sinks and allows placement of features on one SBC that otherwise would need multiple PC/104 modules.

EPIC form factor defines location of the expansion stack (via "104" specifications), bus connectors, and mounting holes for the main board and expansion board. This form factor likewise is divided into zones to standardize connector and component placement (see "Zone specs" diagram). A maximum component height limit applies to most zones, one exception being the "tall CPU and power zone," where processors with a larger cooling fan and/or heat sink are located—along with power supplies and power connectors. Also defined are three I/O connector overhang zones for mating connectors and cables that pose potential interference between "104 expansion" and EPIC modules.

The PC/104 Embedded Consortium also manages EPIC. Its latest version is 2.0.5. More than 20 manufacturers offer EPIC boards and the list is growing.


Defined as "a small 'form factor' embedded PC physical and electrical interface standard," PC/104 was developed in response to users' needs to embed microcomputers for control in their products and systems with tight space limits, according to the PC/104 Consortium. The form factor also seeks to simplify board design and avoid extra costs of custom approaches.

PC/104 delivers architecture, hardware, and software compatibility with PC bus via stackable modules. These board modules come in 8-bit and 16-bit bus types, corresponding to PC and PC/AT buses, respectively. PC/104 further specifies two bus options to help ease tight embedded space constraints. Designated "stackthrough" and "non-stackthrough," these options depend on whether the bus connector can extend through the module or not. A stack can contain a combination of 8- and 16-bit modules.

Some 160 international companies reportedly manufacture products based on PC/104 standards. Current version is 2.5.


PCI eXtensions for Instrumentation (PXI) is a modular instrumentation platform designed for measurement and automation applications in need of a rugged industrial form factor. PXI combines standard PC technology from CompactPCI with integrated timing and triggering capability to deliver up to 10 times performance improvement over older architectures, says the PXI Systems Alliance (PXISA), an industry consortium that promotes and maintains PXI standard. Originally developed by National Instruments as an extension of CompactPCI, PXI retains the same form factor.


Venerable VME is well over 20 years old and still claims dominance as an open standard industrial form factor for embedded computers, especially in military and defense applications. (VME stands for "VersaModule Europa" although variants to this terminology exist.)

The original specification was sponsored by VITA, establishing a framework for 8-, 16- and 32-bit parallel-bus computer architectures to enable single and multiprocessor systems. VME mechanical specifications are based on IEC 60297 and IEEE 1101.10 standard, also known as the Eurocard form factor. VME has standardized on two card sizes: single height (3U) for space limited and vibration prone environments and double height (6U) allowing more component placement when board space is available.

Eurocard connection

Several popular industrial board form factors—CompactPCI, PXI, VME, among others—actually derive their physical dimensions from Eurocard. The Eurocard mechanical standard (IEEE 1101.10) has established a gamut of modular dimensions for subracks and boards; however relatively few sizes have been translated into available products.

Probably most well known are 3U and 6U board sizes, where U refers to "units" of front panel height dimension. 1U equals 1.75 in. (44.45 mm). Board height (or card height) is smaller than rack height to allow space for card guides and front panels. Actual height is 100 mm for a 3U card and a less even 233.35 mm for a 6U card. Modular Eurocard depth dimensions start at 100 mm and grow in 60-mm increments.

Many form factors have been created for board products, but relatively few are firmly entrenched in industrial applications. It takes time to establish user and market acceptance, as well as a reputation for long-term product availability.

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