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At Last, A Proposed Standard Allows Any Sensor Or Actuator To Be Interfaced With Systems And Networks, Independent Of The Network Used.
The advent of microcontrollers and low-cost analog-to-digital converters (ADCs) has given rise to a new type of transducer, characterized by having a digital output in some form of computer-compatible format. Some have a built-in microcontroller that gives them the ability to provide builtin linearization and error correction. This allows inexpensive devices to provide greater accuracy previously unavailable. More importantly, they give the potential for much simpler wiring than is possible using older analog technology. This translates into lower installations costs. So, what does it take to obtain these lower costs?
This is difficult to answer right now, because while there are smart sensors, there are few standard interfaces for them. A popular interface for smart transducers is RS-232. While many transducers support RS-232, they all use a different protocol. RS-232 is a point-to-point interface, so the wiring is almost as complex as the analog wiring, unless receive and repeat protocols are used to allow many transducers to be connected in series. An alternative is RS-485, which supports multidrop buses. Again, while the interface is standard, the protocols are not, so transducers from different manufacturers won't work together. Clearly what's missing is a standard interface.
This was the problem Boeing faced while developing and testing its new 777 Airplane (see "Simplifying testing of Boeing's 777,"p. 98). Various Fieldbus committees have been trying for years to solve this problem, but to no avail. The IEEE P1451 set of standards tries to solve at least part of the problem. IEEE P1451.2 defines an interface that transducer manufacturers can build to, independent of the network the transducer is being used in. Three different entities are defined within the standard: The smart transducer interface module (STIM) that contains the transducers; the networkcapable application processor (NCAP), which is the interface to the larger network; and the transducer-independent interface (TII) that interfaces the STIM and the NCAP (Fig. 1).
The TII, some characteristics of the STIM, and the method of communicating between the NCAP and STIM are all standardized by IEEE P1451.2. This allows the same smart transducer to be used on many networks by providing the appropriate NCAP.