Providers of C5ISR solutions often focus on the powerful deployable computer systems that provide superior performance capability. But to get to that end-design, there were prototyping/test systems that played a critical role in their development. It’s an important time for development systems as the military moves to more advanced modular open standard systems. This includes the SOSA/HOST/CMOSS efforts over the OpenVPX architecture, wideband testing via programable software defined radios (SDRs), and much more.

Market Drivers

The SOSA efforts and the OpenVPX evolution in defense applications are driving the push to utilizing optical (VITA 66) and RF (VITA 67) contacts directly on the module/backplane interface. As such, many prime contractors, research groups, and government installations are rushing to develop new configurations with these capabilities.

These designs are challenging performance barriers in multiple areas from a chassis platform perspective. First, the backplane speed requirements are climbing, with more PCIe Gen4 (16 Gbaud/s) becoming prevalent, up to 100 GbE (with 4 × 25G lanes) and beyond. The heat within the enclosure is also increasing, with boards that are 125W to 180W each being utilized more often. It is no longer unusual to see enclosures that require 2000W of thermal management, often in a MIL rugged design. Finally, with the more advanced I/O and RF/optical cabling, the management of these interfaces along with the cooling, environmental, and space constraints are putting more pressure on enclosure solution providers. This is not to mention all of the challenges that OpenVPX module and software providers face. Prototyping/testing is increasingly an essential step to ensure that a system can meet all of the objectives of the design requirement.

Prototype/Test System Elements

There are all types of prototype and testing tools for the embedded computing/sensor market. This includes development chassis, extender boards, power and ground backplanes, power interface boards, specialty components, and more. Of course, there is a vast market for test equipment/devices, but for our purposes, we’ll focus more on the embedded board market.

A starting point for these systems is the development chassis. These are typically an open frame style, so that test engineers can probe the boards with easy access. An important element is the ability to provide card guides for either air-cooled or conduction-cooled boards.

Figure 1. The open frame development chassis for 3U or 6U OpenVPX backplanes is ideal for easy access to the cards, versatile backplane options, and guide rails that support air or conduction-cooled boards.

Figure 1 shows an OpenVPX open frame chassis with a mix of both types of card guides. The air control dial allows the user to keep the fans quieter for short demonstrations and run faster during long periods of testing. These enclosures typically come in versions that support 3U or 6U boards and a couple of different widths. It’s desirable to not have the open frame be too heavy or bulky, so a width of 42HP (8.4") is a convenient size to allow up to 8 slots at a 1.0" pitch or smaller backplanes. For designs that require more slots, a 63HP wide (12.6") is fairly common. These chassis can be made with removable sidewalls. This allows the unit to be fully enclosed for airflow testing for example, then the sidewalls removed for easy access.

Another key element of the OpenVPX development enclosure is power and ground only backplanes along with Rear Transition Module (RTM) connectors. With the signals all passing through to the rear of the backplane, it can have VPX cabling wafers plugged in to connect slots or to provide I/O with various terminations at the end of the cable (USB, Ethernet, RS-232, etc). This provides a great deal of flexibility and allows the system to be reused across multiple applications. There are also backplanes with various VITA 66 and VITA 67 cutouts for development. The cutouts allow the test/system engineer to load the optical or RF contacts as needed.

Often board vendors and chassis manufacturers will collaborate to provide development systems with interoperable and pre-tested products. This trend will continue with today’s more complex systems.

Other Chassis Types

Of course, open frame chassis don’t have to be utilized. Often, there is a desire for standard 19" rackmount chassis for development in a larger system. This naturally allows for the full 16 slots at 1.0" pitch in the case of OpenVPX.

The rackmount chassis can also provide better cooling. One drawback of the open frame chassis is that the air is not enclosed. Air follows the path of least resistance, so in open frames, the air goes everywhere. This is why they tend to have powerful fans and are so loud. In a rackmount chassis, the airflow can be contained with filler panels for any unused slots. In these designs the chassis can be many dBs quieter and the airflow path is more efficient.

For 3U OpenVPX boards, a 4U vertical chassis can provide a 19" rackmount or desktop approach for up to 16 slots at 1.0" pitch with optional RTMs. The cooling configuration in this approach is typically bottom-to-top, with 1U of space for the fans. Similarly, a 7U enclosure can support 6U OpenVPX boards. But this airflow approach is typically limited in the wattage/ slot it can cool, depending on which fans are used. Bottom-to-top is fine for development, but many deployable systems need front-to-rear cooling. Therefore, powerful front-to-rear airflow systems are often used for higher power requirements. More on this subject shortly.

