VITA-74/ VNX System Prototyping and Development Considerations

Let’s clear up some misunderstandings about a spec that can put you on the path to creating a small, lightweight and low-cost system for SWaP critical deployment.

This article will discuss aspects of the VITA-74 / VNX specification that are sometimes misunderstood as well as considerations for design and deployment of a VNX system.

A common misperception of the VNX specification is that chassis and external connectors are included. This is not the case. The specification details the physical size of the two standard size modules, the module connectors and the pin assignments of the signals on the module connector. How the modules are packaged into a system is up to the system designer. Part of the confusion stems from original proof of concept pictures (see Figure 1) that were used to promote how modules could fit together, and what a VNX compliant system could look like.

Figure 1. The VNX specification does NOT include the chassis and external connectors, but, as shown here in this proof-of-concept depiction, it’s possible to illustrate what a VNX compliant system COULD look like—depending on the system designer.

Going by the original proof of concept pictures, many people assumed that the four-slot backplane, transition panel, power supply and connectors were a part of the specification. They are not. Backplanes can easily range from one to eight slots (or more) and the system designer is free to choose the most appropriate chassis shape, connector style and connector placement to fit the deployment environment. In a VNX system development, the hard questions become, “How do you quickly prototype in the lab?” and “Just what are the deployment considerations?”

Early Stage Time Saving

To address the issue of fast prototyping, a typical D38999 style circular MIL connector panel and wiring harness can take quite some time to prepare. Utilizing commercial connectors and readily available patch cables in a laboratory environment can save a lot of time in the early stages of a program when deciding on which I/O signals will be needed and what the best layout and pin assignments for the connectors will be. Figure 2 shows a transition panel set up for quick prototyping in a laboratory environment.

Figure 2. Transition panel with commercial connectors.

Once all of the required I/O signals are settled, the appropriate transition panel can be designed for the particular application. As with any system, connections to the outside world can be a source of electromagnetic interference (EMI) and electrostatic discharge (ESD) problems. The transition panel design should address these concerns by including necessary pin-hardening and filtering techniques.

Another important consideration to the system designer is the backplane. The VNX bus topology was lifted directly from VPX, specifically VITA 46. It is possible to build a backplane where VNX and VPX cards can be intermixed, and the cards will interoperate. Among the top five items on the system design checklist is the number of VNX 12.5mm and 19mm slots required for the system, as well as any requisite possible expansion capacity, and it’s just as important to consider the function of the cards. The VNX specification is topology-agnostic for the PCIe interconnect bus. The System Controller choice, coupled with standard and specialized peripheral card-to-card data transfer requirements (such as creating an FPGA-to-FPGA data path), will add to the constraints of the exact backplane topology needed.

Thermal Requirements

Thermal requirements are perhaps the most critical considerations for system card placement. Depending on the external cooling methodology, which will be discussed shortly, placing 19mm cards approaching the 20W suggested maximum dissipation in non-adjacent slots could be the way to go. Placing the cards in this way will improve the thermal gradient if the design is pushing the system cooling limits.

Now that we have discussed the transition panel and the backplane considerations, let’s delve into the chassis, power supply and external cooling techniques. Power supplies in VNX designs are typically mounted parallel to the cards or perpendicular to the cards, under the backplane. Shorter chassis designs with four or fewer 19mm slots are typically single-system designs. Chassis designs with six or more 19mm slots can contain one large complex system or could host two complete systems in a single chassis using either a single monolithic or split backplanes. Power Supply Units may be one large design, or be multiple power supplies, potentially mitigating the risks of a single point of failure.

The mounting and cooling of the VNX chassis demonstrates the flexibility of the systems. The cards are designed from the outset to be conduction-cooled. It is not possible or desirable to have an air-cooled module variant. Figure 3 shows the PCB with heat spreader and case assembly. Internally, the system is natively conduction cooled. If a cold plate, or a heat conducting mounting surface with a known maximum temperature is available, the system can be bolted down to that structural plate with no further considerations. Sometimes it is not that easy. An external metal wall exposed to the sun will need to be evaluated for thermal loading. For natural convection cooling, vertically oriented external fins can cool adequately, provided that the ambient air does not exceed 159.8 °F (71°C). If the mounting surface has any thermal conductivity at all, that conductivity will augment the cooling the fins offer. Enlarging the size and area of the fins, or increasing the thermal conductivity between the chassis and the mounting plate, will aid in the thermal calculations and make heat transfer more effective.

Figure 3. 19mm VNX module showing PCB with heat spreader and case.

There are times when the two cooling methods just discussed are not sufficient due to the deployment environment. “Forced air conduction cooling” is a third method, whereby heat exchangers with integral fins and external coverings (or skins) are fitted to the chassis. Air is then drawn down the sidewalls of the chassis by a fan and exhausted through a plenum. This may take on the form of a box with integral fins, and with skins and plenum being connected to a central cooling duct. Similarly, the mounting tray may have a rear- or bottom-mounted cooling fan, pulling air across the integral heat exchangers.

A Natural Fit

The VNX module is a natural fit for use inside other types of embedded systems that do not require a conventional VME or VPX data bus. An example would be to put a VITA 74 module inside an air compressor on an aircraft. In this case, only a small processor is required to check status of the compressor subsystem and report out to a mission computer via a serial data bus. This is where a small one- or two-slot chassis embedded deep within the compressor would fit the bill perfectly.

The modules’ small size and conduction cooling create a simplified system that has an inherent high tolerance for shock and vibration. With its roots the VPX bloodline, VNX has low technical risk. Careful attention to connector and packaging selections can yield a very small, lightweight and low-cost system, putting VNX in an excellent position to provide computing solutions for SWaP critical deployment.

mcgeeWayne McGee is the vice president of sales and general Manager for North American Operations for Creative Electronic Systems SA. Wayne has served in various senior management positions in his career and has more than 30 years of experience in the VME, CompactPCI, ATCA and VPX markets. Wayne is also the chairperson for the VNX VITA-74 Marketing Alliance. Companies Wayne has worked for include Motorola Computer Group, VMIC, SBS Technologies and GE Intelligent Platforms. He holds a BSEE from the University of South Carolina.

RipleyBill Ripley is the director of business development for the North American Operations of Creative Electronic Systems. Bill has served in various Consulting, Business Development, Product Management, Sales and Marketing roles in the Embedded Computing marketplace for over 15 years with Creative Electronic Systems, Themis, GE Intelligent Platforms and SBS Technologies. Prior to these, Bill spent 23 years with Bell Helicopter performing electronic design and integration of avionic, flight control, electrical and electronic warfare systems on a variety of commercial and military aircraft including the CV/MV-22 and M609 tilt-rotor aircraft, as well as the OH-58D, AH-1W, M412 CFUTTH, M407 and M222 helicopters. He holds a BSEE from the University of Texas at Arlington.

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