Protocol conversion: Speaking the same language

Communications (in)compatibility should not weigh into the selection of mission- and safety-critical avionics system components. Communications converters quiet the 'babble.'

2Avionics applications share many similar requirements, including extended operating temperature ranges
and the ability to withstand vibration and high g-forces, but they don’t always “speak the same language” due to different communications standards such as MIL-STD-1553 and ARINC 429. Creating a converter that fits the needs of military and commercial applications and translates between the different standards
used eliminates the language barrier, providing flexibility, interoperability, and an increase in the selection of available devices for various aircraft systems.

Every device on an airplane must meet physical requirements for size and weight; power requirements; heat dissipation; and the ability to withstand vibration, moisture, and g-forces. It is not unusual for an avionics application to need to operate between -40 °C and +71 °C and to withstand 7 g in all three directions. Then there are more mundane considerations such as cost and availability. All of this is before dealing with the requirements of the device’s functionality.

It’s common to find a device that meets all project requirements and specifications, does exactly what’s needed for the application, and comes in below budget, except it communicates over the wrong standard; for example, the device is designed for a military project and communicates over MIL-STD-1553, but isn’t compatible with a project involving an airplane or helicopter application for civilian avionics that communicates over ARINC 429 or serial or Ethernet. While each of these protocols sends the same kind of information and should therefore be interchangeable, their physical and electrical characteristics usually make them incompatible and, as a result, unusable as is the case in many applications.

One option is to compromise on temperature, cost, accuracy, or some other requirement. Developers could also try to get a manufacturer to customize the device, which is unlikely unless it’s for a big name like Boeing or Lockheed Martin. Another option helpful for a wider range of projects is a converter that can translate across multiple communication standards to fit the needs of many applications, military or civilian.

Designing a “multi-lingual” converter

The requirements for designing such a device are intimidating. Such converters have to be small, rugged, EMI and RFI impervious, and tolerant of extreme weather conditions such as lightning storms, yet flexible and configurable enough to serve a large variety of applications. To address these challenges, a device should be designed to conform to widely accepted mechanical and electrical military standards.

Learning the military’s language

The various military standards should be part of any designer’s vocabulary if they plan to be fluent with military and aerospace markets in addition to civilian markets. They include: MIL-STD-810F for mechanical testing, MIL-STD-461E for EMI and RFI susceptibility, and MIL-STD-1275B and MIL-STD-704E for electrical tests. Complying with these standards creates a rugged, versatile unit that acts as a base for a range of communication protocols that can be utilized for a wide range of products over a wide range of applications.

A unit that includes all these standards can enable an Ethernet-based controller to control a MIL-STD-1553 or ARINC 429 device, or a MIL-STD-1553 controller to control an RS-232/422/485 device or any of a large number of other combinations of normally incompatible devices. Ordinarily, this degree of flexibility carries a cost in terms of high parts count with accompanying problems of heat generation and parts obsolescence. These issues are far from ideal in the final design, especially in SWaP-constrained military applications, and need to be kept to a minimum.

Connecting with military standards

For military applications, easy interfacing with military systems and hardware is required. The first point of contact in a device is the connectors. Widely accepted MIL-DTL-38999 Series III connectors help comply with military standards. However, their bulky size limits how small the converter can be. Locating all the connectors on one side minimizes the required access space around the unit. And in order to improve reliability, no internal wires or moving parts should be included.

Putting it all together

Excalibur engineers tackled the “multi-lingual” challenge by developing the Miniature Airborne Communications Converter (MACC). Paying attention to the factors mentioned earlier, the design team developed a 75 mm x 140 mm x 50 mm converter unit. The unit is made up of a specially designed Printed Circuit Board (PCB) to attach to three MIL-DTL-38999 connectors, and two additional boards that snap on tightly to the connector board in a “U” shape (Figure 1). The two boards that comprise the logic board contain all the I/O interfaces, flash, logic, processing, and power supply board that allows a wide range of DC voltage from 9 V to 36 V.

Figure 1: The MACC internal boards make up the logical boards that contain I/O interfaces, flash, logic, processing, and a power supply board.
(Click graphic to zoom by 1.9x)

Processing the conversions

To accommodate flexibility while not sacrificing real-time performance, the converter is designed in three parts. One processor is used for non-real-time operation such as downloading tables or firmware and uploading log information. A second processor implements ARINC 429, MIL-STD-1553, and serial and discrete I/O adapters. A third processor relieves the second processor of some of the more time-consuming tasks associated with MIL-STD-1553. In addition, hardware-implemented FIFOs are associated with all I/O adaptors to keep latency to a minimum – within hundreds of microseconds between receipt of input and transmission of output. And because the unit must be self-sufficient during flight for avionics applications, designers created a flash-based structure that would be automatically loaded on power up.

To reduce the parts list, thereby increasing reliability and decreasing heat generation and integration problems, designers used core-based processors, fitting all logic and processors into a single Field Programmable Gate Array (FPGA). In addition to the more common benefits of a single chip, this also served as a hedge against obsolescence, as acquiring more copies of a core-based processor or core-based memory is rarely problematic. The FPGA provides functionality for Ethernet and serial communications, and allows logic development for MIL-STD-1553, ARINC 429, and discrete I/O on the same chip. The FPGA as well as the firmware running on the processors can all be upgraded in the field, providing a robust environment for adding or adjusting features. Core-based processors also offer free compilers, which allow for custom conversions for convenience or secrecy requirements without added cost.

Configuring the tables

The converter’s tables allow the user to configure the unit in the laboratory (or maintenance facility) through attachment to any PC’s USB port. These tables determine the lists, ARINC 429 labels, or MIL-STD-1553 messages to be transmitted or received, along with timing data and other specification parameters. The formula for translating from one I/O port to another is also included in these tables.

Previous-generation translators taught the design team that the exact labels, packets, or messages to be handled by the converter would likely be very fluid. For example, if ARINC 429 labels were the input, designers would need to assume that the type of incoming labels would likely change over the course of the project, possibly several times. The converter comes with a PC-based GUI program called the MACC Management Tool (MMT) to further guide users through the steps of building conversion tables and uploading data from the unit. It can also store multiple versions of firmware and configuration tables so users can experiment with different configurations.

Speaking the same language across many applications

A flexible converter increases the available selection of devices for any given purpose on an aircraft. The artificial constraint of communications compatibility should not be the driving force in the selection of complex units that have dozens of more important parameters that relate to the safe and efficient functioning of the aircraft.

A converter with diverse communications abilities is a powerful tool capable of translating between different communications types with differences in hardware, protocol, timing, and engineering unit formats. Without the disadvantage of a device developed for a single communications protocol, testing the waters on new markets doesn’t come with a significant R&D investment. It’s difficult enough to find the right device for the job without having to worry about communications compatibility. Having a small, robust, configurable tool to handle these compatibility issues could simplify the search for a device that speaks the same language.

David Koppel is Chief Technical Officer of Excalibur Systems. His responsibilities include the development of new products and software such as the MACC converter system for the ever-changing avionics community.

Excalibur Systems, Inc. (516) 327-0000