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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

ARINC 429 IMPLEMENTATION

One hundred kilobits per second throughput aside, this slug of a protocol is still the most popular on transport aircraft, so letýs take a closer look at its implementation. Its specification is 520 pages thick, so all I can do with this article is merely explain the fundamental aspects of the inner workings. [3]

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Figure 2ýHereýs the definition of input and output voltage levels for ARINC 429 data bus.

The signal is in bipolar, return-to-zero (RZ) format (see Figures 2 and 3). It is tri-state, with the defined levels 1, 0, and null. The transmitter output voltage swing is ± 10 V nominally, at 12.5 or 100 kb/s. Rise and fall times are specified to be 10 ý 5 ýs for the slow and 1.5 ± 0.5 ýs for the fast format. The rise and fall times specification makes the pulses trapezoid in shape, and with their return to zero characteristic, resembling a sinusoid. This results in a significantly lower harmonic content (less radiated interference) than you would get from a rectangular wave shape, such as RS-232.

Figure 3ýThis timing diagram of ARINC 429 data bus shows a bipolar return-to-zero (RZ) format.

The hardware interface is simple to implement with discrete components, as shown in Figure 4, and the protocol timing functions provided by a microprocessor. To create the transmitter, you need only a couple of buffers to drive the lines in bipolar fashion. For slow speed, many small monolithic audio amplifiers will do. For the fast mode, current buffers such as LH0002 can be used. The receiver is more complicated, involving a differential amplifier feeding several window comparators to decode ones (Line A HI), zeros (Line B HI), and nulls. The transmitter output resistance R_o should be 75 ohm. Resistance R_s split between two resistors in series with the buffers provides the necessary adjustment of the usually very low-impedance buffer outputs. The receiver differential input resistance should be greater than 12 kilohm, with >12 kilohm between either input and ground. The linesý and inputsý parasitic capacitances should be kept below 50 pF. Up to 20 receivers can be hung onto one transmission line.

Figure 5 is an example of a different implementation of the receiver. It is simple and robust, built with two optocouplers and a couple of LEDs. The input resistance is less than the recommended 12 kilohm, but the protocol is robust and capable of handling this type of interface. The advantage of the circuit is that it is simple and provides excellent line isolation. The LEDs are needed instead of regular diodes to provide the necessary null voltage window.

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Figure 5ýThe receiver can be also built with optocouplers and LEDs to provide the required null threshold.

If your budget isnýt severely restricted, I highly recommend that you opt for an interface chip, such as Holtýs HI8282, instead of building your own hardware. The interface chip costs around $40 and contains one complete transmitter and two receivers with all the necessary functions, voltage level shifters, line drivers, and serial-to-parallel converters. The block diagrams of the receivers and the transmitter are shown in Figures 6 and 7. ARINC 429 transmitter assembles data in 32-bit words and the message packets are transmitted at a repetition (or update) rate of, usually, 3 to 5 Hz. Every receiver needs to listen to the chatter and determine whether the message is intended for it or some other user. Consequently, this is resource-intensive processing. Supporting your own hardware by generating the properly timed pulse train for one transmitter and decoding the incoming chatter from one receiver, both at the slow data rate, can use most of the 68HC11 microcontrollerýs processing power.

Figure 6ýHoltýs HI 8282 receiver section is shown as a block diagram.

The Holt chip unloads this overhead from the microprocessor. It interfaces directly with the communications lines and accepts data from and sends data to the microprocessor in parallel fashion in 16-bit chunks. By taking care of the timing and all serial data processing, it not only saves a lot of processing resources, but also eliminates software development and certification of potentially critical functions.

Figure 7ý Hereýs a block diagram of Holtýs HI 8282 transmitter section.

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