The one use I can think of is REALLY remote data acquisition (and mebbe supervisory control)over the internet. What if (for whatever reasons) you needed to monitor your controller and communicate with it from outside the facility where its installed... maybe from another country, maybe from your home if you don't feel like going to the site on a weekend :D

However I don't see it being used for communication within a limited area where simple serial communication would suffice... so Mahmood's example seems a bit far-out to me (unless its a TRULY huge hall :D ) But that sort of thing will work well in office Building Management Systems. Offices nowadays have ethernet networks installed all over the place. The temperature control application in such a case might make use of TCP/IP to communicate over the ethernet between different rooms and floors.

I was at an international Instrumentation and Control fair about 2 years back, and they actually had a PLC that allowed an engineer to access and change parameters, and even reprogram the device over the internet. I couldn't help but worry about the security aspects of this concept...Data acquisition and monitoring (be it PLCs or microcontrollers) is fine, but I dont think reprogramming/ parameter changing should be allowed over the net.

The advantage of TCP/IP is that it gives connectivity with the internet, which means connectivity with absolutely anywhere in the world. Its up to you what to do with that sort of connectivity.

kundi