Traditionally, through the 1980s, military electronics utilized military-grade ICs, custom ASICs, custom form factors, and specialized firmware/software. There was little commonality, and each new system came along with a large development cost, high per unit cost, and a variety of design challenges. To answer these challenges, the first COTS systems started emerging in Europe in the 1980s.
However COTS widespread adoption, aka "the COTS revolution," only started after U.S. Defense Secretary William J. Perry circulated his 1994 memorandum titled "Specifications and Standards – A new way of doing business." Motivated by the rising cost of military electronics development and the shrinking budgets available to the military, Perry advocated that the Department of Defense should increase its adoption of commercial specifications and standards. As a result, COTS products started to embed their way into a vast array of military systems ranging from sonar systems in submarines, to control systems in tanks, data processing units in radar systems and aircraft, and even optical sensors in missiles.
These early adopters embedded their COTS products in a benign environment such as a control room, in which their commercial electronics were right at home. After a successful start, this quickly progressed to COTS designers specifying industrial temperature range ICs (allowing operation from -40 ¬∞C to +85 ¬∞C) and including some form of ruggedization to protect the electronics from shock and vibration. This approach facilitated more development of many applications such as digital signal processors, communications cards, and sensors and control units. And the revolution didn’t stop there.
The next evolutionary step was for COTS to be implemented in mission-critical systems such as control computers, flight electronics, fire control systems, and sensor fusion equipment. These applications generally require many additional characteristics such as reliability and maintainability, among others. Accordingly, COTS designers are now being requested to support these additional requirements in their products, which is posing a host of new demands on designers. However, new and advancing technologies such as the creation of SoC technology, advances in semiconductor fabrication and PCB manufacturing, along with the capability to design in reliability and maintainability right from the beginning are presenting engineers with viable answers to these additional requirements, which are essential for mission-critical systems.
System-on-Chip technology
The advent of SoC technology has enabled designers to combine many different ICs into a single FPGA. Whereas designs once included a separate microprocessor, RAM, ROM, discrete glue logic, and external peripheral ICs all contributing to a low Mean Time Between Failure (MTBF), a designer can now insert all these functions into a single chip, thus raising the MTBF (Figure 1). In a single channel of MIL-STD-1553 communication, the MTBF can be raised as much as 25 percent by using SoC design techniques.
Semiconductor fabrication advances
Advances in semiconductor fabrication have lowered IC power dissipation, which enables electronics to run faster and cooler and contributes to raising the MTBF, thereby increasing overall system reliability. In the past five years alone, the power consumption of FPGAs has been reduced by 50 percent, memory capacities have doubled, and clock speeds have more than doubled. This combines to provide at least a fourfold performance boost over previous technology.
Smaller component packaging (such as BGA and flip-chip technology), along with less power supply circuitry due to the lower power consumption, also combine to reduce the overall size of the electronics assembly. Less circuitry also allows for smaller standard form factors to be utilized, such as PC/104-Plus, ExpressCard, and conduction-cooled PMC (ccPMC, aka ANSI/VITA 20), which also enhances the product’s ability to withstand shock and vibration.
Less circuitry and lower power density also allow the designer to increase the channel count. As an example, in 2003, a VME card from vendors such as Excalibur Systems could support up to eight MIL-STD-1553 channels. Using the new technology, the same size card is now available with 16 channels.
Printed circuit board manufacturing
Recent advances in PCB manufacturing also enable the COTS designer to specify thick planes of copper on internal layers that can act as built-in heat sinks and allow the implementation of conduction-cooled form factors without the necessity of a heavy and bulky external heat sink. This internal conduction cooling via the PCB’s internal heat sinks enables card operation inside a sealed system. These internal planes also act as an electromagnetic shield, not only preventing other radiated noise in the system from getting to the internal signals, but also preventing the internal switching signal noise from escaping and disturbing other parts of the system.
Designing in reliability
Increasing the MTBF and ruggedizing the product go a long way, but these modifications alone aren’t enough to answer all the stringent requirements of mission-critical systems. The majority of COTS cards, for example, are based on commercial designs and architectures, which may be fine for non-critical applications. However, for mission-critical systems, the architecture and interconnect needs to be designed from the outset with a view for single points of failure.
Designing to avoid a single point of failure, though, can provide a design trade-off. It’s all very nice to maximize density by placing, for example, eight channels of MIL-STD-1553 into a single FPGA, but that very same FPGA then becomes a single point of failure for all eight channels, as does its power supply circuitry. Although it will adversely impact the MTBF (which will go down due to the extra components used), it is architecturally more sound for a mission-critical system to split the functionality up into separate independent isolated channels in separate FPGAs, where only one channel will be lost in the event of a failure. Additionally, since each independent channel will have its own microprocessor, this will reduce the burden on the central processor.
Factoring in maintainability
Maintainability must also be designed into the architecture from the outset. The ability to carry out Built In Self Tests (BIST) and provide system health reports is mandatory for maintainability in mission-critical systems. Another factor in maintainability is the capability to run total system tests without the need for reconfiguration or recabling the system. Since mission-critical systems are not normally accessible for changing plugs and cables while testing, the ability to carry out a full internal loopback of communication channels without them transmitting externally is required. Although reconfiguration and/or recabling would be acceptable for a commercial system, imagine the extra effort and logistics required to test a system if a technician would need to disconnect system data cables and connect test cables in their place to run maintenance checks. Not only would it be costly in terms of time and money, but the probability of a cabling mistake occurring would also increase.
Today’s vendors are wise to factor in maintainability, along with designing in reliability and implementing the aforementioned technology advances into their mission-critical designs. Accordingly, Excalibur Systems offers a number of ANSI/VITA ccPMC form factor cards (Figure 2). Conduction cooling via the PCB’s internal heat sinks allows the card to operate inside a sealed system. The standardized ccPMC form factor is more resistant to shocks and vibrations due to its small size. Additionally, the combination of SoC technology along with smaller IC packages allows a high channel count to fit into its small envelope (either 4 independent channels of MIL-STD-1553 or 20 channels of ARINC-429).
COTS satisfies mission-critical design challenges
Developers of COTS electronics products for mission-critical systems face a number of design challenges. Accordingly, COTS designers are now able to make use of modern technologies and design methodologies – including System-on-Chip technology, advances in semiconductor fabrication and PCB manufacturing, and designing in reliability and maintainability – to make the step up and satisfactorily meet the additional requirements for mission-critical systems. CS
Excalibur Systems
516-327-0000
http://www.mil-1553.com