Getting the most out of ARINC 429
Four decades old and counting, the ARINC 429 bus protocol is going strong. Its presence on the A310/320, A330/340, B737, B747, B757, B767, and MD-11 means that it will be popular for many years to come. Even more recent products with highly integrated avionics architectures – such as the B777, B787, and A380 – still use 429 buses to transport sensor data. The same is true for military platforms based on commercial aircraft, including the Navy’s P-8 Poseidon.
The question for suppliers, then, is how best to support users – from scientists in test and simulation labs to operators on the flight lines – with the latest, best performing, and most reliable hardware and software. The key attribute is flexibility: both the flexibility to transition from the lab to the embedded world and the flexibility to simulate and test a wide range of bus configurations with a single interface.
The ARINC 429 data bus is a straightforward, legacy bus. It is a one-direction communications channel that transmits 32-bit messages at a rate of either 12.5 or 100 Kbits/sec. As many as 20 nodes can be hooked to a single pair of cables, but often there are only two boxes per bus. One is always the transmitter, while the other always receives.
However, the simplicity of individual bus construction breeds complexity if there are numerous buses on an airplane. Each box that needs to send data to other boxes will have its own bus connecting it to one or more of them. A complex box may need inputs from multiple computers in order to make its calculation and send it on. All of this message-passing occurs on a preset schedule to ensure deterministic behavior, which leads to complex timing and synchronization issues. Simply wiring up the data bus infrastructure is a task in itself.
That’s what happens in the embedded world. The test and simulation world has much more exacting demands: If a laboratory is testing a complex piece of equipment like a flight management computer (FMC), for example, the FMC may be receiving inputs from many 429 buses and sending out data to other buses. In order to find out whether the FMC is working correctly, the tester needs to be able to replicate those data flows as if the FMC were actually in flight.
What’s more, a test rig may be looking at an FMC for a 737 one day but may be evaluating the air data computer for an A320 the next day. One box may require 18 transmitters, while the next calls for only six transmitters. Therefore, the interface that is used to bring all these inputs into the flight box must be adaptable and rapidly reconfigurable. The more simultaneous channels it can handle, the better. If the channels can be software-programmable to function as either transmitters or receivers, better still.
ARINC 429 interface technology has moved with the computer industry from PCbus to VMEbus and on to switched fabrics like VPX and PCI Express. Ever more channels are being squeezed onto cards, along with higher-resolution time tags and enhanced programmability, along with software support. Boards have shrunk in size from PC cards to XMC modules and have added more memory and I/O.
Vendors also can provide support for multiple levels of ruggedization if customers choose to transition a card design from the benign environment of a desktop computer to the more challenging environment of an airplane. These rugged modules can be wired via front- or rear-panel connections with air or conduction cooling.
One example of the latest ARINC 429 products is the GE Intelligent Platforms RAR-XMC, a four-lane XMC card that provides a total of 32 channels – 16 of them programmable as either transmitters or receivers – overvoltage protection, support for ARINC 717 flight-data recorder protocol, and 64-bit time tag resolution (Figure 1).
Although ARINC 429 is a fairly simple legacy bus, the overall infrastructure is a complex wiring scheme with many interdependencies and timing issues. This intricacy requires bus interfaces to provide both flexibility and smarts whether they are used to emulate bus activity in a lab or they are used to connect the cockpit switches to aircraft sensors and computers so that pilots get the information they need.