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Foundation Fieldbus
Difference between FF, HART, 4-20ma communication. Does a FF transmitter require 4-20ma signal?


I would like to know the difference between FF, HART, 4-20ma communication. When I say the transmitter is FF, which means FF is a digital communication protocol, does it mean it doesnt require 4-20ma signal? I am confused between these protocols and signals?

Can I have FF-AI, FF-AO- FF-DI and FF-DO ?

Please advise.

An excellent resource for many questions like this is They usually have very high-level explanations and often have some very detailed explanations, as well. AND, they often have links to excellent websites with more information.

The 'Foundation Fieldbus' entry in is an excellent example of all of the above.

There have also been some good descriptions in previous threads on, available by using the cleverly hidden 'Search' function at the far right of the Menu bar at the top of every webpage.

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1. 4-20mA is an analog signal that is converted to a digital value at the host PLC, DCS, or other controller or similar device.

2. HART is a digital communication protocol that rides on top of a 4-20mA signal without affecting the actual amp value. HART is used to obtain secondary and tertiary data from the instrument, and can be used in an interactive manner to configure the instrument.

3. Foundation Fieldbus (FF) is a digital communication protocol, there is no analog component. Multiple devices can share the same twisted shielded pair of wires.

There are pros and cons to each, there are books on the subject.

For your last question, you can have all of those things you listed in an FF based system. However, the nature of your question to me infers that you have more reading to do. You can, for example, procure an FF device that converts a 4-20mA signal to an AI channel, and on a different channel drive a 4-20mA AO. But with that you are not getting most of the benefits of FF.

Compare a control valve positioner, for example.

1. In the 4-20mA environment, you will have one set of wires connected to an AO channel to send a position demand. That might be it. You can add a second set of wires taking the position feedback to an AI channel. From here you can do some simplistic comparisons in an attempt to determine the health of the asset. But there is not much granularity.

2. Moving to HART, the 4-20mA signal is the AO sending the position demand. You can read the position feedback as a secondary signal superimposed on the 4-20mA carrier signal. The positioner might have 20 other signals available, but you can't get them all.

3. In the FF environment, in addition to position demand and feedback, you can get (depending on make/model) air pressure, friction, stiction, last time calibrated, total inches of stroke, and on and on. You can deploy some very complex asset management techniques.

Lastly you did not mention Profibus PA, which is another protocol in this space.

4-20 mA is an pure analog signal used to transmit process variables to control system.

FF is a fully digital instrumentation technology which employs digital communications over a bus to communicate and control with or without host.

HART is trade-off between these 2 as you could get some additional data over the same pair of wires.

Each have it's features pros/cons. You can always google and read very good literature available on the internet.