Protecting ARINC 429 from Radio Interference
Click for ARINC-429 Avionics Cards or ARINC-429 Analyzers
Given that the ARINC 429 data transfer protocol is frequently used in aircraft avionics, one needs to consider how this system is made to operate optimally. Any such system is at risk of reduced performance due to a number of phenomena. Let's take a look at how the ARINC 429 protocol is protected from radio interference.
How Is Interference Reduced Around ARINC 429?
There are number of ways in which ARINC 429 function can be preserved despite the presence of all kinds of other sources of electromagnetic radiation on board the typical aircraft. There are physical and electrical methods employed, as well as protocol-based actions that are used.
The first way in which radio interference can be reduced right from the installation stage has to do with the signal cabling. The cables associated with ARINC 429 data buses always involve a shielded 78 Ω twisted pair. This approach is taken in order to prevent digital noise affecting any received signals and the fact that it involves near point-to-point wiring means that data transfer is as reliable as it can be.
Output impedance from the transmitting source is set at 75 Ω, which is allowed to fluctuate up and down by up to 5 Ω each way. This impedance is shared equally by Line A and Line B of the twisted pair, providing a balance that is suitable for the cable used.
On a related note, cabling like this is generally limited to 150 feet in most case as increased length would naturally reduce signal quality. However, cable length can be doubled to 300 feet or even longer if conditions are appropriate. Ultimately, this depends on the number of sink receivers, source power and sink drain.
Interference From the Signal Cable Itself
Of course, the signal cable could be a source of interference to other devices as well as to itself. This aspect of electromagnetic interference is minimized by the choice of data encoding method used by aircraft running this data bus protocol. The data encoding process involves the use of a complementary differential bipolar return-to-zero transmission waveform, typically referred to as BPRZ transmission.
Interference Issues With Older Aircraft
Some older aircraft still use the almost completely obsolete LORANC system and it has been found that ARINC 429 can actually be the cause of radio interference in this case. The reason for this has to do with the precise carrier rates used and the solution posed involves the use of fiber cabling, with the rationale being that this kind of cabling intrinsically provides isolation and has EMC qualities.
It is evident that the ARINC 429 protocol has been formulated with a significant amount of thought. Minimization of radio frequency interference both from and to this data bus has been made possible via physical, electrical and protocol-related means.