October 2009

Eventually data arrives and becomes information

By Lawrence Thompson

Data itself is useless until organized, at which time it becomes information.

We organize communication processes in three basic ways: Point-to-point, multi-drop, or networked. While these terms are couched in technical symbology, the concepts behind them are relatively easy.

Point-to-point means just that, from one point to another or from one end user to another directly. Multi-drop is closer to a network than to point-to-point. Generally, multi-drop involves a master of some kind with slave stations, not peers.

A network is simply three or more stations connected by a common media, by which they may share information.

There are three different categories of communications: Simplex, half-duplex, and duplex. These terms may describe all types of circuitry or modes of transmission, whether they are point-to-point, multi-dropped, or networked. It is important to understand these three terms because almost all descriptive language pertaining to data communications uses them. 

A circuit is typically the actual communications channel hardware configuration. Mode can refer to either the hardware configuration or a “virtual” circuit, which is a channel that consists of software or other processes that do not directly connect with one another but are communicating entities. Hence, a virtual circuit refers more to the process of communications than to an actual hardware configuration.

If the hardware is duplex, the software may make it run half-duplex, however, if the hardware will not support a mode of operation, no amount of software will cause it to actually be so, although in the virtual world it may appear to be so to us, an example would be a half-duplex system that appears (due to the speed and message constraints) to be duplex to the user.

Simplex or unidirectional mode: In this mode, communication occurs only in one direction, never in the opposite direction. We called the circuit that originally provided this operation simplex, but this leads to confusion with telephony terminology. Unidirectional is the name of the mode of transmission, and using it for this circuit is much more descriptive.

Half-duplex mode: In this mode, communication may travel in either direction, from A to B or B to A, but not at the same time. Half-duplex functions much as human conversation does, that is, one speaker at a time and one (or more) listener(s).

Duplex mode: In duplex or full duplex, commu-nication can travel both directions simultaneously: A to B and B to A at the same time.

Serial, parallel transmission

Serial transmission has one channel (one medium of transmission), and every bit follows one after the other, much like a group of people marching in single file. This means the expense of only one channel is required to send bits at much higher speed in order to achieve the same throughput as parallel transmission.

Parallel transmission is where the signal must traverse more than one transmission channel, as in a group of people marching four (or more) abreast. For a given message, a two-channel parallel will transmit twice as much information as a serial channel running at the same line speed.

However, a parallel transmission running any appreciable distances will encounter two problems. First, the logistics of having two (or more) parallel media is sure to double (or more) your costs. Second, assuring their simultaneous reception over some distance is technically quite difficult, along with ensuring cross talk (a signal from one transmission line coupling on to another) remains low.

Cross talk increases with signaling rate, so trying to go faster with multiple conductors becomes increasingly difficult. Note for the two 4-bit combinations, it took two timing periods to transmit all eight, whereas the serial signal took eight timing periods.

The catch is the serial signal only needed one media channel, while the parallel signal needed four media channels.

ABOUT THE AUTHOR

Lawrence Thompson is an ISA adjunct instructor and has designed, developed, taught, and maintained industrial controls and networks in many varied applications. His new book is Industrial Data Communications: 4th edition, ISA Press 2008.