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Turn a HART PV into Two Identical Analog Outputs

Q: How can I take a HART Primary Variable (PV) from an oxygen transmitter (with only one HART output available), and output two identical analog signals?

A: Here are two approaches you can use by utilizing Moore Industries SST HART Functional Safety Isolator and Splitter or the HIM HART Loop Interface and Monitor.

SST Splitter Solution:

Since the oxygen transmitter has a HART output, we know that this particular output is a 4-20mA signal plus the HART digital signal. We also know that the 4-20mA signal is the HART Primary Variable. Therefore you can wire your oxygen transmitter to the SST Splitter to split the 4-20mA signal and send it to your two destinations as a 4-20mA signal. This is the less expensive approach. The SST solution could also send the HART digital signal to both destinations, if that had value to the user.

SST Oxygen Transmitter

HIM Solution:

Since the O2 transmitter HART output digital signal also contains the Primary Variable, you can wire your oxygen transmitter to the input of the HIM. The HIM will read all the HART Dynamic Variables, but we will assign the Primary Variable to two HIM 4-20mA outputs which go to the two destinations. The HART digital signal does not get passed to the HIM analog outputs.

HIM Oxygen Transmitter

While this is the more expensive strategy, it does offer significantly more flexibility in your system. The HIM can send up to three analog output signals, as opposed to just splitting an analog signal in two; these analog signals can all be the same Primary Variable, or they could be the Second, Third, or Fourth Variables, which are also contained within your oxygen transmitter’s HART output signal. In addition, the HIM can provide you with two relays for Process or Diagnostic alarming and has a large 5-digit display for ease of use. Because the HIM is a configurable device it can easily be repurposed in the future.

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