Barbara SchmitzAlthough system designers can't always control the operating conditions of embedded computing applications, they now have new cost-effective options for delivering 3U CompactPCI (cPCI) board compatibility in harsh, extreme temperature environments. A new modular rack system design that adapts affordable, standard convection-cooled boards to demanding conduction-cooled applications offers off-the-shelf availability for both PowerPC and Intel-based platforms, with versatile I/O-intensive capabilities for applications such as graphics, touch screens, serial interfaces, fieldbus controllers, binary I/O and more.

Figure 1. This new modular rack system from MEN Mikro Elektronik provides greater flexibility for building more affordable embedded computing solutions by adapting standard 3U cPCI boardsThis conduction-cooled rack system concept, designed by MEN Mikro Elektronik GmbH, enhances the performance of standard 3U cPCI boards by using custom conduction cooled assembly (CCA) adapter frames and heat sinks to divert waste heat. A directly coupled thermal conductivity path – from CPU to external rack housing – permits the adaptation of affordable convection-cooled boards to conduction-cooled applications in operating environments ranging from -40 to +85 °C1, while maintaining the full usable space of the PC board. (Figure 1.)

Modular compatibility for extreme environments
In response to constant demands for versatility, economy and flexible implementation in extreme embedded computing applications, the new rack concept uses popular standards and commonly requested capabilities: 

• Rugged design in a familiar format. The new wall-mounted Eurocard rack system is designed to support single-board computers (SBCs) using the popular 3U cPCI format. Each rack backplane includes an H15 connector for a power supply, plus three cPCI slots to accommodate one SBC, one side card and one I/O card (or, optionally, a second I/O card in place of the side card). While all board connectors adhere to the 3U cPCI standard, the mounting spacing between connectors on the rack's backplane is adjusted to accommodate the added thickness of the CCA frames. (Figure 2.). 

Figure 2. The board mounting layout of the conduction-cooled rack system provides space for a single-board computer

Figure 3. This new convection-cooled F50C PowerPC-based SBC is already adapted for use in extreme temperature applications when mounted in the 3U cPCI conduction-cooled rack. A custom adapter frame• Off-the-shelf availability. The latest standard board designs compatible with the new rack and CCA frame systems include PowerPC SBCs based on MPC8543 or MPC8548 processors. These offer clock frequencies up to 1.5 GHz and are capable of supporting up to 2 MB SDRAM, 2 MB non-volatile SRAM, 128 KB non-volatile FRAM and up to 16 GB of Flash solid state disk storage. Existing Intel-based SBCs ranging from Celeron M processors up to power-saving Core 2 Duo processors are also compatible with this type of frame design. Each unique board/processor combination is matched to a customized frame system with a tailor-made heat sink designed to transfer heat efficiently from the CPU to the frame and housing of the convection-cooled rack enclosure. (Figure 3.) 

• Versatile I/O capability. Beyond the sheer number of available connections in the new rack system design, there is versatility in wiring configurations and diversity in the I/O connector options of compatible off-the-shelf boards. These include standard USB and Ethernet ports, plus field programmable gate array (FPGA)-controlled connections to enhance customized I/O functionality. Serial ATA (SATA) ports with transfer rates up to 150 Mbytes/s are also available for mass storage via rear I/O. 

• Compatibility with commonly-used embedded system software. SBCs compatible with the new conduction-cooled rack design support a variety of popular embedded computing software systems, including Linux, VxWorks, QNX, Green Hills INTEGRITY and OS-9.

Built for rugged performance
The robust rack enclosures constructed for this application are hermetically-sealed against environmental effects – including EMC protection, plus compliance with IP-65 standards to resist dust and water intrusion. The inner shape of each CCA frame is unique to the cooling requirements of its matching board, accommodating specific hot spots that need thermal contact with the CCA frame. Those frames are mounted with wedge-lock connectors for high vibration resistance (2g/10 Hz-150 Hz, sinusoidal) and shock resistance (15g/11ms), as well as for reliable thermal coupling. The 200 mm x 350 mm x 145 mm rack housings also accept a range of power supply options, including 9 V to 36 V (24 VDC nominal) and 35 W.

Predictable thermal performance
The total thermal resistance (Rth) of the design is 0.4 °C/Watt, allowing system designers to calculate the performance of their real-world applications according to the following formula:
?T = Tinternal – Tambient = Pv * Rth
As an example, this formula would dictate a maximum electrical power of 37.5 W to be installed in a CCA frame with a maximum allowable temperature of +85 °C and an ambient temperature of +70 °C. (Pvmax?T/Rth = 15°C/0.4°C/W = 37.5 W). Likewise, given an installed electrical power of 22 W and an outside temperature of 60 °C, the component operating temperature could be calculated as 68.8°C (Tinternal = Pv * Rth + Tambient = 22 W * 0.4°C/W + 60°C = 68.8°C).

Ample processing and I/O power
The processing speed, power, memory and mass storage capabilities available to end use applications of the new conduction-cooled rack system all vary according to the specific board and processor combinations selected. One common advantage of the modular board-and-frame design, however, is that multiple standard board options are readily available for immediate prototyping operations. Another advantage of using frame-mounted convection-cooled boards – beyond the obvious cost and availability benefits – is the fact that there is no need to sacrifice board space to achieve the convection-cooled performance. The frame-mounted approach typically maintains from 20% to 30% more available space than traditionally engineered VITA-conforming products.

Standard front connectors of the boards are wired to four MIL-C-38999 connectors at the back of the rack system, with different outer connector styles available as options. Three of the connectors (a total of 59 pins) are available to support both front and rear I/O connections, while one connector (7 pins) is devoted to the power supply. Unique mechanical coding of the connectors is provided to ensure foolproof installation.

Keeping system designers in control
Adoption of this new rack system offers embedded computing designers broader options for implementation in environments that demand sealed, conduction-cooled housings. The modular execution of the adapter frame design provides greater freedom to accommodate the thermal characteristics necessary for their unique embedded applications in harsh, high-temp environments.

With the ability to adapt familiar, proven convection-cooled boards for use in conduction-cooled environments, designers can streamline new application development while avoiding the expense of custom conduction-cooled SBC development. Also, by concentrating any new component development to only the CCA frames customized for the desired standard 3U cPCI boards, designers can achieve their desired physical performance at minimal costs, and shorten implementation timeframes. Equally important, they can accommodate the diverse I/O demands of specific applications within the preformatted capabilities of standard hardware products. 

1 +85°C is the maximum temperature inside the rack. The maximum ambient temperature is +70°C.