Protocol Conversion - Letting You Speak the Same Language

Every device on an airplane must meet several requirements—physical requirements for size and weight; power requirements; heat dissipation; and the ability to withstand vibration, moisture and G forces. Then there are more mundane considerations such as cost and availability. All of this is before dealing with the requirements of device’s functionality.

Imagine finding a device that meets all your requirements and specifications, does exactly what you need and fits in your budget—happiness and joy!—except that it communicates over MIL-STD-1553 and your airplane or helicopter communicates over ARINC 429 or Serial or Ethernet.

What do you do? Do you keep looking for another device? Do you give up on some of your requirements: extended temperature, cost, functionality? You could try convincing the manufacturer to modify the product to meet your needs, and if your company’s name is Boeing or Lockheed you may succeed, but even then it will take time to develop your custom version of the product.

This is the challenge that Excalibur Systems took upon itself when deciding to develop the MACC—Miniature Airborne Communications Converter. The requirements for such a device were intimidating. The MACC had to be small, rugged, EMI and RFI impervious, tolerant of extreme weather conditions, yet flexible and configurable to serve a large variety of applications.

The MACC is highly configurable and enables you to use (for example) 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.

The MACC’s Hardware Design

The limiting factor with regard to size is the connectors. In order to comply with military standards, we chose the widely accepted MIL-DTL -38999 Series III connectors. We selected two 22-pin connectors for the I/O signals and a 3-pin connector for the power. All the connectors are on one side of the MACC to minimize the required access around the unit. This determined the first two dimensions of the MACC to 50 mm x 140 mm. In order to increase the MTBF, the MACC was designed without any internal wires. A specially designed PCB was developed to attach to the three connectors. The remaining two boards that comprise the MACC snap in to the connector board and are held tightly together in a “U” shape.

These two boards are the logic board that contains all the I/O interfaces, flash, logic and processing power; and the MIL-STD-704E power supply board that converts the 9–36 Volts generally received on an aircraft to the 5 Volts required by the MACC.

The unique design of the logic board accommodates the complex functional requirements of the MACC. The MACC is built with a generic structure yet has the flexibly to appeal to a large variety of applications. The balance of flexibility and structure was the driving force behind the system design. This allows the MACC to meet each project’s exacting demands while not requiring custom code for each application.

How It Works?

Our experience with previous generation translators taught us that the exact labels, packets or messages to be handled by the MACC would likely be very fluid. For example, if ARINC 429 labels were the input for the MACC, we would need to assume that the type of incoming labels would be likely change over the course of the project, possibly several times.

We already had experience with flexible structures for ARINC 429 and MIL-STD 1553 though the development of our 1553Px and 429RTX line of adaptor boards. We decided to incorporate these devices into the MACC to retain their flexibility. Since the MACC would need to be self-sufficient during flight we created a flash-based structure that would be automatically loaded on power up. We designed tables that allow the user to configure the MACC in the laboratory (or maintenance facility) through a simple attachment to any PC’s USB port. These tables determine the lists or ARINC 429 labels or MIL STD 1553 messages to be transmitted or received along with timing data and other specification-specific parameters. The formula for translating from one I/O port to another is also included in these tables.

Still, we realized that in order for the MACC to be a truly powerful tool, we needed to allow the possibility of customizing the code. This meant that the MACC had to support downloading new firmware and even new hardware logic.

To accommodate this flexibility, the MACC is designed in three parts. The ARINC-429, MIL-STD-1553, Serial and Discrete I/O adaptors are all based on their equivalent Excalibur M4K modules used on our 4000 family of boards. The Ethernet interface is core based and similar to the design used on our AFDX board. All are designed to minimize processor intervention which is critical for this real-time application.

There is one processor solely used for reading the I/O from the various adaptors and converting between them. An independent processor is tasked with less real-time tasks such as firmware download, configuration table download and error logging to flash, or if available, real-time logging over a Serial debug port.

To reduce the parts list, thereby increasing the MTBF and decreasing heat generation and integration problems, we used core based processors, fitting all the logic and processors into a single field programmable gate array. In addition to the obvious benefits of a single chip, this also served as a hedge against obsolescence. There’s never a problem acquiring more copies of a core-based processor or core-based memory.

Another advantage of the core-based processors is that the compiler is free, giving our customers the option of writing their own custom conversions for convenience or for secrecy requirements of the program.

The MACC Management Tool

The MACC comes with a PC-based GUI program called the MACC Management Tool (MMT). The MMT guides the user through all the steps necessary for building the conversion tables, downloading them to the MACC and debugging them in the early stages of the project. The MMT provides a set of spreadsheets with column headers explaining the use of each field. Once the tables are completed, the MMT checks them for range error and logic errors. For example, if a user tries to convert data into an ARINC 429 label that was not scheduled for transmission, it will be flagged. Once the tables are complete, the MMT downloads them to the MACC where are stored in flash. The MMT is also able to upload data from the MACC to check configurations and to gain a better understanding of the real-time operation of the system.

The MMT can store multiple versions of firmware and configuration tables, so that users can experiment with different configurations simultaneously in a streamlined environment.

The result is a powerful tool capable of translating between different communications types with differences in hardware, protocol, timing and engineering units formats.

The more important effect of the MACC is to increase 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.

From the device manufacturer’s point of view, for devices that have been developed for a single communications protocol, delivering a system with a built in MACC offers the option of testing the waters on new markets without a significant R&D investment in an area that is not the manufacturer’s area of expertise.

It’s difficult enough to find the right device for the job without having to worry about communications compatibility. Fortunately, you don’t need to any more.