Rapidly evolving processors, memory devices, and other commercial off-the-shelf (COTS) component technologies provide rugged military computers with state-of-the-art performance. Most COTS components, however, cannot meet the stringent environmental requirements of rugged applications without special packaging or modification. In addition, rugged-computer designers seeking to take advantage of evolving commercial technology must build systems to "open" standards. This allows for an easy expansion or upgrade, and it helps to avoid obsolescence and a shorter product life.
The PCI, VME, and CompactPCI open-system architectures all accommodate COTS components with varying levels of performance and ruggedness. Therefore, the choice of a rugged-computer architecture depends upon cost, performance, and expansion requirements, as well as the quantity of systems to be produced. For example, the architecture that is selected for an inexpensive computer needed in large numbers on a digital battlefield is typically different from that of a high-value signal processing workstation that's used on a warship.
Makers of rugged computers require experience with all of the open architectures to provide the best solution for military users. Moreover, users have to understand the advantages and limitations of the standard architectures to pick the best long-term system for their applications.
Environmental requirements for military computers are more demanding than those applied to commercial equipment. Some of the specifications for rugged systems in three broad application classes—airborne, ground-based mobile, and shipboard—are presented in Table 1.
Environmental Hazards Airborne applications of rugged computers, particularly aboard propeller-driven airplanes and rotorcraft, are characterized by sustained vibration that can shake components loose or break them. Additionally, airborne systems cannot be allowed to emit sparks that may ignite fuel fumes, or to emit electromagnetic interference (EMI) that might disturb navigation or communications equipment. Furthermore, rugged computers in aircraft are commonly required to withstand altitudes up to 40,000 feet, and they must be able to survive rapid decompression.
Ground-based mobile computers are subject to vibration and shock. In vehicles or shelters that don't have heat or air conditioning, they also must contend with sustained temperature extremes. Temperatures can range from the −40°C cold that's encountered in Greenland up to the 70°C heat that's experienced in Saudi Arabia. Portable or partially exposed mobile computers must stand up to rain, sand, and dust. Plus, they are expected to operate from unregulated power with irregular waveforms, changing voltage, and frequent "brown-outs" or "drop-outs."
Shipboard applications make shock or impact resistance critical for the protection of computers aboard large vessels hit in combat or smaller frigates and patrol boats slammed by rough seas. Electronics that are used on deck or otherwise exposed to the elements cannot allow contamination by corrosive salt fog. Also, EMI must be contained to shield the tightly packed electronics of modern vessels.
Implementing COTS components in rugged applications often requires a mix of packaging and component modifications. Enclosures for hot climates, for instance, can incorporate active-cooling solutions that range from forced air in a clean environment to sealed heat pipes or heat exchangers in sandy, dusty deserts. For applications in extremely cold environments, heaters are incorporated into the de-sign to prevent hard-drive lubricants from thickening and liquid-crystal displays from freezing.
Regardless of the environment or ap-plication, the long life cycle expected from most military systems demands that rugged computers be engineered to accept COTS technology upgrades without costly redesign. While many manufacturers use commercial components, not all have integrated them into open-system architecture designs. A comparison of common architectures for rugged systems appears in Table 2.
The ATX form factor is an industry standard that's widely employed by original equipment manufacturers. It requires that ATX computer boards conform to a uniform set of physical and electrical requirements. By using common-size parameters, input/output (I/O) locations, and power connections, ATX computer boards can be readily adapted to common enclosures. Plus, they can easily accept many COTS power supplies and other third-party accessories.
Most commercial computer manufacturers have accepted the ATX standard, so there's a wide variety of PCI/ATX-compatible add-on devices. Power supplies, storage peripherals, and I/O boards, for example, are interchangeable among ATX computer manufacturers. Because of their widespread use, commercial PCI/ATX components are available at low cost.
Leveraging COTS hardware in the ATX form factor, PCI open-system architectures offer rugged-computer designers and their end users the most current technology that's readily configured for specific applications. The potential for upgrade has been a key design feature of the ATX computer board. With socket memory, PCI expansion slots, and modular central processor units, ATX computers provide users with an upgrade path long after the initial purchase.
Many Platforms Accommodated One important feature of the ATX standard is its applicability to numerous platforms. While enclosure systems were once designed around specific hardware, such as Intel, Sun, or HP microprocessors, ATX versions of all these processors are adaptable to the same enclosure. For example, the DRS Explorer MP Multiplatform Portable Rugged Workstation was originally designed around a PCI bus and Intel microprocessor. But in production, this workstation has been built mostly with Sun UltraSPARC processors.While the ATX form factor with its card-edge connectors may be the least rugged of the open rugged-computer architectures, it also is the least expensive choice for military computers. To date, over 1000 Explorer MP workstations have been ordered by the U.S. Army and Marine Corps to host the Advanced Field Artillery Tactical Data System (Fig. 1).
