Last month, I presented a DDC controller categorization scheme to show how their capabilities can vary greatly between manufacturers. The unfortunate reality revealed is that no reasonable categorization approach can ever accurately reflect all variations between manufacturers’ controller capabilities. The conclusion was that a BAS spec’s controller requirements will always contain some (or a lot of) inaccuracies for the chosen manufacturer. This also means that enforcement of the spec during construction may be a bit challenging. Why do these variations exist, and what does this say about the goal of BACnet to provide “open systems” and “interoperability?”