The Difference Between FFtype Sensor and Hart Protocol Sensor

For any given brand/model instrument, the sensing part of the instrument is the same, the difference is how it communicates its data back to the control system/DCS.

HART is normally a 4-20mA analog signal, with 'home run' wiring back to the controller's input. Digital data is superimposed on the analog signal and can be 'read' by a 'HART' enabled analog input (only DCS and a select few high end PLC analog inputs qualify as such). HART can be intrinsically safe.

HART is very widely used to configure devices (with separate hardware/software), far less widely used for data communication.

Foundation Fieldbus (FF) is strictly a digital protocol with specific comm hardware requirements. Instruments can be daisy chained, without home run wiring. There are engineering considerations as to the number of devices per segment. FF instruments are configured directly over the comm network by the control system. FF can be intrinsically safe.

The control system has to be FF capable and that means quite high end controls, DCS level controls. There is a signifcant learning curve in implementing FF for those already familiar with analog/HART.

For existing controls, the existing comm mode, 4-20mA or FF will determine the replacement or addition to the system.

For new designs, consideration must be given to the ability of the plant to maintain FF.