Watch Out! Open Standard Architecture Pitfalls



Use these tips for avoiding heat, complexity, cost and other issues.

For today’s modular COTS embedded computing requirements, the main options have been whittled down to a few choices. The venerable VME and CompactPCI enjoy some tech refresh business in some applications and a new design here and there, particularly overseas. However, even the Mil/Aero tech refreshes are finishing up on the legacy buses and upgrading to serial-based systems for future designs.

To focus the discussion, let us limit the system types to backplane-based designs for Mil/Aero and other mission-critical and high-reliability applications (including Communications, Test & Measurement, Transportation, and high-end Industrial.)   For these applications, the primary architectures are OpenVPX, MicroTCA, and AdvancedTCA. CompactPCI Serial also has some inroads in Transportation and less demanding Industrial applications.

When choosing between these architectures, there are some pitfalls to avoid. Each architecture has its advantages and disadvantages, but sometimes the latter are not immediately apparent.

Figure 1: The connector interface for VITA 62 power supplies can be incorporated into a monolithic backplane, but that requires customization. Many applications (as well as prototype systems) utilize these types of standard power interface boards for simplicity.

Figure 1: The connector interface for VITA 62 power supplies can be incorporated into a monolithic backplane, but that requires customization. Many applications (as well as prototype systems) utilize these types of standard power interface boards for simplicity.

OpenVPX

OpenVPX is the primary choice for Mil/Aero applications. There is a wealth of board options, particularly in conduction-cooled formats with proven Industrial and Mil-grade components for extreme temperatures from -40 ºC to +85 ºC. Backplanes are moving from standard 3.125, 5.0, 6.25, and 10 Gbps signal rate options (x4 for 40G speeds) and beyond. There are also standardized options for optical and RF contacts via VITA 66 and VITA 67 respectively. With a 3U or 6U Eurocard format, many designers are familiar with the mechanicals. However, there are some issues to be forewarned about:

  • Compatibility/Ease of Use: Many readers are aware of the compatibility issues that OpenVPX was designed to address. But to ensure this compatibility, matching which profiles work together can be complex. Further, what many engineers do not realize are the other “little things” for compatibility. For example, the SATA lines between modules and backplanes or any dedicated lines going to the Rear Transition Modules (RTMs). Great care should be taken to ensure all of the boards are compatible.
  • Lots of Specifications: There are VITA 46, 48, and 65 for the backplanes and chassis. Then there are VITA 66, 67, and 68 for further backplane iterations and details. Then there are the “dot specs” (VITA XX.1, XX.2, XX.3, etc.) for them as well. It’s no wonder so many VPX customers are often looking for the full solution from vendors. VITA 62 power supplies are often required in Mil apps (meeting MIL-STD-704 requirements). Standard power interface boards per VITA 62 can be a convenient tool (see Figure 1).
  • Watch the Heat:  With the packaging demands getting more compact, the heat from many VPX boards can skew your cooling requirement. This is especially concerning for conduction-cooled requirements. Watch the wattage of any given board and account for a way to cool it —the board could change your cooling solutions and increase your costs.
  • RTM Slots: Plan your RTM requirements carefully. The VPX connectors are extremely expensive, and full RTMs on the backplane could nearly double its cost. Know what you need and plan carefully.


MicroTCA

MicroTCA (“single module” size) has the advantage of being a little more than one half the size and weight of 3U VPX modules. This is attractive for many applications. There is inherent compatibility written into the core specification, including for multiple fabric options (PCIe, SRIO, and GigE), SATA lines, and GigE out-of-band communication.

For smaller systems, a switch is not required (connected via point-to-point). Plus, the performance density is outstanding. You can have up to 12 slots in a 1U chassis (via a midplane)!  There are powerful digitizing, FPGA, and storage modules available off the shelf. MicroTCA has unique features such as electronic keying. It also has inherent shelf management. Rear I/O is available on MicroTCA.4 systems, but the size increases to the “double module” size. For rugged Small Form Factor applications, there are application-specific designs that take advantage of the small module size. (See Figure 2 for an example). But, there are some considerations for this versatile architecture:

  • Power Limitations: The max power is around 85-90W/slot. With a clever multi-tongue design, this can be stretched to a little above 100W. This is pretty high, but those used to OpenVPX and the very high-power options should keep this in mind. To reduce costs for Power Modules, many chassis have the PSUs built-in.
  • Complexity: MicroTCA is very powerful and can be overkill. Its complexity was very frustrating for early adopters and the resulting problems (much like in the original VITA 46 VPX) soured many on the specification. However, today designs have been simplified and only the expert vendors are still offering the technology. The bells and whistles in the specification require some practice and support from the MCH vendor. It is recommended to work closely with your MCH or system vendor.
  • Vendors: There are several vendors who sell an AMC board here and there, but only a couple of key players. The risk of a smaller (but very dedicated) ecosystem is something to consider.

