Problem: When sharing a Process Variable (PV) with single or multiple systems, the connection needs to be thought out carefully especially when considering additional scenarios to this application: HART functionality and Safety Instrumented Systems.
Solution: Creating a series connection for the 4-20mA signal is not a best practice for sharing the PV. In part 2 of this series, sharing one Process Variable (PV) was discussed as well as defining a primary and secondary loop.
Part 1 in this series discussed sending a signal to more than one PLC with a Quad Isolator. Now, in the third post in this series, let’s look at including HART functionality.
Many isolators, like the SIX and ECT, do not allow HART protocol to pass across the isolator. If you are using HART, it is important to remember that the HART digital signal can be accessed on the Primary Loop, but not the Secondary Loop. But other isolators like the 2-wire (loop powered) HIX, are specifically designed to provide the same isolation but also pass the HART digital signal bi-directionally, across the isolator.
Figure 1. Analog Isolation with HART Using HIX
As seen in Figure 2, isolators can be extremely useful at sharing PV values from your Safety Instrumented System (SIS) loops too. Since the isolator input is in series with your safety function, you may want to use an isolator that has been designed and built to the Global Safety Standard IEC 61508, with the purpose of being used in a safety function.The SSX SIL 3 capable isolator passes the HART digital signal while providing isolation so an auxiliary monitoring system, like an Asset Management System (AMS), can communicate bidirectionally with the critical field transmitter.
Figure 2. Analog Isolation with HART and SIL Using SSX
Read Part 1 in this series: Quad Isolator
Read Part 2 in this series: Analog Isolation
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