01 - What is the difference between HART and Fieldbus?

That is like answering "what is the difference between a bicycle tire and a car?" HART is only meant for communication protocol, whereas Fieldbus is actually a system architecture including control strategy etc.

Not merely better, but radically different. Indeed, mistaking Fieldbus for a "digital 4-20 mA" or a better DCS is like mistaking the computer for a better typewriter. It cannot even be compared to "smart" transmitter protocols. Imagine for a moment: A system that renders obsolete all separate signal conditioners, isolation amplifiers, input cards, output cards, CPU cards, I/P converters, and their web of interconnecting wires, almost an entire DCS. A system completely self-contained; expressed simply as field devices and a man-machine interface (MMI) like an operator console. A system where all controls, alarms, computation, selection, Totalization and much more - performed by the field devices' microprocessors. A system so powerful it may overcome a process’ controls problems with a few clicks of a mouse.

Fieldbus is the ultimate. The signal transmission has evolved to completely digital, system architecture has evolved to completely distributed to the field. I.e. Fieldbus not only replaces 4-20 mA, but also the by now hopelessly outdated DCS architecture of the seventies.