We have and continue to experience this. We are having to go as high as 500
Ohms because of the low resistance in the FBMs. Our instrument guys connect
the HART communicator across the resistor which is hooked in series not
parallel. We found some terminal strips (from Phoenix Contact I believe)
that have modular resistor inserts to avoid having to splice resistors
inline. Rosemount states that at least 250 Ohms of loop resistance is
required for the HART communicator to communicate to the smart transmitters.
I believe they were assuming a certain amount of resistance at the input
modules when they stated that also. Maybe some other DCS's have higher
input resistance and it is not a problem. Foxboro's is pretty low.
Chad Airhart
Senior Instrument, Electrical & Control Systems Engineer
Equistar Chemicals, LP.
Victoria, TX
Wk. (361) 572-2568
Pgr. (361) 270-2214 alpha messaging at [EMAIL PROTECTED]
Fx. (361) 572-2541
> -----Original Message-----
> From: Loupe, Rory (RJ) [SMTP:[EMAIL PROTECTED]]
> Sent: Thursday, August 30, 2001 10:52 AM
> To: '[EMAIL PROTECTED]'
> Subject: FBM1, HART 275 and Rosemont 3051 Transmitter
>
> Our instrument guys are having trouble communicating with a HART 275
> communicator to a Rosemont 3051 level transmitter while connected to the
> Foxboro I/A FBM1 input point. The instrument is being loop powered by the
> FBM1 (P and P+). The only way we can get it to communicate is to put a
> 250
> ohm resistor across the pair of wires (parallel). This allows us to
> communicate with the transmitter but we will not want to do this will the
> instrument is in service. I have tried this with several different HART
> 275s, FBM1s and 3051 level transmitters. If we disconnect the loop from
> I/A
> and use an Altec to power the loop, everything works as it should. This
> eliminates our cabling as a problem, in my humble opinion.
>
> Has anyone else experienced this problem? Is there a solution to allow
> the
> communicator to work without a resistor across the pair of wires?
>
> Rory Loupe
> The Dow Chemical Company
> LHC-3
> P.O. Box 150, Building 6801 21255 highway 1,
> Building 6801
> Plaquemine, LA 70765-0150 USA Plaquemine, LA 70764 USA
> Phone: 225.353-6409 Fax: 225.353.6968
> E-mail: [EMAIL PROTECTED]
>
>
>
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