Does the Bus Stop Here?

Communicating in the digital age


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.

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