Figure 2. The 19" horizontal mount enclosure is desirable for saving rack space. The approach is also efficient for providing a mix of slot sizes as shown in this 1U tall version that holds either three 3U OpenVPX slots or one 6U slot and one 3U slot.

To save rack space, a horizontal-mount enclosure is often a good solution, where the boards plug in to the crate sideways. The horizontal enclosures also commonly utilize split 3U/6U configurations. For example, the 1U chassis shown in Figure 2 can hold three 3U OpenVPX slots or one 6U slot and one 3U slot. These types of enclosures can be very attractive for designs with lower slot counts. They are also lightweight. The chassis shown in the Figure is about 16 lbs. with the chassis, backplane, and PSU. This type of chassis can be front-to-rear cooled, but are often side-to-side cooled. The drawback for the front-to-rear orientation for horizontal-mount systems is that they will nearly always allow less space for slots and the airflow path is not as efficient.

SOSA & High-Power OpenVPX Development Systems

SOSA/HOST systems typically utilize high-power boards along with VITA 66 for optical and/or VITA 67 for RF contacts. It’s not uncommon for multiple slots to be over 100W each. When the slot counts are high, as these types of systems often are, this can create the need for a system that can dissipate a high amount of heat. The 6U chassis in Figure 3a features dual 191 CFM fans that pull the air from below the card cage and blow the heat ninety degrees out the rear of the system. This allows RTMs to be plugged in all of the slots in the rear of the enclosure, maximizing density for testing or deployment.

Figure 3a. For SOSA/HOST or other high-power development, a front-to-rear 19" rackmount chassis with powerful cooling can be employed. This RiCool example has dual 191 CFM each fans above the card cage allowing the use of full RTMs and over 2000W of cooling. Figure 3b: The airflow path of the RiCool enclosure is illustrated.

The capability to handle the RF/optical cabling and any RTM or VPX cabling interfaces in this type of front-to-rear cooled chassis is important for SOSA systems. The chassis can cool 2000W in the 6U size, but with a taller enclosure can cool up to 4000W. In Figure 3b, the airflow path is illustrated.

For 6U OpenVPX boards, a 9U tall RiCool chassis can be utilized. It is also possible for a divider plate to be used to split the enclosure into two rows of 3U slots (Figure 4). Alternatively, the designer can have some slots as 3U along with other slots at 6U.

Figure 4. For larger systems, a divider plate can be used to create dual rows of 3U slots or a mix of 6U and 3U in a vertical-mount chassis platform.

Prototyping and Development Tools

SOSA systems employ the use of system management per VITA 46.11. A VPX Chassis Manager, as shown in Figure 5 can provide many functions for the system. The Tier 2 requirement of SOSA and HOST includes:

Figure 5. The VPX Chassis Manager can be utilized to monitor and control fans, power supplies, and other FRUs in the system, among other functions. Per VITA 46.11, these managers are being required in more applications.
  • Discovery of all FRU (Field Replaceable Units) in the chassis

  • Event logging, generation, reception

  • Management of the power supplies, fans, FRU recovery

  • FRU reset, power cycling, graceful reboot, initiating diagnostics

The unit in the Figure can be plugged into any backplane slot in the P0 connector. For more I/O and performance options, P1 and P2 can be utilized. In that case, the backplane needs to be customized.

In non-SOSA/HOST systems, simpler devices can be utilized such as alarm cards for identifying trouble with thermal management or voltage levels in the enclosures. Going even simpler, there are also very basic voltage monitors and similar devices that add little cost to a development system.

Ruggedized Testing

It’s not as common in the early test/development phase, but sometimes there is a desire to do basic flight or hover testing with the development enclosure. In these cases, it is possible to utilize semi-rugged for fully ruggedized chassis platforms. The standard RiCool or horizontal enclosures can be reinforced for moderate levels of shock and vibration. The horizontal enclosures may or may not utilize MIL-grade fans and filters, depending on the application requirements.

