Does the Bus Stop Here?

Communicating in the digital age


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.

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