ANSI/VITA makes computer-on-modules mission-critical - Embedded.com

ANSI/VITA makes computer-on-modules mission-critical

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

In the PC world, “case modding” is quite common to enhance the thermal behavior of computers. You can buy a mainboard completely dipped into cooling oil, with a pump making the liquid circulate. This rather extravagant idea would make embedded systems designers smile, but it would not solve any of their design problems.

Facing space-conscious requirements, they need to use smaller form factors, and they are continually being pressured to do more within that reduced space. As if that is not enough, embedded computers now also conquer markets that bring in extreme application demands. These include medical, industrial and especially mobile applications.

Beside small footprint and ruggedization, any system needs specific functions. In the most ideal case, enough flexibility is left for technological upgrades, which in turn makes component compatibility a nice-to-have treat. This sounds as if the costs of embedded designs will soon be exploding because they have to go into so many details. This is where COMs come in.

Computer-On-Modules (COM), or “System-On-Modules” (SOM), incorporate a complete computer on a plug-on mezzanine card. The carrier board customizes the system: it is individually tailored to each application, including specific I/O.

Since the introduction of the COM concept, embedded system designers have reaped a variety of benefits from that technology. Making it easier to replace modules of compatible form, fit and function has enhanced design flexibility, extended application versatility, reduced development costs, and optimized time-to-market.

But despite standardization efforts, these benefits still were not readily accessible for rugged system performance in harsh operating environments. Mobile and safety-critical applications face the same constraints as less harsh applications, but the costs of failure are higher.

The transportation industry, which encompasses both land and air vehicles, has stringent standards itself in terms of the needed ruggedization and manufacturing specifications. Mobile applications, found in areas as diverse as medical engineering, commercial vehicles and ships, are rugged by nature. A growing number of cranes, construction machinery, trucks and tractors are equipped with small, robust human-machine interfaces.

Medical engineering includes safety-critical systems such as movable or portable systems for imaging or to monitor, anaesthetize or treat patients. Outdoor computers are used for telecommunication, in traffic monitoring, on oilrigs or in the security sector for video surveillance applications.

Many environmental aspects come to mind for electronics systems. Computers are subjected to high levels of shock and vibration. For the COM concept with multiple PCBs and board-to-board connections, this is a serious issue. Other factors are dust or moisture, for instance in outdoor applications, but also EMC and finally heat as one of the most typical problems.

Heat needs to be drawn away from the components to prevent them from overheating. And increasingly dense embedded systems with more and more components and complexity equal more heat to be dissipated from deeper within the system. For semiconductors there is a maximum junction temperature above which the semiconductor ceases to work.

Convection cooling is the easiest method to cool board assemblies, by guiding an air flow along the surface to be cooled. The mechanical set-up is simple, but flowing air can bring impurities and liquids into the device, which can cause damage. The complex filtering equipment necessary to reduce this risk and the cooling fans themselves, having a limited lifetime, create the need for maintenance.

Conduction cooling optimizes the thermal contact to lead the heat from the source to the outer wall of the enclosure. Suitable materials minimize the thermal transfer resistance from the electronic components to the enclosure wall. The boards inside the housing need to be placed in the right way, and the heat-conducting cooling blocks need to have such masses that optimum heat transport is guaranteed.

System designers with a mobile, critical application in mind are likely to build upon the principles of conduction cooling, because it is more calculable and reliable. The bad news is: the methods necessary to implement this need to be undertaken at the onset of PCB design, including custom aluminum parts or additional copper layers inside the PCB to transfer heat.

Other aspects of rugged design can be addressed more easily: soldered components and suitable connectors withstand jolts and jars, and coating the PCB protects against dust, humidity and corrosive materials. Handling temperature, in turn, is a science, and is one of the biggest challenges for COM concepts. It is obvious that a standardized COM module with a well-designed cooling concept could take much heat out of the designer’s work, also by reducing design costs.

Most of the COM concepts available on the market are not standardized, lack rugged aspects or lag behind in interface technology. COM Express (PICMG COM.0) as the first PCI Express based computer-on-module standard specifies an operating temperature range of -40°C to +85°C but has its shortcomings in the details: shock and vibration are not fully taken account of, and cooling and EMC are still rather complicated issues.

A more recent variety of the computer-on-module concept known as ESMexpress builds upon the concept behind COM Express and takes its implementation to a higher level regarding rugged performance. Its specification is being prepared by a work committee including system vendors as well as avionics manufacturers and other rugged electronics experts to be standardized by VITA according to ANSI under the official designation ANSI/VITA 59, RSE Rugged System-On-Module Express.

