Objects Have Properties

Although objects can represent physical points, they can also describe processes or internal operations. Each object has a listing of properties that tell us about the nature of the object. This object listing varies with the object type. What the BACnet standard does is define the object types and the property fields for each object type. In this way, uniformity is ensured among vendors that will lead to interoperability between different vendor systems. The intent is to make the objects “network visible” — any object can be queried from any point along the network. As the standard has evolved, object types have been added — bringing the total from the initial 18 to the current 25. These 25 object types are referred to as standard objects. A BACnet device need not support all object types, but if an object type is supported, it must comply with the standard object model for that object type. Each object type has a list of required properties and optional properties. Optional properties can be included at the manufacturer’s discretion.

Table 1 — There are 25 BACnet object types. The BAS Remote uses five which are highlighted.

There is one object type that must be included in any BACnet-compatible device and that is the Device object. There must be one, and only one, instance of the Device object whose structure is shown in Table 2. An instance number is the way of identifying items using object modeling. The Object Identifier must be unique within the complete BACnet network. It is comprised of the object type DEVICE and an instance number. The default instance is 2749, but there is no significance to this value since it is changed by the installer through the web page during commissioning. The Object Name can be set as well to some meaningful description such as the location of the device. The remaining fields are set by the manufacturer.

Table 2 — The required properties of the Device object are listed. From this object, much can be learned about the device.

From the other properties you can learn much about the device. All the object types present in the device are listed under Protocol_Object_Types_Supported. All the objects within the device along with their instance number are included under Object_List. In our example of the BAS Remote, there would be five object types and nine objects listed. We will study one of those objects of the type Analog Input.

In Table 3 you will see all the required properties of one Analog Input instance which the manufacturer identifies as AI, Instance 2, meaning that the second physical point on the BAS Remote happens to be configured as an analog input. All properties in any object must be able to be read. Point 2 on the BAS Remote is connected to a thermistor located on the roof of the building in order to measure outside temperature. The actual temperature appears in the Present_Value property as 69.3. The unit of measure is degrees Fahrenheit as indicated in the Units property. The installer has identified the point as “Outside temperature” in the Object_Name property meaning that another BACnet device on the network can search for “Outside temperature” and obtain the actual outside temperature without caring which device or which point had the result. This demonstrates the power of object modeling.

Table 3 — The required properties of the Analog Input object for Instance 2. Notice that the value of the point is maintained in engineering units.