COMs vs SBCs

COMs vs. SBCs: Forward Thinking Helps Make the Best Embedded Choices

Each embedded form factor solution has a best fit, and designers should look at “customized modular versus off-the-shelf with no customization” as a foundation. This, in consideration of the end-use application, is an excellent starting point for the critical thinking that kicks off any design.


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The end application of any design typically helps define the choice of the computing form factor used. From there, the desired level of performance and scalability can be built into the design. But, when the application’s end use needs to offer future flexibility, and designers can choose form factors of varying size and shape, other issues can have an important influence on early design decisions.

For many applications, designers will need to make their embedded computing choice between modular Computer-on-Modules (COMs) and off-the-shelf single board computers (SBCs). Their decision can depend largely on the future of the design rather than just the current requirements for the application. In addition to meeting existing application requirements, designers are well served with a solid understanding of product evolution beyond the present generation on their design table. A little forethought in anticipating changes and upgrades from generation to generation, and even within a single generation, will help a designer determine the best and most effective computing form factor option.

One of the first questions asked is if the design can utilize an off-the-shelf SBC that offers minimal customization with standard I/O, or does it need a specialized customizable computing solution for non-standard I/O that will also enable the application to scale in the future.

A modular embedded computing solution such as a COM is scalable, interchangeable and allows for extensive customization. When integrating a COM design, however, OEMs must allow space and budget for a two-board design, as COMs work in tandem with a custom carrier board that handles all the unique functions of the specific end-use application. Ideally for this choice to benefit an OEM, the design volume over time will be in balance with the cost of developing a two-board solution as compared with using other off-the-shelf, less-customizable standard products.

In contrast, an SBC is a static single board solution that is designed to be readily available in large volumes and is usually a more cost-effective solution. Small form factor SBCs have become increasingly important in the embedded environment and there are many options, simple or complex, that are dictated by particular application demands. But it would be a mistake to think that the latest SBCs do not offer cutting-edge features. While all SBCs are built using similar standards, different technologies can be included such as video capability, Ethernet access functionality and digital computer technologies to meet the needs of a specific application.

The PICMG 1.3 specification is the latest generation SBC standard. It not only delivers outstanding performance and bandwidth, but also features a PCI Express on-edge connector to allow I/O headroom for emerging system requirements. PICMG anticipated the need for increased networking and audio/video capabilities, and the current PICMG 1.3 specification includes a strong feature set in a standard long-life solution. For example, many more applications have requirements for 3D graphics, high performance and communications capabilities. A pitfall many designers have fallen into is that adding custom features is beyond the scoped options of an SBC. This can become costly unless the volume needs of the OEM justify a custom board and/or a custom bios. SBCs may not be the best solution in cases when significant ongoing design flexibility of frequently adding parts is going to be an ongoing requirement.

Applications such as pharmaceutical manufacturing require good, straightforward communication capabilities and a solid level of horsepower–but not much more than that. PC/104-compatible SBCs hit the mark very effectively for pharmaceutical manufacturing by balancing computing functions with the right level of power, performance and cost. Certain medical imaging applications, by comparison, tend to have a lot of special peripherals that need to be added to the system. Designers frequently choose COMs because the plug-in ports on many SBCs do not typically accommodate the computing requirements of many of today’s medical devices.

Furthermore, newer infotainment applications may require computing controls that are very small and thin. This market is very closely tied to consumer expectations of fast evolution and regular performance increases, so COMs may be the ideal embedded solution. But designing computing controls that run the dancing fountains at key tourist venues around the world is a different story. These applications require a very consistent level of performance that would match the standard features provided by a small form factor PC/104-compatible or 3.5-inch SBC.

Computer-on-Module Solutions

COM Express is a PICMG standard that defines a Computer-on-Module, or COM. The standard includes defined interfaces that enable smooth transition computing solutions from legacy parallel interfaces to LVDS (Low Voltage Differential Signaling) interfaces, which include PCI bus, parallel ATA, PCI Express and Serial ATA. For future scalability, COM Express defines five different pin-out types. High-end applications that require high performance need the computing power provided by the new COM Express open standard.

To keep up with emerging applications and computing trends, the PICMG-defined COM Express standard is also evolving. Initially, COM Express was designed to accommodate the next generations of PCI Express (5 GHz) and Serial ATA (300 Mbit/s) interfaces, effectively doubling existing data rates to 160 Gbits/s and 1.2 Gbytes/s.

Kontron’s ETXexpress family of COM Express Computer-on-Modules was introduced to allow the application of high-speed COMs for PCI Express Bus and PCI Express chipsets. There is also a compact footprint (95 mm x 95 mm) version of COM Express modules–the microETXexpress family of COM Express-compatible modules. The “micro” form factor keeps the same footprint of existing PCI modules (such as PC/104) but provides the needed pin-out compatibility with the existing “Basic” and Extended” form factors that will ensure future scalability. This second round of standardization gives developers the option to utilize and pay for only what they require for a particular application. More recently, the nanoETXexpress family of COM Express compatible modules has been introduced, which has a footprint of just 39 percent of the original COM Express standard “Basic” form factor module (Figure 1).