ESMexpress sets out to make COMs a stable, cost-effective solution for a wider number of areas by addressing mechanical stability, rugged physical performance, and aggressive fanless cooling. To achieve this, the populated PCB with a size of 85 mm by 115 mm is mounted into an aluminum frame with a final size of 95 mm by 125 mm, which completely encloses the assembly.

This allows for conduction and convection cooling and enables power dissipation of up to 35 W. Instead of being limited to the -40°C to +85°C operating range, ESMexpress can offer performance up to -55°C through +125°C. (Figure 1 below )

Figure 1: ESMexpress ANSI/VITA 59 frame and cover structure for conduction cooling

The mechanical set-up of the COM module greatly contributes to the thermal performance of the overall design. The high pressure caused by the safe joints between the housing and PCB supports the thermal connection of the components.

The PCB edges transport heat to the frame and then to the housing cover. Also, the hottest components within the unit are coupled directly to the cover. The eight mounting screws for the carrier secure the entire physical assembly in place and ensure optimum heat transfer.

If a module needs additional cooling, the aluminum housing can be connected to an external heat transfer device (conduction) or combined with a heat sink for heat dissipation (convection). The closed metal case also ensures optimum EMC protection conforming to EN55022 for the electronic parts, hermetically sealing them on all six sides.

To make it resist shock and vibration, the COM module has fully soldered connections and is rated to withstand 15 g / 11 ms for shock and 1 g / 10 to 150 Hz for vibration (sinusoidal). Eight screws and two robust 120-pin board-to-board connectors make the COM and its carrier a tight unit. The connectors are qualified to railway and MIL-compliant applications and specified for -55°C to +125°C. Supporting differential signals with up to 8 GHz, they provide ample connections for I/O intensive applications.

For particularly harsh operating environments, where dust or moisture is a potential concern, conformal coating is optional for added protection.

In addition to the 125 mm x 95 mm ESMexpress, an even smaller format using the same rugged-design concept is available in a 55 mm x 95 mm module size. This more compact COM format called ESMini is not specifically part of the VITA 59 standard but follows the same specifications for rugged performance and cooling as its bigger brother.

In terms of functionality, ESMexpress defines electrical signals only for modern serial buses. The pin assignment of the two 120-pin connectors is fixed to guarantee complete interchangeability of ESMexpress modules. (Figure 2 below )

Figure 2: ESMexpress connector side with two 120-pin board-to-board railway-compliant connectors

For PCI Express, there are four single lane ports (4 x1) and one configurable port (1 x16, 1 x8, 2 x4 or 2 x1) for connection of external high-end graphics. If only one single-lane port is needed, one of the 120-pin connectors can be omitted entirely. Three Gigabit Ethernet (also as 10-Gigabit), eight USB, three SATA, SDVO, LVDS, HD Audio, several utility signals and the single 12 V power supply round out the connection ports.

Building on IP cores in an FPGA, you can add further individual I/O to the design. The FPGA in ESMexpress is added to the carrier board where needed and is controlled over a fast PCI Express link, ensuring a future-proof system.

Unlike other COM standards limited to x86 chipsets only, ESMexpress and ESMini are processor independent, making the CPUs scalable and flexible. Currently available modules accommodate a variety of Intel and PowerPC chipsets to satisfy application-specific requirements ranging from low-power mobile apps to high-end communications platforms. Despite a set threshold of 35 W for power dissipation, many of the specific CPUs in current modules actually draw only a fraction of that power, minimizing cooling concerns.

ESMexpress complies with the COM Express Basic Form Factor and has the five screw holes defined for COM Express. MEN offers an ESMexpress to COM Express carrier adapter board that accommodates the pin-out and mechanics of the two standards, for example to evaluate ESMexpress solutions without producing a fully developed carrier board. Systems that currently employ COM Express could benefit from the added rugged attributes of VITA 59. (Figure 3 below )

Figure 3: Chart comparing COM Express with ESMexpress

ESMexpress and ESMini offer a unique combination of economy and performance for increasingly difficult environmental challenges. They open up technological capabilities for mission-critical applications not previously addressed by COM designs and bring together a rugged board concept and cost-effective, timely development.

The ANSI/VITA 59 RSE standard offers new promise for real-world applications from the high-profile security of video surveillance and biometric access control, to robotic and industrial control systems, to mobile test, control, and display systems across a variety of vehicle and transportation environments.

Susanne Bornschlegl is a member of the technical support staff at MEN Mikro Elektronik .

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