Of course, full MIL-grade 19" rackmount enclosures or ATR (Air Transport Racks) can be utilized. However, they are much more expensive than the other types for prototyping and often quite complex, leading to long lead-times. Utilizing a semi-rugged design is a much more attractive approach, when it is an option.

With the very high speeds across the backplane, sometimes even faster than 100 GbE (four lanes of 25 Gbps signals), backplane signal integrity pre-design simulation is desired. While for moderate speeds this type of simulation is not required, as the speeds get extremely fast, it is a way to mitigate the risk of delays from a backplane or board not performing as expected. Similarly, thermal simulation for the chassis can be helpful when a design is pushing the boundaries of cooling physics. Figure 6 shows a simulation model of an 800W OpenVPX ATR conduction-cooled chassis with airflow heat exchange. The thermal simulation can help the engineer make critical decisions to optimize the cooling in the enclosure.

Figure 6. Thermal simulation, as shown in this OpenVPX ATR, can be used to identify potential hot spots in an enclosure and allows the engineer to make adjustments to optimize cooling.

Components & Specialty Items

During prototype testing there are several tools that help make a system designer and test engineer's life easier. As many OpenVPX boards are conduction-cooled, it is helpful to have individual card guides that can be plugged into any slot. It is very common for some boards that are air-cooled and other boards that are conduction-cooled to be used in the same test chassis. The chassis shown back in Figure 1 shows an example of the conduction-cool card guides.

Air slot baffles to redirect airflow and air slot blocker boards are helpful for prototyping. The baffles can be used to block off unused slots or direct air towards hotter boards in the enclosure. The air slot blocker is a 3U or 6U × 5HP (1.0") board that plugs into an otherwise empty OpenVPX slot to block the airflow. This is important during prototyping to help optimize cooling by forcing the available airflow to pass through utilized slots.

Figure 7. Many deployed OpenVPX systems utilize VITA 62 power supplies to meet avionics and/or ruggedized requirements. For prototyping, a separate 3U or 6U power interface board can be employed in an open frame or other test chassis.

Another prototyping tool is the power interface board, which are individual (or sometimes dual) slots for a PSU to be plugged into the development chassis. A VITA 62 power interface board can come in 3U or 6U sizes. (See Figure 7 for an example.)

Extender boards are used to extend the signals outside of the card cage for the ease of probing signals. These were popular in bus-based applications such as CompactPCI and VME. The design for OpenVPX is much more complex and it's very hard to test plug-in boards at higher speeds. Therefore, engineers will typically use open-frame chassis for the access and forgo the use of an extender.

Other critical tools for prototyping are backplanes simulation and chassis thermal simulation. Before finalizing a development system, the designer can use simulation to validate the design first. This can save critical time and money if a design is expected to be on the edge of either backplane performance or cooling physics.

Battlefield Monitoring & Wideband Testing

Figure 8. A ruggedized, weatherproof version of the National Instruments programmable SDR allows prototyping/testing or deployment in the field, including outdoor applications.

So far, we have limited the topic of prototyping to backplane-based systems. But there are important prototyping requirements for Mil/Aero systems available in other form factors. This includes a cost-effective and versatile way to do wideband research, prototyping, and testing of new techniques. The programmable software defined radio (SDR) is used for sensors, communications, and electronic warfare, almost everywhere on the battlefield. Much of the prototyping/testing can be done in labs, but field testing is a critical requirement. This is true whether it is drone detection, signal detection/spoofing, or testing new communications systems and protocols. Figure 8 shows a ruggedized version of an Ettus Research (a National Instruments Brand) X310 Software Defined Radio (SDR) for wideband testing. The IP67 weatherproof enclosure can be placed outdoors on the top of shelters, mobile vehicles, pole-mounted, etc.

The Importance of Prototype Systems

We’ve seen that there are a significant number of tools for prototyping/testing in the embedded computing market for C5ISR systems. As the signal speeds increase and hotter processors are utilized, the complexity of the development system also continues to adapt. For SOSA/HOST systems, there are chassis platforms for development that provide relatively cost-effective, versatile, and powerful solutions.

This article was written by Justin Moll, Vice President, Sales & Marketing, Pixus Technologies (Waterloo, ON, Canada). For more information, visit here .

Aerospace & Defense Technology Magazine

This article first appeared in the December, 2020 issue of Aerospace & Defense Technology Magazine.

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