Does the Bus Stop Here?

I recently traveled commercially to attend a conference in Europe. After arriving at an international gateway in the early morning hours, I had to take an automated shuttle from one terminal to another in order to rendezvous with my connecting flight. My mind was running at full speed as a result of the potent and highly caffeinated coffee I had consumed during the transatlantic crossing.

The shuttle station was fairly typical of many modern airports. As the transport vehicle arrived, one set of doors opened for a specific amount of time to allow passengers to exit. The doors then closed and a second set of doors opened on the opposite side of the tram, allowing a new group of travelers to enter. Once again the doors closed after a predetermined time and the shuttle departed the station.

The ample glass windows enabled me to view the cars ahead of and behind the one I was in. There were about two dozen people occupying my car, about half that number in the lead car, and the same number of people in the rear one. I then scrutinized my companions and deduced that everyone probably had a unique story to tell.

About that time the tram arrived at the destination and the doors opened. All the passengers departed to complete their individual quests. The thought came to mind that assembled on that tram were people of various nationalities and dialects, yet they all understood the protocol and timing required to get them to their destination. This episode triggered a revelation and a digital data bus immediately came to mind.

Digital technology
Ah, the digital age. It seems like almost everything today is controlled by electronic pulses. Most current production aircraft include digital technology in operating systems and may have 50 or more processors all tied into various networks. This produces situations that could drive even the most savvy computer technician to the brink.

Data buses make the delivery of electronic information possible. Aircraft offer more challenges than wiring the standard business office.

Soon after the invention of the telephone, open wire lines were used for transmission; two wires were strung on either side of cross bars on poles and usually shared with electrical lines. At first, interference from power wires limited the practical distance for telephone signals. Eventually the phenomenon became understood and engineers devised a method referred to as wire transposition. The telephone wires crossed over each other periodically in order to cancel out the interference. In this way, the two wires would receive similar electro magnetic induction (EMI) from the power lines. Today, such open wire lines with periodic transpositions can still be found in rural areas. This represented an early implementation of today’s twisted pair data bus.

In the design of digital systems, it is often necessary to have one or more devices communicate information to and from other devices. One advantage of the digital concept is that it tends to be far more resistant to transmitted and interpreted errors than information symbolized in an analog circuit. This accounts for the clarity of digitally encoded telephone connections, compact audio discs, and for much of the enthusiasm in the engineering community for digital communications technology. However, digital communication has its own unique pitfalls, and there are multitudes of different and incompatible ways in which it can be sent.

Common networks
Some of the common networks used in aviation include:

USB (Universal Serial Bus): used to interconnect many external peripheral devices (such as keyboards, modems, mice, etc.) to personal computers.

FireWire: a high-speed serial network capable of operating at 100, 200, or 400 Mbps with versatile features such as “hot swapping” (adding or removing devices with the power on). Designed for high-performance computer interfacing.

Bluetooth: a radio-based communications network designed for office linking of computer devices. Provisions for data security come designed into this network standard.

20 milli-Amp (mA) current loop: this is a digital communications network based on interrupting a 20 mA (or sometimes 60 mA) current loop to represent binary data. Although the low impedance gives good noise immunity, it is susceptible to wiring faults (such as breaks) which would fail the entire network.

RS-232C: the most common serial network used in computer systems, often used to link peripheral devices such as printers and mice to a personal computer. Limited in speed and distance (typically 45 feet and 20 kbps, although higher speeds can be run with shorter distances). RS-232 can run reliably at speeds in excess of 100 kbps, but this is only valid for very short distances of around 6 feet. RS-232C is often referred to simply as RS-232.

RS-422A/RS-485: two serial networks designed to overcome some of the distance and versatility limitations of RS-232C. Used widely in industry to link serial devices together in electrically “noisy” environments. Much greater distance and speed limitations than RS-232C, typically over half a mile and at speeds approaching 10 million bits/second (Mbps).

Ethernet: a high-speed network which links computers and some types of peripheral devices together. “Normal” Ethernet runs at a speed of 10 Mbps, and “fast” Ethernet runs at 100 Mbps.

ARINC 629, Multi-Transmitter Data Bus: serial data bus which operates over cable at 2 Mbps. Developed by Boeing.

ARINC 429, Digital Information Transfer System: point-to-point, two-wire Bi-Polar Return-to-Zero signal, 32 bit data, 100K or 12.5K bit rate.

ARINC 659, Backplane Data Bus for Integrated Modular Avionics: which operates at 60 Mbps as a commercial aviation bus. SAFEbus architecture, developed by Honeywell, is based on ARINC 659.

ASCB (avionics standard communications bus): a high-speed, bi-directional digital data bus.

The important factors of avionics buses include operational behavior, fault tolerance, and redundancy. Most avionics buses are serial in nature, using only a few sets of wires to keep the point-to-point wiring and weight down to a minimum.

Flow of information
Buses and networks are designed to allow communication to occur between individual devices that are interconnected. The flow of information, or data can take a variety of forms:

• With simplex communication, all data flow is unidirectional — from the designated transmitter to the designated receiver.

