Comparing Analog and Digital Output Transmitters in Safety Applications - ISA TECH 1999

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Use of analog output transmitters in safety interlocks and shutdown applications allow other conditions to affect the validity of the signal going into the trip device and therefore the validity of the trip action. Because analog output transmitters can cause high false alarm rates, over the years, manufacturers have provided configurable features within shutdown devices to compensate for those uncertainties with
hopes of avoiding false shutdowns. Unfortunately, those same precautions cause delays in the response to a valid shutdown. With digital output transmitters, most of the uncertainty can be eliminated, shutdown response time can be improved and false alarm rates reduced. With the variety of industrial digital communication protocols on the rise, the use of digital process alarm trips are fast becoming the logical and safer choice. This paper compares the differences in safety performance and availability between analog, hybrid and all-digital safety solutions.

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