Re: [foxboro] HART on I/A -- thoughts?

Dave,

FBM214 - group isolated

FBM215 - total isolation

FBM216 - redundant HART - group isolation

FBM218 - redundant HART - total isolation

For any installation that has group isolation you must ensure that the
potential (voltage) between any two channels never (as in 24 X 7 X 365)
exceeds some small voltage. The Foxboro PSS does not specify a voltage.
Other manufacturers do publish the voltage in their specification.

I have seen voltage limits from 0.25 V to 2.5 V from various
manufacturers, so the voltage is not high and that means it is difficult
for a user to ensure that the voltage is never exceeded.

If you run the transmitters in the 4-20 mA mode there is obviously an
A/D converter. The Foxboro PSS does not state whether there is an A/D
per channel or per group. I have seen products from various
manufacturers where a common A/D is used per group and if the voltage is
exceeded, the A/D shuts down and you lose all signals.

For these and other reasons, many users demand total isolation for all
input and output circuits. I happen to think it is worth the extra cost.

Terry Doucet, Eng.



So, is there an galvanically isolated HART input card?  Cause, uhh, I 
didn't realize that this was just group isolated, and that kind of 
sucks.  See I was hoping to avoid, "is usually only found to be 
deficient at installation and is a pain to fix."

Just wondering.

David
 
 
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