The venerable MIL-STD-1553B bus has survived remarkably well even as other more advanced solutions gained wide acceptance in the last few years. However, the fact remains that its maximum data rate of 1 Mb/s is orders of magnitude too slow for today’s data-intensive systems, so logic dictates that it will soon fade away. That may be a logical assumption, but it’s likely to prove wrong, for several reasons.
The most obvious is that MIL-STD-1553B continues to fly on at least 30,000 aircraft, as well as on commercial and military ships, and is widely used in industrial and other applications. The situation is analogous to the International Space Station (where it’s also present). The ISS was expected to “last” until 2015 but when it arrived, ISS lifetime projections were extended to 2020 and then to 2024, and the latest consensus is 2028.
Basically, until its weaknesses blatantly outweigh its strengths, the ISS will still be up there, along with MIL-STD-1553B. Down here on Earth, it will take decades before all the platforms MIL-STD-1553B controls are either obsoleted or worn out, and combined with the slow process of defense technology insertion and the high cost of retrofits, MIL-STD-1553B will be here longer than many of the readers of this article.
The standard is so valuable that there are still many sources of every type of component used by MIL-STD-1553B and many IC suppliers have been supporting it for decades. There are even bridges between MIL-STD-1553B and Gigabit Ethernet that allow the existing standard to transfer data to the world’s most widely used networking standard.
Many of its key and often unique benefits can be found in its architecture, which makes MIL-STD-1553B reliable and fault-tolerant for connecting processors with real-time sensors and controllers. It’s arguable that the most important reason MIL-STD-1553B still retains its stature for mission-critical systems is its command/response protocol that ensures real-time determinism (Figure 1).
Avionics and other systems that operate in real time require determinism to ensure predictable behavior, every time, without fail. That is, a real-time system must behave in a way that can be mathematically predicted, executing functions with no concern that they will be degraded in an unexpected way. For real-time systems in which surprises are intolerable, MIL-STD-1553B is nearly perfect in its ability to predictably perform functions in real-time with microsecond accuracy and very low jitter.
The standard was created to operate in hostile environments that include lightning, wide temperature ranges, high levels of vibration, and the potential for significant interference. The latter is the result of galvanic isolation that its transformers provide to fend off lightning, which has become even more important as many newer aircraft are made from composite materials, reducing and sometimes eliminating the benefit of having a Faraday shield inherently created by an aluminum skin. Finally, MIL-STD-1553 has refined its criteria for validation testing over the years and has not encountered issues with interoperability, even though massive numbers of designers have implemented it in diverse systems.
Beyond these points there is the fact that MIL-STD-1553B, or its protocol, is used in a variety of other standards, and it’s a long list (Table 1). The world of communications bus standards – and MIL-STD-1553B nomenclature – is deep, wide, and often obscure, with countries and defense agencies within them tweaking the bus and renaming it. For example, the upper-layer protocol of MIL-STD-1553B is also used in FC-AE-1553 and High-Speed 1760.
FC-AE-1553 uses the MIL-STD-1553B command and response protocol and supports all its core elements including command and status, sub addresses, mode codes, transfers between remote terminal, error checking, and broadcast. As a result, it allows the reuse of MIL-STD-1553 and MIL-STD-1760 commands and legacy software. In addition, FC-AE-1553 includes extensions and optimizations supporting RDMA to provide direct memory access of remote systems over Fibre Channel.
MIL-STD-1760 is typically used for interfacing weapon stores to an aircraft’s control systems, but an enhanced version called High-Speed 1760 (SAE standard AS6653) has a high-speed interface based on Fibre Channel that can deliver data rates up to 1 Gb/s over two 75-ohm coaxial cables. The Fibre Channel upper layer protocols are based on FC-AE-1553, MIL-STD-1553B for command and control messaging, and FC-AV for transferring images, video, and audio files.
The final reason for the standard’s longevity it that a lot of time and money has been invested in making MIL-STD-1553B viable in the future. In fact, variants of the standards today are actually delivering data rates of 100 Mb/s – 100 times that of MIL-STD-1553B – and have demonstrated their ability to reach 200 Mb/s.
So, why haven’t these variants transformed the standard into something like MIL-STD-1553C? Well, they have, but in a much more limited fashion than might be expected. To better understand this, it helps to trace the long, winding path that this standard has traveled in the last 15 years or so.
