MIL-STD-1553

In U.S., allied, and compatible electronics, the MIL-STD-1553 bus, now at MIL-STD-1553B revision level, is a standard interconnection bus inside an airplane, vehicle, or ship. It is widely supported by commercial components. Its basic data flow rate is 1 Mbps, a very slow speed compared to COTS local area networks, but its role is different than a typical LAN. It operates in an electrically noisy environment, and usually passes simple commands and responses:

• Weapons computer to Missile 4, here is your target...
• Missile 4 to weapons computer: understood
• Weapons computer to Missile 4: launch and kill.

This is quite different than a file transfer or video stream, which were not major requirements when 1553 was developed.

To get away from the weight and complexity of point-to-point wiring for avionics, in 1968, the United States Air Force joined the Society of Automotive Engineers (SAE) (now SAE International) in forming the A2K committee for the purpose of developing a networking standard. In 1973, MIL-STD-1553, the Military Standard Aircraft Time Division Multiplexing Data Bus, was issued. In 1978, the standard was re-issued as MIL-STD-1553B, its current revision level.

While some higher-performance interconnects are also being considered for specialized applications, MIL-STD-1553 remains the most common military electronics bus. For fault tolerance and greater bandwidth, a given vehicle may have multiple 1553 buses; a given device can connect to more than one. Some other alternatives include In addition to MIL-STD-1553, High-Speed 1553, Fibre Channel, Gigabit Ethernet, and ARINC 664, a form of profiled Ethernet. <

Evolutionary improvements

There is too much legacy 1553 use for it to go away, but the core can have incremental improvements. New integrated circuit packaging in plastoc allows extended temperature ranges, and almost as much moisture resistance as the traditional and more expensive ceramic chip cases. DDC, for example, is developing plastic encapsulated chips that provide almost as much protection against moisture as do hermetically sealed ceramic encased chips. New manufacturing techniques and materials also let them operate through an extended temperature range.

Miniaturized connectors and equipment improve 1553. The standalone, miniature transceivers (e.g., National Hybrid Inc. (NHi)) allow developers either a third-party or their own 1553 intellectual property firmware in a Field Programmable Gate Array (FPGA) with an external transceiver and random access memory.

There can be both unintended consequences and paradigm shifts in using more COTS technology. Much as mainframe business computers controlled the network, the general model for avionics is that mission computers control the bus. When going to more complex, perhaps nondeterministic technology such as routed packets over Ethernet or Fibre Channel, there start being needs for devices such as routers and bridges, common in general networking but a new idea in tightly coupled systems. Designers used to tight coupling can feel a loss of control.

The ability to transfer large amounts of data also has to consider the load that this places on a mission computer. It may well be that image processing, target recognition, etc., need to be on different computers than are running fly-by-wire and engine monitoring.

Standard for existing weapons

Most precision-guided munitions take pre-launch programming via MIL-STD-1553, so it is not only a question of whether to build the core networked architecture of new aircraft with an alternative question, but how to interface legacy inventory.

Whether 1553 remains the standard, while higher-performance COTS networking technologies are widely available, is an oft-asked question. all of the world’s most advanced fourth- and fifth-generation fighters — F-35, F-22, Gripen and Eurofighter — were designed and are being fielded with alternate technologies like Ethernet and fiber channel, not because E1553 wasn’t worthy but because those aircraft were developed long before E1553 became a reality. The F-35, for example, uses gigabit-rate Fibre Channel for some of its imaging systems that don't really need a 1553 interface. The answer will most likely come in 2 to 3 parts/:

• Basic 1553 will be needed to include nondevelopmental avionics, but a wideband 1553 will come into service
• High-speed IEEE 802.3 Ethernet, as well as routers and bridges, will start appearing in new designs, but require a paradigm shift among system integrators and the government.

1553 limitations and growth

As a bus, it has more rigid interface conventions, number of device, etc. than a local area network. It manages medium access control by a positive command-response mechanism.

"Enhanced" 1553

In April 2006, Notice 5 for MIL-STD-1553B requested 200 Mbps over 300 feet of bus.

The "Enhanced" 1553 (E1553) was developed by the United States Air Force and Edgewater Computer Systems of Ottawa, Ontsrio.

According to Edgewater, individual flows are faster than the bus designers had planned, such as > 1 Mbps for good-quality video. There are also more flows than expected.

Edgewater essentially put an additional, nondeterministic wideband flow capability on top of the existing 1553 cabling, using the nondevelopmental connectors and passing the interoperability tests.

"HyPer 1553

Edgewater competitor Data Device Corp. (DDC) offers an alternative, "HyPer" 1553, which it believes to be a less risky approach that builds transmission speed incrementally 50 Mbps at a time rather than an E1553 that starts at 200 Mbps and is envisioned to reach 500 Mbps. DDC's incremental approach stresses reliability over performance, with stepwise improvement of performance.

The traditional role 1553 technology plays in weapons delivery will remain strong, even with the latest fighters. For example, the F-35’s fiber channel network will be complemented by the usual 1553 technology for weapons delivery. The Mil-Std-1760 communications bus being developed for smart bombs includes 1553 in its signal set.

Open standards and 1553

Edgewater E1553 is proprietary, which makes use of COTS products more difficult. People in the standards community also feel as if they were excluded from the process. What began as the Society of Automobile Engineers has evolved to SAE International, which deals with other industrial interfaces for high-noise, extreme-temperature environments, such as engine compartments. "We haven’t accepted it very well because the government went about developing the standard without SAE," said George Sponsler, chairman of SAE International's AS-1A Avionics Network Subcommittee. "We’ve always been the caretakers of 1553."

Alternatives

Without doing away with 1553, it may be quite practical to have other interconnection technologies in the aircraft.

Hybrid

One approach is to have at least a protocol converter, and possibly a computer that does some application processing, with both 1553 and COTS LAN or device (e.g., Universal Serial Bus (USB)) interfaces. If the signal comes in at high speed on Ethernet, but can be reduced on the to 1553 speeds, a computer with dual interfaces may do the job.

Additional buses/networks with COTS technology

A designer could create a cluster of such computers, linked by 100 Mbps or Gigabit Ethernet, which then hand off a smaller volume to the 1553 bus. Such devices might be at both the sensor and display side, doing such things as video processing and decompression in the cluster, transferring compressed imagery to 1553, and have a 1553 decompressor computer that talks to a COTS-interfaced display.

Fibre Channel, at 1 and 2 Gbps, are on platforms and systems including, including F-18 Super Hornet, F-16 Fighting Falcon, F-35 Joint Strike Fighter, B-1 Lancer, B-2 Spirit, E-2 Hawkeye, the AH-64 Apache Longbow, and several AESA radars. ARINC 664, or AFDX, is a profiled version of switched Ethernet and Gigabit Ethernet used for commercial avionics.