Networks and busses
This collection of wires that I keep referring to between the tank and the monitoring location can be called a bus or a network. The distinction between these two terms is more semantic than technical, and the two may be used interchangeably for all practical purposes. In my experience, the term “bus” is usually used in reference to a set of wires connecting digital components within the enclosure of a computer device, and “network” is for something that is physically more widespread. In recent years, however, the word “bus” has gained popularity in describing networks that specialize in interconnecting discrete instrumentation sensors over long distances (“Fieldbus” and “Pro¯bus” are two examples). In either case, we are making reference to the means by which two or more digital devices are connected together so that data can be communicated between them.
Names like “Fieldbus” or “Pro¯bus” encompass not only the physical wiring of the bus or net- work, but also the speci¯ed voltage levels for communication, their timing sequences (especially for serial data transmission), connector pinout speci¯cations, and all other distinguishing technical features of the network. In other words, when we speak of a certain type of bus or network by name, we’re actually speaking of a communications standard, roughly analogous to the rules andvocabulary of a written language. For example, before two or more people can become pen-pals, they must be able to write to one another in a common format. To merely have a mail system that
is able to deliver their letters to each other is not enough. If they agree to write to each other in French, they agree to hold to the conventions of character set, vocabulary, spelling, and grammar
that is speci¯ed by the standard of the French language. Likewise, if we connect two Pro¯bus devices together, they will be able to communicate with each other only because the Pro¯bus standard has
speci¯ed such important details as voltage levels, timing sequences, etc. Simply having a set ofwires strung between multiple devices is not enough to construct a working system (especially if the
devices were built by di®erent manufacturers!). To illustrate in detail, let’s design our own bus standard. Taking the crude water tank measure-ment system with ¯ve switches to detect varying levels of water, and using (at least) ¯ve wires to conduct the signals to their destination, we can lay the foundation for the mighty BogusBus:
The physical wiring for the BogusBus consists of seven wires between the transmitter device (switches) and the receiver device (lamps). The transmitter consists of all components and wiring connections to the left of the leftmost connectors (the “{>>{” symbols). Each connector symbol represents a complementary male and female element. The bus wiring consists of the seven wires between the connector pairs. Finally, the receiver and all of its constituent wiring lies to the right of
the rightmost connectors. Five of the network wires (labeled 1 through 5) carry the data while two of those wires (labeled +V and -V) provide connections for DC power supplies. There is a standard for the 7-pin connector plugs, as well. The pin layout is asymmetrical to prevent “backward” connection. In order for manufacturers to receive the awe-inspiring “BogusBus-compliant” certi¯cation on their products, they would have to comply with the speci¯cations set by the designers of BogusBus (most likely another company, which designed the bus for a speci¯c tsk and ended up marketing it for a wide variety of purposes). For instance, all devices must be able to use the 24 Volt DC supply power of BogusBus: the switch contacts in the transmitter must be rated for switching that DC voltage, the lamps must de¯nitely be rated for being powered by that voltage, and the connectors must be able to handle it all. Wiring, of course, must be in compliance with that same standard: lamps 1 through 5, for example, must be wired to the appropriate pins so that when LS4 of Manufacturer XYZ’s transmitter closes, lamp 4 of Manufacturer ABC’s receiver lights up, and so on. Since both transmitter and receiver contain DC power supplies rated at an output of 24 Volts, all transmitter/receiver combinations (from all certi¯ed manufacturers) must have power supplies that can be safely wired in parallel. Consider what could happen if Manufacturer XYZ made a transmitter with the negative (-) side of their 24VDC power supply attached to earth ground and
Manufacturer ABC made a receiver with the positive (+) side of their 24VDC power supply attached to earth ground. If both earth grounds are relatively “solid” (that is, a low resistance between them,
such as might be the case if the two grounds were made on the metal structure of an industrial building), the two power supplies would short-circuit each other!