The original Embedded Technology eXtended (ETX) Computer-on-Module standard was introduced in 2001 to provide an open standard to meet the needs of embedded industrial applications. ETX provides a highly flexible mechanical design with performance scalability, and a number of board manufacturers have supported it making ETX the de facto standard for customizable designs. ETX modules are very compact (100 mm square, 12 mm thick), highly integrated computers. All ETX modules feature a standardized form factor and a standardized connector layout that carry a specific set of signals. This standardization allows designers to create a single-system baseboard that can accept present and future ETX modules.

The addition of the ETX 3.0 specification provides a highly integrated and compact form factor (3.7 x 4.4 in/95 x 114 mm). ETX 3.0 adds two Serial ATA interfaces without changing any of the ETX pin designations, making new modules 100 percent pin-to-pin compatible with previous versions. This ensures long-term support for the vast number of embedded applications already designed for the medical, gaming and entertainment, military and aerospace markets. The trade-off between ETX 3.0 modules and COM Express is that ETX does not accommodate PCI Express (PCIe). If the communication interface in the application needs to change in its next generation, then a transition to a COM Express module would be warranted to take advantage of the PCIe standard.

To satisfy the demands for high-performance and low-power solutions, Kontron offers the best performance-per-watt ETX Computer-on-Module based on the Intel Atom processor. With 2.5 watts TDP for the processor, 6 watts TDP for the Intel 82945GSE Graphics Memory Controller Hub and 1.5 watts TDP for the Intel I/O Controller Hub 7-M (ICH7-M), the ETX-DC COM requires a maximum TDP of 12-15 watts. The module’s performance-per-watt features make it an optimal choice for point-of-sale and industrial control applications as well as for use in harsh environments that require passive cooling and completely sealed housings (Figure 2).

SBC Solutions

PICMG specifications provide the backplane interface standards for SBCs. The PICMG 1.3 specification is the latest generation SBC standard, which resolves the bandwidth limitations of parallel bus technology by replacing the parallel bus interfaces with high-speed serial links. PICMG 1.3 not only delivers outstanding performance and bandwidth, but also features a PCI Express on-edge connector to allow I/O headroom for emerging system requirements and board to board interface. PICMG anticipated the need for increased design flexibility with support of interfaces for SATA, USB, IPMB, SMB, geographic addressing and power management.

Featured in the latest generation PICMG 1.3 SBCs, such as the Kontron PCI-760, is the Intel Q35 Express Chipset, which addresses many embedded computing designs requirements including graphics, low-power consumption, noise reduction, manageability, data protection and security (Figure 3). The chipset is designed to provide a responsive, high-performance, low-power platform when combined with the Intel Core2 Duo processors on 45nm and 65nm process technology.

The current PICMG 1.3 specification includes a strong feature set including 3x 1 Gbyte LAN, 6x SATA II, 12x USB, 4x PCI Express and more in a standard solution with long life availability. For example, many more military and medical applications have requirements for 3D graphics and video to meet the need for visual/audio-on-the-spot data or simulations. This flexible new SBC also offers up to quad core processor and up to 8 Gbyte 667/800 MHz DDR2 SDRAM memory options, so designers can develop cost-effective products to fit the needs of the application.

PC/104 has been a popular standards-based SBC form factor for applications that require rugged hardware and high reliability for maximum uptime where repair or replacement may not be possible. For industrial, military and point-of-sale applications, PC/104 is an optimal choice. Because it can be expanded with additional I/O sockets through its unique stacking bus using reliable onboard pins, multiple modules can be added to a system without the burden of backplanes or card cages. The design also enables PC/104-compatible SBCs to work well in rugged environments because of its tremendous shock and vibration tolerance and wider operating temperature range.

Satisfying space-constraint designs plus its low power make PC/104 highly suitable for designs that do not need game-quality graphics processing. While PC/104 is a robust form factor, it has current confines in both processor performance and power management. Even with its limitations, military, aerospace and medical customers are especially interested in using PC/104 systems in signal, image and data processing applications, in particular with real-time processing and data transfer.

To avoid the pitfalls or consequences of choosing one embedded computing form factor or platform over another, it is important to have a solid understanding of the exact needs of the application over time. So asking the right questions and including longevity as a design element have become more important than ever in achieving a successful design.

While a COM or SBC solution will typically fit similar application areas, deciding on the right form factor comes down to the specific OEM or application requirements. Considerations such as production volume, size, power and overall costs can be deciding factors. But other things come into play such as end-user expectations, future tools or interface requirements; modularity and the ability to easily change the design also influence a designer’s choices.

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