BACnet

Lesson 6.1

What BACnet Actually Is

BACnet stands for Building Automation and Control Networks. It is an ASHRAE standard (ASHRAE 135) developed specifically for building automation.

Before BACnet, every manufacturer used a proprietary protocol. BACnet's promise was interoperability: buy controllers from different manufacturers and have them communicate on a common network.

The honest caveat: BACnet interoperability in the real world is not seamless. Each manufacturer implements the standard differently.

Objects and Properties

BACnet organizes data into objects. Every object has a type and a set of properties. The most important property is always Present_Value.

Analog Input Object: Represents a physical or virtual analog measurement. Key properties: Present_Value, Units, Out_Of_Service flag, COV_Increment.

Analog Output Object: Represents an analog command to a field device. Has Priority_Array for multiple command sources.

Binary Input Object: Represents an on/off input. Polarity property lets you invert the meaning.

Binary Output Object: Represents an on/off command. Same priority array concept.

Analog Value Object: A software point. Used for setpoints, calculated values.

Binary Value Object: A software binary point. Used for mode flags, occupancy state.

MS/TP vs. BACnet/IP

BACnet MS/TP: Master-Slave/Token-Passing. Runs over RS-485 twisted pair. Standard for field-level controllers. Max master must be set correctly or devices will not be discovered.

BACnet/IP: BACnet wrapped in UDP packets over standard Ethernet. Standard at the supervisor level. A JACE or network controller acts as a router between BACnet/IP and BACnet MS/TP trunks.

Device discovery: BACnet devices announce themselves with Who-Is and I-Am messages. Duplicate Device Instance numbers cause conflicts.

COV subscriptions: Change of Value. The supervisor subscribes and the controller sends updates only when the value changes by the COV_Increment threshold. Dramatically reduces network traffic.

What Goes Wrong

Duplicate Device Instance numbers: Two controllers with the same Instance interfere with each other. Fix: change one to unique.

Token timeout on MS/TP: Max Master set lower than the highest MAC address. Fix: set Max Master to the highest MAC address.

No BBMD on multi-subnet network: Discovery broadcasts blocked between subnets. Fix: configure a BBMD on each subnet.

Mismatched baud rates: Every device on a trunk must match. Fix: hard-set the baud rate on every device.

Missing termination resistors: RS-485 requires 120-ohm termination at each physical end.

Reversed polarity (A/B swapped): MS/TP is polarity-sensitive. Kills communication downstream.

Cut or damaged cable: A physical break severs all devices downstream.

Diagnosing with a Multimeter

A to B (differential bias): Normal: ~200 mV. Below 200 mV = shorted. Above 500 mV = no termination. Above 1.5-2.0 V = cut cable. Negative = reversed polarity.

A to Shield and B to Shield: Normal: 2.0-2.5 VDC each. A below 2.0 V with B above 2.5 V = reversed. Below 700 mV = shorted to shield.

A to B (resistance, power off): OL = no termination. ~120 ohm = one terminator. ~60 ohm = two terminators (normal). Near 0 ohm = shorted.

Shield to Earth Ground: 0 ohm = properly grounded. Greater than 3 ohm = not grounded.