Beginning to understand Industrial Communications Protocols
Protocols is a communication language between systems/devices. Just like different countries have different languages, different device groups have different languages. If you were in spain you could get by knowing only english, but you would be more efficient and effective if you used spanish language.
Communicating computer to computer is different than communicating PLC to PLC, which is different than communicating between a PLC and a smart sensor. Lets take the two extremes, difference between computers and smart sensors. Computers require a lot more information communicated, more security, typically sit in a office. Smart sensors require less, and typically are out in the field or an industrial type environment. So HART has attributes hard&soft designed specifically for the devices it serves.
Yes, you could use RS232, but it would be slower, have distance restrictions, etc. so you pick the protocol that best fits your specific system. If you were using Allen Bradley (AB) equipment primarily, you may chose ControlNet and/or DeviceNet because they were designed by AB, so most likely best suited for their systems and devices. (More efficient and effective)
Another example, HART was designed to communicate over a sensor's 2-40ma signal, you may chose it if you need to interface with old systems. For a new system, you would rather use the new Fieldbus protocol (Profibus DP/PA with better diagnostics). Mostly AB in one new facility? ... you may chose DeviceNet which is based on Fieldbus.
Because protocols can be layered, linked, derived from another protocol, I understand it can get confusing. Just like someone in one country may speak two different languages, and within that single country, different dialects. Your best bet is to use the internet to study all the industrial communication protocols. See http://en.wikipedia.org/wiki/Common_Industrial_Protocol for a start with your Industrial Communication Protocol Comparison.
Don (Follow me on Industrial Skills Training Blog and on Twitter @IndTraining .) Be sure to add me to your Google+ circles to stay on top!