Thursday, November 1, 2001
Product Focus: Data Bus Test
The predominant avionics data bus standard in commercial aviation today is ARINC 429. The first widespread application of digital data bus technology in commercial aviation, ARINC 429 has proved resilient over nearly 30 years of use. Problems, when they are found, typically occur with the input/output ports of ARINC 429’s transmitter/receiver integrated circuits, engineers say. But "often…the failure reason is outside the LRU [line replaceable unit], like wiring shorts or high voltages," adds Franz Ilse, workshop engineering section manager with Lufthansa German Airlines.
Other interconnects include the Boeing 777’s ARINC 629, ARINC 659 and optical link. According to comments from several airlines, ARINC 629, a more recent technology than ARINC 429, appears to be more robust, and ARINC 659, a B777 backplane bus, seems to be relatively problem-free. The B777 optical link, the Fiber Distributed Data Interface (FDDI), has shown high reliability, Boeing says, but will be replaced by Ethernet.
British Airways (BA) finds that most ARINC 429 faults in its B757 and B767 aircraft equipment "are evident from the loss of [an avionics] function or from the BITE [built-in test equipment] of the system, which is not receiving the information that it requires," says Bernie Tremain, a BA avionics design engineer. BA uses handheld bus analyzers and automatic test equipment (ATE) for troubleshooting. There has been a trend away from Level-3 (component-level) repair, Tremain notes. "In many cases our facilities only filter-check or replace cards, with confirmed faults often being referred to the manufacturer."
Modification programs introducing new equipment can sometimes raise issues with source destination identifier (SDI) settings, Tremain explains. But "when this happens, it is usually because the equipment specification is not tight enough rather than [because of] a failing of ARINC 429 itself." Bus refresh rate specs occasionally have caused problems. "However, problems are invariably ironed out ahead of operational use of the equipment," Tremain says.
"ARINC 429 is a very, very small problem" in the larger scheme of things, says Mark Sorensen, a project engineer in Northwest Airlines’ avionics shops. The carrier relies on card exchange programs.
Bucking the trend away from component-level repair, Air France has been identifying ARINC 429 failures at the chip level for 10 years. "We have designed an A429 receiver/transmitter connected with a PC in order to…troubleshoot [components] in our avionics shop," says Jean-Paul Deleporte, shop avionics engineering manager.
The other often-mentioned commercial data bus standard is ARINC 629. Introduced in the mid-1990s, this 2-Megabits/sec (Mbits/sec) link was developed by Boeing and is deployed only on the B777. ARINC 629 achieves bidirectional connection of multiple systems without the need for a bus controller, an element that could be the source of single-point failure. This data bus runs 20 times faster than ARINC 429’s high data rate.
ARINC 629 "has proven very reliable to date," says BA’s Tremain. "BA [has] only experienced one fault so far, and that was resolved by replacing a coupler."
ARINC 629 is more robust than ARINC 429, says Paul Prisaznuk, a data bus expert associated with ARINC’s Airlines Electronic Engineering Committee (AEEC). He notes that ARINC 629 employs distributed control, avoiding single-point failure modes; extensive self-monitoring, where each terminal monitors its own transmissions; and non-intrusive, inductive coupling. (ARINC 429, by contrast, uses wired transformer coupling.)
"The diagnostic capabilities of the B777 onboard maintenance systems make investment in complex equipment to diagnose such problems as timing unjustifiable," BA’s Tremain says. On the 777, failures are isolated to the LRU level. Problems on the 429 and 629 buses "are identified by the receiving systems’ reporting a loss of input from the [avionics] system concerned."
The B777 uses a tightly coupled backplane bus, ARINC 659, for the Aircraft Information Management System. "To date [BA] has not experienced any failures of the backplane data bus," Tremain says. The 60-Mbit/sec bus contains advanced self-monitoring features. Faults are reported through the central maintenance computer function.
B777’s Optical Interconnect: Obsolete
The Boeing 777’s fiber optic interconnect, known as the Fiber Distributed Data Interface (FDDI), is up for replacement on recent versions of the aircraft, but not because of maintenance or performance issues, the company says. The reason is "technology obsolescence," says Chuck Royalty, lead engineer for network systems with Boeing’s 777 program. The 100-megabits/sec (Mbits/sec) optical system links maintenance terminals with the central maintenance function in the Aircraft Information Management System (AIMS). The original maintenance network also included links using 10-Mbit/sec Ethernet.
For a system that uses glass fiber and electromechanical components in a severe environment, the B777’s FDDI has a reputation for reliability. "I am aware of maybe one [optical fiber] break because a cable was bent," Royalty says. British Airways, a longtime FDDI user, considers the bus "very reliable in service." The carrier has replaced only one feeder cable assembly so far, according to Bernie Tremain, a BA avionics design engineer.
When Boeing introduced FDDI to the B777, the company considered the interconnect a "premier commercial bus." But now the technology is becoming more expensive and less available, Royalty says. Boeing plans to replace the optical system with 10-Mbit/s, copper Ethernet, concurrent with its implementation of an AIMS-2 upgrade on the B777-300ER and 777-200LR airplanes, expected in the third quarter of 2003 and the first quarter of 2004, respectively. "Instead of being part FDDI and part Ethernet, the maintenance network will now be all-Ethernet," Royalty says. He predicts that the replacement, all-Ethernet system will be "a wash"—comparable in maintainability and reliability to the optical system.
Civil aviation appears to be moving toward faster Ethernet technology. Boeing already employs a 10-Mbits/sec Ethernet network on the 767-400ER aircraft for flight-critical displays. And Airbus plans to deploy a 10-Mbits/sec/100-Mbits/sec, full-duplex, switched Ethernet communications system on the A380 super jumbo aircraft. ARINC’s Ethernet standardization effort can be found in Project Paper 664, including both 10 Base-T and 100 Base-T implementations. No date has been set for the completion of that work.
Aeroflex Circuit Technology www.aeroflex.com
AIM GmbH www.aim-online.com
AMP of Great Britain Ltd. www.webzero.co.uk
AMPOL Technologies Ltd. www.ampol-tech.com
Ballard Technology www.ballardtech.com
Condor Engineering www.condorengineering.com
DAC International www.dacint.com
Data Bus Products Inc. www.databusproducts.com
Data Device Corp. www.ddc-web.com
Excalibur Systems Inc. www.mil-1553.com
Holt Integrated Circuits www.holtic.com
L-3 Telemetry & Instrmentation www.ti.l-3com.com
Max Technologies www.maxt.com
National Hybrid Inc. www.nationalhybrid.com
North Hills Signal Processing www.northhills-sp.com
Pentar Avionics www.pentar.com
PowerCom USA www.power-comusa.com
Radstone Technology Corp. www.radstone.com
SBS Technologies Inc. www.sbs.com
SCI Systems Inc. www.sci.com
Sima Engineering www.simeng.com
Smiths Aerospace www.smithsind-aerospace.com
Systran Corp. www.systran.com