Shootout At The VPX Corral

July 16, 2010
Technology Editor Bill Wong checks out three competing form factors looking to bring a smaller VPX to the market.

VPX Software Radio

Conventional ATR

Reduction in volume

VPX boards

Six-slot chassis

Rugged Mezzanine

Processor boards

Front panel


Size is relative. But almost all aspects of military electronic technologies are looking to get smaller, lighter, and more power efficient as well as faster and smarter. The 6U and 3U form factors used by VME systems for decades remain the form factor for standards such as VPX, VXS, and CompactPCI.

Other standards-based solutions such as AdvancedTCA and MicroTCA are found in the military and avionics environment because of the importance of communication. For example, the RadiSys Promentum C2 AdvancedTCA Server is designed to bring communication to the front line (see “Rugged Communications,” p. xx).

Systems like the Promentum C2 AdvancedTCA Server are ruggedized. However, they normally don’t compete with their more rugged cousins in deeply embedded systems where extreme shock, vibration, and temperature ranges are encountered. Still, the extremes are much wider than most industrial environments.

AdvancedTCA systems tend to be rather large. 3U systems are much smaller but still substantial in size. Even smaller form factors are needed in many applications such as small unmanned aerial vehicles (UAVs) and small unmanned ground vehicles (SUGVs). Until now, most of these solutions have included custom-built electronics.

This often meant custom cases as well as custom boards. Developers would reuse their own designs, and there was rarely commonality beyond a project or organization. Computers-on-modules (COMs) are popular in this environment, allowing developers to move much of the hard processor design work to the COM boards. Unfortunately, tying the COM boards into a conduction cooling system isn’t always easy.

This trend toward smaller systems isn’t unique to military and avionic environments. Compact industrial applications have a range of standard or de facto standard solutions (see “Rugged Stacks,” p. xx). Many of these solutions wind up in military and avionic applications that are in harsh environments but not necessarily at the extremes. In some cases, this is because the underlying boards are Class 2 while hardier solutions are Class 3.

The Air Transport Rack (ATR) is standard fare. An ARINC 404A 1 Short ATR is 257.05 by 320.5 by 269.88 mm, and a 1 Long ATR is 257.05 by 498.3 by 269.88 mm. There are smaller and larger ATR versions as well. These days, they’re often filled with VPX boards like Pentek’s 5300 series of VPX Software Radio boards (Fig. 1). The OpenVPX standard has made VPX board utilization even easier by providing standard backplane and system configurations (see “OpenVPX Accelerates Military Time-To-Market,” June 17, p. 33).

Smaller tends to be better if a designer can create a solution within other design constraints. This leads to more compact ATRs like Curtiss Wright Controls Embedded Computing’s MPMC-5931. It’s smaller than a 1/2 Short ATR, which is essentially half the size of a conventional ATR (Fig. 2). What used to fit into a full-size ATR box can now fit into an MPMC-5931. Of course, this means that even more can fit into a smaller package.

Rugged, Compact proposals
Three companies look to fill the low-end void where custom implementations currently reign. Themis, PCI-Systems, and Curtiss Wright Controls Embedded Computing have submitted competing proposals to the VITA Standards Organization that are related to the popular VPX standard, which utilizes high-speed serial fabrics on the backplane (see the table).

In all cases, the boards are smaller and the power limitations of the system are lower, but that’s what the designs call for. They all provide a substantial reduction in volume (Fig. 3). The proposed standards all mirror the VPX backplane in some fashion. In some cases, though, the connectors differ from the MultiGig RT-2 connectors found on standard VPX boards (Fig. 4).

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Each approach has advantages and disadvantages. They have slightly different roots like the PCI-Systems 2.5-in. hard-drive form factor and Themis’ nano-ETXexpress support. One or all may wind up being VITA standards. That is to be determined. VITA has working groups for each, including VITA-73 (PCI-Systems), VITA-74 (Themis), and VITA-75 (Curtiss-Wright Controls Embedded Computing).

PCI-Systems based its proposal around a 2.5-in. SATA hard drive. In fact, one of its connector configurations is for a SATA drive. In this case, a backplane would route the SATA signals to the processor card. The VITA-73 group now includes major organizations such as Raytheon, NASA, and Israel Aerospace.

The 78- by 101-mm boards come in 11.3-mm high and 23-mm high versions. Logic boards employ ERNI MicroSpeed gas-tight connectors. The can handle 10-GHz signals, allowing the system to easily meet VPX signalling requirements.

A Six-slot chassis (Fig. 5) is only 101 by 114 by 152 mm. The boards incorporate an aluminum X-frame to provide rigidity and conduction cooling surfaces. The frame can accommodate a circuit board or a 2.5-in. disk drive. A typical system weighs less than1.5 kg. I/O connectors are designed to mount on the backplane, eliminating any internal cabling. A typical system includes a power-supply slot, a processor slot, and one or more I/O or storage slots.

Storage slots have a SATA connector on the backplane. Processor and I/O slots have three connectors designated P0, P1, and P2. P0 provides power but no signaling, as with VPX. P1 has PCI Express connections for I/O support. P2 is for differential signals for Gigabit Ethernet. It also handles SATA support, USB, and audio and video connections. I/O cards are similar, but the P2 connection is for I/O. Four coax connectors can replace the P2 connector.