    Figure 2:  MicroTCA’s small module size (approx. 75mm x 180mm) is conducive to application-specific designs for Small Form Factor (SFF) requirements. Figure 2 shows a conduction-cooled example with point-to-point connections.

    Figure 2: MicroTCA’s small module size (approx. 75mm x 180mm) is conducive to application-specific designs for Small Form Factor (SFF) requirements. Figure 2 shows a conduction-cooled example with point-to-point connections.

AdvancedTCA

AdvancedTCA  (ATCA) has the advantages of inherent shelf management and redundancy. The architecture is popular in Communications systems using Ethernet (GbE to 40GbE, with 100GbE in planning stages). With its high reliability and standard form factor, ATCA has also become popular in Mil/Aero communications applications and is even used in some airborne systems like the P-8 Poseidon. The 8U x 280mm form factor provides plenty of space for powerful processors, with the cooling defined in the specification to address it. ATCA has its limitations, including:

  • Large Form Factor:  The large size is a negative for taking up rack space. The speeds of MicroTCA and OpenVPX are not far behind, and many engineers only choose ATCA when they must (massive throughput demands, board real-estate needed, etc.).
  • Cooling and Backplane Claims: Some of the claims of the backplane speeds and cooling capabilities are best-case scenarios, which doesn’t mean much. Does the backplane meet IEEE requirements and pass for Crosstalk, Fitted Attenuation, Insertion Loss, Return Loss, etc.?   Does the “watts per slot” account for CPTA guidelines such as a fan tray removed for 2 minutes? Watch the “too good to be true” claims—they probably are.
  • Versatility: As ATCA has thrived in one core market—communications systems—it doesn’t have a lot of versatility. That said, you may be surprised to see some of the standard offerings of FPGAs, Graphics, and Digital I/O boards in the architecture. Further, with ATCA Carriers for AMCs, a lot more versatility is opening up.

CompactPCI Serial

CompactPCI Serial is a little bit lower-cost and simpler design for less demanding requirements. It has the same mechanical format, but the serial architecture doesn’t offer any electrical compatibility with the CompactPCI parallel bus. However, CompactPCI PlusIO is a stepping-stone to CompactPCI Serial that offers compatibility.

  • Narrower Niche: With less bandwidth capabilities and features than MicroTCA and OpenVPX, CompactPCI Serial fills a fairly narrow niche. It has gained some inroads in primarily Transportation (High Speed Rail), and some other moderate-to-low performance requirements in Industrial Control, infotainment/signage, etc.
  • Vendors: There are only a couple of main players in CompactPCI Serial, so caution should be taken.
  • Versatility: With fewer vendors in the market, the ecosystem is relatively small and there are fewer standard choices.
  • No Special Interconnects:  Standard interconnects such as PCI Express, Gigabit Ethernet, USB, and SATA are supported.  However, Serial RapidIO and Aurora are not.


Ruggedization

Ruggedized OpenVPX systems are commonplace, but for AdvancedTCA and MicroTCA they are less common. MicroTCA.2 and MicroTCA.3 are dedicated for ruggedized solutions and have gone through extensive lab testing. But, there are only a few ruggedized MicroTCA.3 designs in the field. Alternatively, there are a growing number of application-specific MicroTCA solutions that are ruggedized.

The set of application-specific MicroTCA solutions includes Small Form Factor (SFF) designs in conduction-cooled formats. However, these typically have not gone through detailed live testing. AdvancedTCA also has some rugged solutions, but they can be very costly, and the choices are limited. Its size and powerful processors make conduction-cooling impractical. Typically, the chassis are bolted in a ruggedized cabinet rack in airborne applications.

All of the architectures discussed here have their pros and cons. It is important for designers to realize and avoid the pitfalls of each architecture. Doing so will save your company time, effort, and money.
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moll_headshotJustin Moll is VP, US Market Development, Pixus Technologies.

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