BACnet Basics - BACnet Objects

Overview

BACnet departs from traditional industry conventions with its object-oriented nomenclature. The industry has long used the general-purpose term "points", which could refer to sensor inputs, control outputs or control values, with different characteristics according to manufacturer. BACnet instead defines a standard set of "Objects", each of which has a standard set of "Properties"; that describe the Object and its current status to other devices on the BACnet internetwork. It is through these properties that the Object may be controlled by other BACnet devices.

One of the standard BACnet objects is the Analog Input Object, which represents an analog sensor input such as a thermistor. Figure 1 shows a diagram of just such an Analog Input Object as it might be seen over the network through five of its properties. Some of the properties-such as Description, Device_Type and Units-are set during installation. Others, including Present_Value and Out_Of_Service, provide status about the sensor input represented by the Analog Input Object. Yet others (an Analog Input Object can have up to 25 Properties) may be set by the equipment manufacturer. All may be read; in this example, a query about the Present_Value Property of this Analog Input Object would get the reply "68.0".

Example of Analog Input Object

 

Point-related BACnet Objects