• In a duplex communication circuit, the flow of information is bi-directional for each device. Duplex can be further divided into two sub-categories:

Half-duplex communication is like two tin cans on the ends of a taut string. Either can may be used to transmit or to receive, but not at the same time.

Full-duplex communication is more like a true telephone, where two people can talk at the same time and hear one another simultaneously. The mouthpiece of one phone transmits to the earpiece of the other, and vise-versa. Full-duplex is often facilitated through the use of two separate channels (or networks) with an individual set of wires for each direction of communication. It is sometimes accomplished by means of multiple-frequency carrier waves, especially in radio links, where one frequency is reserved for each direction of communication.

Twisted pair cables are often shielded in an attempt to prevent electromagnetic interference. Because the shielding is made of metal, it may also serve as a ground. However, a shielded or a screened twisted pair cable may have a special grounding wire (called a drain wire) added. This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is applied to the collection of pairs, this is referred to as screening. The shielding must be grounded for the shielding to work. Shielded twisted pair (STP) cabling includes metal shielding over each individual pair of copper wires. This type of shielding protects cable from external EMI (electromagnetic interferences).

Screened shielded twisted pair (S/STP) cabling, also known as Screened Fully shielded Twisted Pair (S/FTP), is individually shielded (like STP cabling) and has an outer metal shielding covering the group of shielded copper pairs (like S/UTP). This type of cabling offers the best protection from interference from external sources.

Screened unshielded twisted pair (S/UTP), also known as fully shielded (or foiled) twisted pair (FTP), is a screened UTP cable. It is a thin, flexible cable that is easy to route. UTP is small, so it does not fill up wiring ducts quickly.

There are some disadvantages as well. Twisted pair’s susceptibility to the electromagnetic interference greatly depends on the pair twisting schemes staying intact during the installation. As a result, twisted pair cables usually have stringent requirements for maximum pulling tension as well as minimum bend radius. This relative fragility of twisted pair cables makes the installation practices an important part of ensuring the cable’s performance.

Troubleshooting digital networks will often require a bit of finesse. Depending on the type of system, diagnostic devices may be built-in and can direct the technician to a potential fault on the bus. In an ARINC 429 circuit where one transmitter supplies information for up to 20 users, a physical observation of what works and what does not is often a good first step in fault isolation. Fault isolation can often be accomplished using common place electrical testing equipment.

It should be noted that digital networks operate on very low current flows and generally do not provide adequate power to illuminate test lamps. For example, ARINC 429 operates on a 10v DC range with 5 volts positive representing a digital 1 and 5 volts negative representing a digital 0. A digital volt meter is a better choice here than an analog meter as it puts a lesser load on the bus.

An oscilloscope can be an effective tool as it allows a real-time observation of bus traffic and enables the user to identify possible impedance problems or observe the presence of electrical noise.

As with the handling of any electronic equipment precautions, ensuring against electrostatic discharge is paramount. In the event a problem is detected on the network that requires further investigation to isolate the fault, all devices that are connected to the specific bus should be disconnected prior to the introduction of any test equipment that could introduce potentials outside the normal range of operation. In other words, if you plan to use a megger to locate an insulation breakdown, consider that the high electrical potentials will damage any unit still connected to the bus.

Termination resistors are sometimes installed at each end of the bus to compensate for the impedance. Although generally reliable, a failure of a terminating resistor can render the entire bus disabled. When troubleshooting, consider that terminating resistors are wired in a parallel circuit to each other; the overall bus resistance should be half the resistor value.

Bus splices are another common area for faults to occur. With the sensitivity and construction of digital networks in aircraft, proper handling techniques are essential. Pulling a twisted pair cable too tight can alter the pitch of the wire twists and increase EMI susceptibility. Installing a wire tie or cable clamp where it compresses the bus could alter the impedance, as could routing the cable with too tight a bend radius.

There have even been situations where bus cables have been routed through bungs at pressure bulkheads. The bung would squeeze the bus when the aircraft is pressurized, altering the impedance and producing a crippling effect.

Shielding is another area that can cause anomalies. Improper grounding or termination can sometimes allow a free passage for electrical noise to invade the bus or can alter the impedance by changing the capacitance value.

Many technicians are intimidated when it comes to troubleshooting data transmissions circuits. With some basic knowledge and precaution, it is not that complicated. After all, it is usually only two wires that could be either shorted or open! What could be difficult about that?

I have just come to the realization that comparing a digital data bus to an international airport terminal shuttle is probably not normal and could be considered strong evidence that a long, airplane-free vacation is probably warranted. Au revoir!

Jim Sparks has been in aviation for 30 years and is a licensed A&P. His career began in general aviation as a mechanic, electrician, and avionics technician. Jim has created and delivered educational programs for several training organizations. Currently, he is the manager of aviation maintenance for a private company with a fleet including light single engine aircraft, helicopters, and several types of business jets.