Communication Protocols and SCADA

Unfortunately due to the phrasing of the questions I'm going to need to make some guesses as to your meaning.

Are you asking that if a plant has a SCADA system, an instrument shouldn't be required to support a communications protocol? 4-20 mA only transmits one piece of information. Unless it also supports HART on top of that 4-20 mA signal, you don't know anything else about that instrument. You can infer some other information if the signal goes below 4 mA or above 20 mA indicating an over/under range, instrument failure, wire break or power failure, but you won't see secondary variables or details of instrument errors or configuration settings. Most plants live with these limitations now.

However, most new instrument installations and new plants standardize one one or two communications protocols (or at least try to) even if is just HART. The cost of adding Profibus or DeviceNet or any of the many protocols out there is a small cost adder that will pay itself back many times over the course of an instrument's life.

If the plant already has a SCADA system then it likely already has one or more communications protocols supported by the instrument manufacturer. The final decision should be left to the end user. Just because a SCADA system supports Profibus for example, doesn't mean you should spec that because you know it when the plant has no existing Profibus instruments.

I am confused on what you mean by threshold though? Can you clarify?