Process instruments consist of two main components: a sensor and a transmitter. The sensor is sometimes part of the instrument assembly, as with some pressure instruments, but is more often separate, as with analytical instruments, such as those used for pH measurement. Before sensors could become smart, transmitters had to gain intelligence by adapting digital technologies. It would not have been practical, or sometimes even possible, to connect a smart digital sensor to a simple analog transmitter. Industrial instrumentation has progressed significantly since the 1970s, when the vast majority of instruments simply had a single 4-20 or 0-20 mA (analog) output proportional to the process variable. Some sensors had the inherent ability to measure multiple process variables, but they required multiple analog outputs to access this additional information. With an analog transmitter with a single analog output, secondary variables remained stranded, as did data regarding the configuration or health of the instrument. The process variable was relegated to a dedicated analog signal transmitted from the instrument over two wires to an indicator or control system, with a multidrop configuration.
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