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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

TIMING IS THE SECRET

How is bus access controlled in a distributed system? In the centrally controlled system, the BC has a list of addresses of all bus users and sends a command to each one of them at a predetermined rate, giving each user a chance to access the bus and send data required by other users. The MIL-STD-1553B data bus, as I already mentioned, represents this command/response method of control.

The core of any distributed control is a contention protocol to ensure that only one transmitter speaks at a time. In aeronautical networks, three fundamental approaches to controlling bus access are used, including contention resolution, time slot allocation, and token passing.

The contention protocol allows any bus user to start transmitting at any time, provided the bus is idle at the moment. If two users start transmitting at the same time, however, a collision occurs. This isnýt an efficient use of the bandwidth and the throughput suffers during heavy traffic. The time slot allocation, on the other hand, assigns a specific time slot for every user. The user listens to the bus for inactivity and if the bus is inactive during the userýs assigned slot, the user may begin to transmit. If the bus is busy, access is not attempted until the next time slot. The token ring passing protocol works by allowing the user to transmit only after it has received a unique bit pattern, called token. So, these are the three main distributed control principles. And, of course, there are many variations on the basic themes.

Contention protocols need collision detection to determine whether or not their attempt to transmit was successful. The users monitor their own transmissions, comparing them to the transmitted data internally stored. If they detect data corruption, they stop transmitting. Then, when the users detect that the bus is idle again, they wait for a random time and, provided the bus is still idle, attempt to access it again. The reason for the random delay is to make sure that two identical transmitters donýt continue colliding indefinitely.

Another problem you need to take into account is propagation delays on the transmission line. The waiting user may see an idle bus while another user is already transmitting. Random time delays alleviate this problem, but the result is poor bus utilization and severe throughput problems under heavy traffic.

The strive for better throughput of avionic networks resulted in sophisticated time allocation methods, such as in ARINC 629 protocol. Generally, the bus control methods are so critical that they are made transparent to the designer of a user terminal by providing him with a dedicated chipset and couplings to build the interface. Token ring protocols are not yet commonly used in avionic systems but do exist, such as the LTPB and HSRB protocols developed by SAE (Society of Automotive Engineers). So, what are the predominant protocols used on aircraft?

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