FF Question

[roydm]

Hi All,
I am trying to get my head around how Foundation Fieldbus operates and have few questions, hopefully someone can answer.
Is the digital signal from transmitter to DCS in engineering units (scaled inside the transmitter) or is it just a digital number e.g. 0-32000 and the scaling done in the DCS?
Is there any difference in resolution if you pick a small EU e.g kPa over a large unit like PSI?
If a transmitter has a cell rated 0-100# and you only need a portion of the range e.g. 0-50# is this done in the transmitter i.e. does the transmitter have a calibration range or will it still send the equivalent of 100# if it's over-pressured.
Thanks & Regards
Roy

What are the advantages/disadvantages of doing PID in the field device rather than in the DCS?

 

[JimCasey]

1. Not in engineering units.
2. No, 0-100% of calibrated span is divided into the available number of points of resolution.
3. PID in the field takes a lot of load off the DCS, Lets the process continue to operate even if the DCS communication is lost. DCS then issues "Memos from the head office" to the controllers distributed throughout your process. Then the controller have their instruction and focus on controlling a few loops. Your distributed loops can be very fast since the communication only has to be passed around a very few devices, as opposed to running the entire plant with hundreds of instruments from a centralized DCS where any one instrument might be interrogated only a few times per minute.