[doubleg72]

That's exactly it.. i work at hospital system and just had to deal with this myself. The HVAC guys were in to upgrade the controllers for the air handling systems. They are basically PLCs that connect to a server like anything else. The issue is interfacing with the system. These pre 2000s air handler units generally communicate with the controller over serial, and there are specific configurations to make it work. We almost had a very costly issue of needing to upgrade the controls in the one system but they ended up getting it going.