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COMMUNICATIONS PROTOCOLS IN AERONAUTICS

by George Novacek

Start ý Data Bus Systems ý Timing is the Secret ý ARINC 429 ý CSDB and ASCB ý MIL-STD-1553B ý ARINC 629 and Beyond ý ARINC 429 Implementation ý Data Format ý Wrap Up ý Sources and PDF

DATA BUS SYSTEMS

On aircraft in general, you will find two primary types of data bus architecture. The accompanying communications protocols will then support one or the other system. The simpler, and to this day the most prevalent onboard commercial airplanes, is the unidirectional data bus represented by ARINC 429 (see Figure 1a). The system consists of a single transmitter and a number of receivers, each monitoring the line and listening for messages identified as their own. Communication back to the transmitter, if needed, is performed by a separate transmitter, receiver(s), and wires. It may appear primitive and redundant, but because of its simplicity, the system is robust, failure-tolerant, and extremely simple to design and implement. The disadvantage is a lot of labor-intensive wiring, which is also expensive and heavy.

Figure 1aýTwo data bus architectures are found in avionic systems, the first is unidirectional. býThe second data bus architecture in avionic systems is bidirectional.

Figure 1b shows a typical bidirectional architecture. This is found on military and some of the newer commercial aircraft, such as Boeing 777 and Airbus 330/340. Here, any member (or user) of the system can transmit, receive, or both. Messages are time-multiplexed, thus the network wiring is simple. Several years ago, Lockheed upgraded its famous Hercules-to-C130J version. Avionic systems, which previously used discrete wiring, were upgraded to interface with the MIL-STD-1553B bidirectional data bus. This resulted in a significant weight saving, and the labor to run wiring harnesses was reportedly reduced by thousands of hours. However, the bus control became complex and the engineering effort to integrate the system was no small task.

Every bidirectional protocol must be able to arbitrate data bus transmissions to ensure that only one transmitter is operational at a time and that the receivers are listening to their intended messages. And, unlike the unidirectional bus, a lot of thought has to go into the system design and integration. There are two approaches commonly used for traffic control of bidirectional buses, central control and distributed control.

The advantage of the central control, or command/respond approach, is that only one bus component has control of the bus traffic. All users are directed by the BC. If the data bus architecture has to change, only the bus controller has to be modified to support the new configuration. But the most significant weakness of such an architecture in an aircraft environment, where the bus controller or network failure (we donýt use "crash" for its obvious connotation) could have catastrophic repercussions, is that the bus controller represents a single point failure. When it goes, the entire data bus goes. Therefore, you will find systems with redundant bus controllers, only one having control of the network at a time.

In a distributed control system, all members of the network are in charge of their own access. Here, the advantage is that if any member fails or happens to operate erratically, the rest of the system will not be affected and will continue operating correctly. The weakness of the distributed control is exactly the opposite of the central control strength. It is weak in bus access sharing, and a change to the system configuration may require a change to every user of the system.

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