Native BACnet/IP in an HVAC equipment controller, across a standard product line

The challenge

Commercial and industrial buildings run on a building automation system, and in most of them that system speaks BACnet. An HVAC unit that cannot answer it is a unit the controls contractor has to wrap in a third-party gateway, map point by point, and support for the life of the building.

For an equipment manufacturer, that is a cost on every job and a question in every bid. The controller already knew the unit’s heating mode, fan speed and setpoints. It needed to publish them as BACnet objects that any building automation system can discover, read and write, on every unit in the product line.

Why the CAS BACnet Stack

• The CAS BACnet Stack is a royalty-free C/C++ library with the source code included: one licence, no per-device fees, however many units ship.
• The controller’s own firmware stays in charge of its data. The stack calls back into the product for each value a client reads or writes, and handles the protocol: discovery, ReadProperty and ReadPropertyMultiple, writes with priorities, and change-of-value subscriptions.
• The engineers who maintain the stack answer integration questions directly, and the free CAS BACnet Explorer lets the manufacturer discover and test each unit on the bench.

The solution

The stack runs inside the controller as a BACnet/IP server. The controller registers its callbacks, adds one BACnet device, and adds the objects the building system needs: values for the heating mode and fan speed, and writable setpoints. A regular call to the stack’s tick does the rest.

Because the stack is part of the standard product line, every unit leaves the factory as a native BACnet device. BBMD and foreign device registration are built in, so a unit can be reached across subnets on a larger site.

Results

• Building automation systems monitor and control heating modes, fan speeds and setpoints over BACnet/IP.
• BACnet ships as standard across the product line for commercial and industrial buildings.
• No gateway is needed between the unit and the building system.