The VITA-73 cards are designed to fit into a VPX carrier board as part of the VITA-71 Rugged Mezzanine Standard (Fig. 6). VITA-71 differs from the more conventional mezzanine standards like PMC and XMC, as the VITA-71 cards are potentially field removable. This approach won’t necessarily be used for processor cards, but it can be handy for storage devices. This would allow something like a 2.5-in. hard drive or flash drive to fit into either type of system.

The Themis Nano-ATR is built around the nano-ETXexpress module form factor. The 89- by 75-mm board can hold an 84- by 55-mm nano-ETXexpress module. A processor carrier board can hold one of these modules so designers can utilize nano-ETXexpress modules. Of course, native processor boards can be created as well. The form factor also lends itself to FeaturePak peripheral carrier boards (see “Module Packs I/O Features” at

The processor boards (Fig. 7) are designed to be 19 mm high. Peripheral boards are 12.5 mm high. Each board is housed in a metal carrier frame. The boards have a Samtec Searay 200 connector that handles 19-Gbit/s single-ended signals and 21-Gbit/s differential signals. Power is brought across on the same connector. Pins handle up to 2 A at 40°C.

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Themis maps a subset of the VPX signals onto sections of the Searay connector designated S0, S1, and S2 versus the P0, P1, and P2 connectors found on VPX boards. There are single and differential signal mappings. The use of VPX signals makes replacing a VITA-46 system with a VITA-74 much easier.

Each board is housed within its own sealed box. Sealed front-panel connectors, which are optional, can be exposed on the case’s front panel (Fig. 8). The module provides conduction cooling on three sides with an internal chip cooling path via the skyline.

Systems tend to be very compact. A five-slot cube measures 90 by 95 by 90 mm, which allows a system to fit into SUGVs and small UAVs. The self-contained nature of the boards also makes single-board systems amenable to use without a case.

Curtiss-Wright Controls Embedded Computing’s VITA-75 was designed to match more closely with the VITA-46 standard. This starts with the use of the MultiGig RT-2 connector found on VPX boards. The approach builds on VITA-46 instead of switching to a different connector system. It essentially reduces one dimension of VITA-46. The approach offers advantages and disadvantages compared to VITA-73 and VITA-74.

The RT-2 connectors tend to be more expensive than the VITA-73 and VITA-74 connectors, but the higher volume expected with the lower-cost, smaller form factors will make this less of an issue. On the other hand, the RT-2 connections are already defined by VPX, and it’s possible to eliminate wafers from the RT-2 connectors to reduce their size when not all connections are required.

Matching VPX connections will also open up the use of high-speed serial other than Ethernet such as Serial RapidIO. Likewise, the existing standards for coax and fiber connections in the P2 slot are defined by VITA already and can be used with VITA-75.

In a sense, VITA-75 is simply a smaller, shorter form-factor VITA-46 versus the competing VITA-73 and VITA-74 standards. VITA-75 also looks to support higher-performance processors with a power envelope on the order of 50 to 75 W. The other standards target low-power processors like Intel’s Atom and enable systems with low-power requirements.

VITA-75 also can handle low power while allowing higher-performance processors like high-end Xeon and Power processors to be used as well. Likewise, VITA-75 boards can easily accommodate high-end FPGAs often found in military and avionic applications.

The smaller VITA-75 cards won’t handle standard mezzanine cards because of their size, though, so Curtiss-Wright is proposing a 74- by 74-mm uXMC mezzanine card. This size is similar to the VITA-57 FMC (FPGA mezzanine card) standard that’s 63 by 84 mm. FMC is primarily an I/O front end, so the interface is more conducive to FPGA support rather than interfaces like PCI Express that are found on XMC anc uXMC.

VITA-73, VITA-74, and VITA-75 all bring something to the table. VITA-73 boards can double as pluggable mezzanine cards for VPX systems. They also could be used in other systems. VITA-74 boards are self-contained and permit single-board solutions that the other two approaches do not. And, it’s the more compact of the three.

VITA-75 maintains compatibility to a much higher degree. Additionally, it accommodates higher-power solutions that may be necessary in many environments while still providing a more compact solution than a standard 3U VPX system.

All three should make it out of the VITA Standards Organization (VSO) process in some form. Each offers unique features that are beneficial in a number of applications that the others do not address. Which standard, if any, becomes dominant won’t be apparent for a number of years.

It also will be interesting to see whether something like OpenVPX will be extended to these platforms. VITA-75 obviously benefits the most given its matching backplane and connector sizes. The other two are likely to need yet another standard to provide standard backplane definitions simply because they are much different than VITA-46.

And, it will be interesting to see what compact standards emerge for military and avionics use. There is always the question of whether the form factor is too small so each system is completely custom anyway. These standards are more likely to move into industrial and rugged consumer applications where this level of rugged standard doesn’t exist.

This is where discussions about Class 2 versus Class 3 boards, lower-cost connectors, and standard backplanes come into play. Moving to the less demanding side reduces cost and potentially increases the number of systems that would be built.

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