PLC Direct PLC and Rosemount Transmitters Won't Work Together?

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Thread Starter

Mcgyver

I have been told that a rosemount 3051s smart transmitter with hart cannot work with the analog input card on a PLC direct 405 controller. Does anyone know if this is fact? Been asked to work on this. But before I make a 600 mile drive, just want to make sure what I was told is a fact so I am not wasting time trying to make it work. Someone said there is a issue with the hart signal interfering with the signal input when using two transmitters on the same input card. Any help?
 
First time I've heard that and I don't believe the explanation.

99% of industrial analog inputs are not HART pass through inputs that are looking for the HART signal. Industrial analog inputs (as opposed to an oscilloscope test & measurement input) 'see' the DC 4-20mA signal but not the HART signal (which is a 1200 baud signal) because the HART signal gets stripped off by low pass filtering.

There can be problems connecting two different signals to the same analog input card due to common mode voltage or a ground loop, but neither of those problems is HART related.

If I were driving 600 miles I'd take along a couple loop isolators for a problem like you describe.
 
> First time I've heard that and I don't believe the explanation.

I agree. It's hard to believe a 300 - 500 millivolt signal is going to mess things up on the PLC end. Now it may be true that the HART signal might not survive the trip through the input card (but that is hard to believe as well).

And to the best of my knowledge, your HART device will only respond to an incoming HART request. So if you don't want HART messing things up at the PLC, well...
 
> HART device will only respond to an incoming HART request.

Correct. HART is a master/slave protocol and the field transmitters are slaves who wait for an instruction from a master.

The analog input spec sheet for the 404 does not include the word HART, so the AI card is not a HART master.

That pressure transmitter is not generating a HART signal because there's no master to communicate reply to.

Something else is causing the problem; it isn't HART.

Correction to the earlier post: HART is a 1200/2400 Hz (not baud) FSK signal.
 
> Something else is causing the problem; it isn't HART.

Yup. I can't see how it would be a HART issue.

> Correction to the earlier post: HART is a 1200/2400 Hz (not baud) FSK signal.

Not to quibble, okay, I'm quibbling, but I believe it's 1200 & 2200 hz.

Now this is interesting, one 1200 hz cycle is a logic 1, while it takes two 2200 hz cycles to make a logic 0. Who knew?

http://www.ti.com/lit/an/slyt491/slyt491.pdf
 
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nirmalya1986

If the transmitter gives 4-20 mA output and if the PLC analog input card can accept this then any transmitter can directly connect with PLC. HART has no issue with that. The only problem is that the distance. at that long distance may be the signal gets dropped and you can't get the accurate signal. so its better to have RIO panel for long distance and optical cable connection with RIO and Main PLC.
 
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