Why 3U VPX has an edge over CompactPCI for FPGA/DSP military applications

Rob argues that on the 3U front, support for high-speed serial fabrics has 3U VPX winning out over 3U CompactPCI for military applications, and he then describes trade-offs to consider when comparing 3U VPX to 6U VPX.

For more than 20 years, the VMEbus architecture has dominated the 6U DSP and board market. One distinct shortcoming of VME was that it lacked a small form factor equivalent to its popular 6U form factor that could specifically address smaller systems, including those used in ground vehicles for vehicle protection and imaging systems, and in UAVs in the sensor payload for such as SAR, , and SIGINT.

About 10 years ago, in response to the growing demand for small form factor COTS boards, started to gain the interest of the military and aerospace community. But as the appetite has grown for higher bandwidth and higher computing densities, such as those needed for processor-intensive DSP and FPGA signal processing applications, the market has turned to higher-speed serial communications (for example, Serial ) that 3U was unable to support. While there have been efforts to address this through CompactPCI Express and CompactPCI Plus, neither has matched the performance targets desired by the military COTS market; thus, neither has gained much traction in the military and aerospace community.

3U () has a standards-based architecture and is able to support the desired high-speed serial fabrics while maintaining features familiar to users of both VME and CompactPCI such as 0.8" pitch. In addition, 3U VPX also supports 0.85" and 1" pitches, which is attractive for higher-density computing applications. The larger pitches help dissipate heat because they permit larger heat sinks, and, in the case of conduction cooling, larger cooling frames. 3U VPX also provides high-speed Tyco MultiGig-2 connectors that support high-speed serial links and alignment and blocks unavailable with 3U CompactPCI. Even better, more user-defined signal pins can be found on 3U VPX than on 3U CompactPCI. 3U VPX also has connectors and framing that build in ESD protection, persistent fail LEDs, and front and back metal protective covers that enable two-level, in-the-field maintenance.

In addition, integrating VPX systems is about to get easier. A new cooperative effort called will soon introduce a standards-based set of profiles for VPX boards and backplanes to improve system-level interoperability between modules. OpenVPX will establish design practices intended to ensure that all adhering modules can work together.

When to choose 3U over 6U?

There are certain application constraints that dictate when 3U VPX is a better choice than 6U VPX. Some systems are simply Space, Weight, and Power (SWaP) constrained. For these systems, packing as much processing and I/O density into the smallest possible package is important, and thus 3U VPX is ideal. In the case of a system with a relatively small footprint and limited high-speed I/O requirements, 3U can also be a superior choice. Where heat dissipation is critical, 3U VPX can help. It can be easier to cool a 3U VPX card compared to 6U because of the lower power density versus the amount of airflow in an air-cooled board or card-edge length in a conduction-cooled format.

3U VPX fits the bill as a powerful, small-footprint choice for smaller-sized systems that require high-speed for interconnects, serial fabrics, or general I/O. The smaller size of 3U VPX slightly lowers efficiency with regards to board real estate or function compared to 6U VPX. When considering the trade-offs between a 3U and 6U design, the break-even point tends to be about five 3U boards. With more than five 3U boards in a system, depending on the application, it’s likely that more processing density, considering both available board real estate and power efficiency, will be achievable with 6U than with 3U.

At half the size (see Figure 1), 3U VPX provides fewer signal pins than does 6U VPX. For some designs, fewer pins can be problematic. Also, many 3U VPX systems implement PCI Express (PCIe), which, while a great fabric for interprocessor and processor-to-I/O communications, can often be difficult to scale. If the application requires a fairly large number of processors all communicating with each other, say in the tens or hundreds of processors, PCIe may not the best choice. In comparison, many 6U VPX implementations employ Serial RapidIO, which is far better for systems of that size.

Figure 1: 3U VPX VPX3-450
(Click graphic to zoom by 1.9x)


SIGINT in small, portable places

An example of a DSP/FPGA-based application that benefits from 3U VPX is signal intelligence, which is often deployed on small platforms, sometimes portable, with low-power constraints. Ground vehicle sensors, software radio, and vetronics also benefit from 3U VPX. Military ground vehicles, while large, often have very limited available space for electronics. Integrators also want to take advantage of higher-performance systems by fitting new capabilities into a fixed space. Similarly, UAVs typically have severe SWaP limitations that can make 3U VPX a perfect fit, especially for smaller UAVs. One additional application area where we’ve seen a lot of interest for 3U VPX is electronic warfare and countermeasure systems. These systems are often deployed in pods or located in out-of-the-way places on an aircraft, such as on wing roots, where there simply is not a lot of space on the airframe.

3U VPX hits the sweet spot for embedded military applications that need maximum processing power in SWaP-constrained environments. Even better, the OpenVPX Working Group, which has been hard at work addressing system-level interoperability issues for VPX, is near completion and will soon be handed off to the Working Group. This effort will make it much easier to integrate 3U VPX systems using cards and backplanes from multiple vendors. At Curtiss-Wright Controls, we see 3U VPX as very important and growing in popularity. We offer a broad portfolio of 3U VPX products including high-performance SBCs, FPGA accelerators, Gigabit Ethernet switches, and carrier cards, as well as fully integrated 3U VPX Packaged COTS (PCOTS) turnkey systems. We have invested and continue to invest in 3U VPX products for a wide range of applications as we anticipate that the market niche for these powerful small form factor cards will become increasingly significant.

Robert Hoyecki is Director of Advanced Multi-Computing at Curtiss-Wright Controls Embedded Computing. Rob has 15 years of experience in embedded computing with a focus on signal process products. He has held numerous leadership positions such as application engineering manager and product marketing manager. Rob earned a Bachelor of Science degree in Electrical Engineering Technology from Rochester Institute of Technology.

Rob can be reached at info@cwcembedded.com.