Toward a Better Bus
In the 2000s, the Air Force recognized that, as it would cost (at that time) more than $1 million per aircraft to replace MIL-STD-1553, the logical step would be to enhance MIL-STD-1553 to increase data rates, hopefully to 200 Mb/s or even higher while allowing simultaneous transfer on existing MIL-STD-1553B cable. To this end, Data Device Corp. (DDC) and Edgewater Computer Systems expended considerable effort to develop versions of MIL-STD-1553B that would allow it to remain viable. Both were successful, achieving excellent results without significantly modifying the standard’s fundamentals. The first of DDC’s efforts resulted in what the company called Turbo 1553 that increased the data rate of the bus to 5 Mb/s on standard MIL-STD-1553 terminals over 430 feet with 10 stub connections to three remotes.
The second, called “High Performance 1553” or “Hyper 1553” uses frequency-division multiplexing and other techniques to allow higher speed data to be simultaneously carried along with standard 1-Mb/s data on the same cable. It was envisioned to implement a multidrop bus and eliminate the need for active hubs or switches. In Hyper 1553, the 1-Mb/s signals are limited to lower frequencies while the higher speed signals occupy higher ones, similar to DSL. DDC determined that enough bandwidth was available to allow the parallel signals to be reliably transferred at higher speeds, depending on the length of the bus and number of stubs.
DDC demonstrated HyPer 1553 in a 2-hr. flight on an Air Force F15-E1 Strike Eagle fighter in 2005, where it was used to transfer imagery between a computer in the forward avionics bay and a bomb mounted on a pylon. The data was transferred without errors at 40 Mb/s over existing cabling along with 1 Mb/s traffic.
Meanwhile, Edgewater was producing similar results with the major benefit of being under contract to DoD to develop Extended 1553 (E1553). Edgewater, along with researchers from the Air Force and Navy, worked on the project for several years, and the technology was flight tested in an Air Force F-16 and Navy F/A-18. The results were very promising, but as the program didn’t have the visibility and priority of others, the Air Force cancelled it. It did this even though the goal of a 200 Mb/s data rate was achieved, again without the need for huge changes, while also simultaneously transferring standard data at 1 Mb/s. The company believed it had the potential to reach 500 Mb/s.
However, all of this work had not gone unnoticed, and the Assistant Secretary of Defense for Research and Engineering and a consortium from Canada, the U.K., Germany, and others successfully petitioned to complete E1553 development work. It was ultimately tested on fixed-and rotary-wing aircraft, which led the NATO Avionics System Panel (AVSP), chaired by the U.S. Navy, to sponsor standardization efforts within NATO. The ratification process began in 2010.
The result was a NATO Standardization Agreement (STANAG 7221) and in 2015 the “Broadband Real-Time Data Bus Standard” was unanimously ratified. STANAG defines everything from processes and procedures to terms and conditions for design and manufacture of military equipment among NATO member countries. Its goal was to create a NATO-specific set of standards so that all members could work off the same page, so to speak. It was a logical and, perhaps, essential approach considering the alternative.
So, E1553 is still alive and well in its standard form as well as delivering 100 Mb/s performance using its infrastructure in avionics and other defense applications it could previously not serve. However, E1553 (i.e., STANAG 7221) has an additional result: limiting the use of a new and improved version of MIL-STD-1553B only to those with permission: NATO countries and their representatives. So, while MIL-STD-1553B has advanced over the years, the odd result is that the hundreds of other applications in which MIL-STD-1553B is used can’t benefit from the one delivering the highest performance.
MIL-STD-1553B has nine lives, and about eight have been used up. However, the sheer breadth of the applications in which this standard is deployed makes it virtually certain that it will continue to be around for many years. Nevertheless, new systems all use something more modern typically based on Ethernet or one of its variants such as Time-Triggered Ethernet (SAE AS6802) or AFDX, as well as Fibre Channel, or IEEE 1394 (FireWire).
Looking back, it’s unfortunate that the evolution of MIL-STD-1553B ultimately wound up as a rather closeted standard, highly unlikely to find its way into commercial markets, even though it delivers data rates high enough to serve many applications today and tomorrow.
This article was written by Mark Hearn, Product Manager, MilesTek Corporation (Lewisville, TX). For more information, visit here .