Understanding Smart Sensors - Nomads.usp

120 Understanding Smart Sensorscommittees and individual companies have expended considerable effort togenerate acceptable protocols that will support the requirements for distributedcontrol. System functionality and the ability to use available silicon hardware,software, and development tools must be considered in the selection of a protocolfor a given application.Several terms and definitions are used to describe the communication ofsystem information. A protocol is an agreed-on set of rules for communications.The bus connects internal or external circuit components and can be serial orparallel; the serial approach is more common. A device-level bus connects basiccontrol elements, the sensors and actuators, to a host controller. Multiplexing(MUX) is the combining of several messages for transmission over the same signalpath. Access to the bus is obtained through an arbitration process. Bit-bybitarbitration is also called collision detection. Contention is the ability to gainaccess to the bus on a predetermined priority. Latency is guaranteed access (withmaximum priority) within a defined time. A deterministic system can predictthe future behavior of a signal. The data link controller (DLC) is the siliconimplementation of the protocol that handles all communications requirements.The system goal of interoperability permits sensors or actuators from onesupplier to be substituted for one from another manufacturer. Different typesof networks are shown in Figure 6.1. Star, ring, and linear (tree, multidrop) arecommon topologies, that is, methods of connecting data lines to system nodes.The use of master-slave relationships, although an important part of controlnetworks, is giving way to the use of more distributed intelligence in manyapplications.The 4- to 20-mA standard of the Instrument Society of America (ISA)has been the analog data transmission standard for over 30 years. However,digital techniques can provide more system functionality and increased noiseimmunity for signal transmission. Many digital interface formats already existfor point-to-point or multidrop communications, as shown in Table 6.1 [1].These protocols have seen limited use in computer-controlled systems. TheEIA-485 is most often used for digital field bus applications. It is the physicallayer in Profibus, Topaz, and Bitbus protocols. The EIA-485 is a balanced-line(differential) data transmission over a single twisted-wire pair [1]. However,examining the different market segments, an even wider variety of standardsexists.6.2.2 BackgroundThe International Organization for Standardization (ISO) has defined theOpen Systems Interconnection (OSI) model that describes seven layers for datanetworking. The layers and their functions are shown in Figure 6.2 [2].