BACnet, published as ASHRAE Standard 135 and ISO 16484-5, models building equipment as objects with properties: an analog input with a present value, a schedule, an alarm. Services read, write and subscribe to those objects over several transports, of which BACnet/IP on Ethernet and BACnet MS/TP on RS-485 are the most common. BACnet/SC adds TLS-secured connections, addressing the lack of security in the original design.
It is the backbone of most commercial building automation systems, tying HVAC controllers, chillers, air handlers and meters to a supervisory system. For smart building IoT projects the usual move is a gateway that reads BACnet points and forwards them to a cloud or analytics layer. The trade-off is that BACnet networks were built for closed facilities, so exposing them beyond the building requires care, segmentation and the newer secure profile.
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