Q. I want to share a critical pressure signal with my DCS that is now part of our safety instrumented system (SIS) and have a two part question. First, do your isolators pass the HART diagnostic data that comes from my SIS smart pressure transmitter? Second, if there is a total failure of the pressure transmitter, where will the output of the isolator go to that is feeding my DCS?

A. Yes, we have isolators that were designed specifically for these types of applications. The SSX and SST are exida approved SIL 3 capable isolators that pass HART data between any HART host or handheld device and the field transmitter. The second part of your question has two answers.

The SSX is an output loop powered isolator and as such when the input goes to 0mA the output will only drop to a minimum of about 2-2.5mA. The floor or minimum output is necessary because the SSX will need some level of current to keep its electronics circuit powered.

The SST is a four wire isolator so it behaves a bit differently. The SST’s output will follow the input through the entire range. For instance, if the input goes to 0mA or open circuit the output of the SST will follow the input all the way down to 0mA. Depending on your DCS settings, you can then choose when and how you want to alarm on the pressure transmitter’s health or loop status. 

Download the data sheet to learn more about the SSX and SST.