An Open Interoperable System is a system that claims to possess complete compatibility with other systems also claiming to be "open." In principle these products are wonderful - they allow for system growth using the programs you choose instead of the few compatible with some programs. They also help strengthen system integration through better optimization in standardization and maintenance data, both of which lead to improved efficiency. Interoperable devices allow for devices from multiple vendors on one central network to coexist, using the same protocol and sharing data.

Open protocol, such as is found in LonTalk and BacNet, is an open-standard, serial, packet protocol specifically designed for control networks. Open protocol offers the greatest opportunity for an open system with parts from different vendors coexisting on the same media, and also offers the greatest opportunity for data sharing, interoperability, and interchangeability. Unfortunately it does not unconditionally guarantee any of these. New hardware that meets different requirements may not necessarily work to replace old hardware, but such instances are rare.  Some "open" products are not, in fact, open to other "open" products.