In this application, the large number of units required made cost an important design consideration for these customers. Also essential was the ability to upgrade the computer with expanded memory, faster CPUs, and newer COTS expansion I/O cards. These features were necessary to accommodate application software developed for true commercial systems.
Ruggedness is another essential feature for a battlefield computer. To withstand the abuse encountered in ground-mobile applications, workstations like the Explorer MP incorporate stiffening bars to strengthen ATX boards (Fig. 2). Also, shielding and filters are included to block EMI effects, as well as a robust power system to condition unregulated power and negate random spikes. Unlike components on commercial circuit boards, all tall or heavy components in the Explorer MP are specially secured to maintain reliable connections despite rough handling.
While the first personal computers used the ISA passive backplane architecture (the first high-performance rugged), open systems were usually based on the VME architecture. VME systems are inherently more rugged because they employ pin-and-socket connectors rather than the edge connectors found in ISA systems. ISA has evolved into the PCI standard to provide in-creased performance, but edge connectors are still more common than pin-and-socket connectors in these systems.
ISA/PCI systems typically cost less than equivalent VME systems, but additional enclosure structure is often needed to ensure reliable operation in even mild shock and vibration environments. Consider the Genesis line of rugged computers, built with dual commercial Pentium processors. These computers are used to track and process data from sensors dropped by the hurricane-hunting WC-130 aircraft of the U.S. Air Force.
The WC-130 turboprop induces a harsh vibration environment for all of the plane's equipment. Stiffened circuit boards and vibration-isolated peripherals make the Genesis a reliable performer in this difficult airborne application. To prevent electrical arcing between power and ground at higher altitudes—where the density of insulating air is less—additional insulation is incorporated around high-power components.
Compatible with any number of card slots, VME has been the architecture of choice in high-performance rugged systems for many years. Until recently, the highest performing components were available only in VME systems. The VME physical structure additionally ensures that cards remain in place. OEM manufacturers and third-party suppliers offer a variety of standard input/output and expansion capability boards for VME systems.
VME-based rugged computer systems can be configured to meet the needs of most users. For instance, using the VME architecture, DRS developed a series of rugged computers for the U.S. Navy's P-3 maritime patrol aircraft. In this product design, 21 VME slots were provided for application-specific expansion boards to process acoustic, radar, and other airborne sensors. To withstand the sustained vibration of the P-3 turboprop, the entire computer, as well as its peripherals, had to be vibration-isolated.
The inherent robustness of VME made it the architecture of choice for the U.S. Navy's AN/UYQ-70 family of mission-critical tactical workstations, one of the first standard combat systems with an open-system architecture (Fig. 3). The Q-70 family of consoles and servers has been deployed aboard AEGIS cruisers and destroyers, Hawkeye airborne warning and control aircraft, and attack submarines. Its rugged design incorporates stiffened circuit boards, shock isolators (see Fig. 3 inset), and other shock and vibration attenuators to ensure that the equipment operates both during and after an attack on the ship.
The space-saving Com-pactPCI architecture combines many features of the VME and PCI architectures, and it is comparable to PCI systems in performance and cost. Like VME, CompactPCI boards use pin-and-socket connectors for better reliability in harsh environments. The CompactPCI computer architecture accommodates seven cards, or fourteen cards with a bridge.
When compared to the VME architecture, CompactPCI has two drawbacks. At 66 Mbytes/s, the CompactPCI databus is slower than the 500-Mbyte/s VME. In addition, the CompactPCI architecture provides less room for expansion cards. Nevertheless, the broad commercial availability of CompactPCI boards means that such systems cost half as much to build as VME-based computers. Additionally, CompactPCI performance is usually adequate to handle data that doesn't require intensive processing.
Among COTS systems, CompactPCI equipment has been designed primarily for the telecommunications user. Telecommunications applications require multiprocessor capabilities with impeccable reliability and performance. With adaptations by integrators such as DRS, CompactPCI can be an ideal solution for military applications requiring flexibility, high performance, and value-based pricing.
One recent application of the CompactPCI architecture was in a portable workstation designed to support the four-nation Eurofighter Typhoon databus test equipment. This ruggedized portable computer was sealed to resist fuel and chemicals.
At about half the cost of CompactPCI and a quarter the price of VME architectures, systems based on the ATX form factor and PCI architecture are expected to be around for some time to come. While costlier open systems are inherently more rugged, ATX cards can be ruggedized by experienced military computer designers. With several viable standards available, the choice of an architecture for COTS-based rugged computers requires designers and users to understand military budgets, missions, and operating environments.