The Group object is described by the BACnet standard as a collection of other objects, and one or more of their properties.

The Group object is essentially used to simplify the information exchange between BACnet devices through a shorthand method to specify all group members at once.

The developer should be aware that a group can be formed through any combination of object types.

The following table will present the actual properties and associated datatypes of the Group object from the BACnet standard:

The developer should pay close attention to two properties in the preceding table. The properties include:

• List_of_Group_Members property
• Present_Value property

The List_of_Group_Members represents a list of read access specification(s) that defines the group members that will be referenced when the object is specified in a standard transaction.

The Present_Value property represents the result from the read access specification(s). This property is a list that contains the values of all the properties specified in the List_Of_Group_Members. This is a “read only” property; and it cannot be used to write a set of values to the members of the group. The Present_Value list is reconstructed each time the property is read by retrieving the member properties. This requirement is required to reduce problems that could result if the Present_Value were stored.

Each of these read access specification(s) contains two essential parts:

• Object_Identifier
• Property Reference List

In addition, all group members should be objects that are in the same device that maintains the Group object.

The following example from the BACnet standard will present the Group object structure from an actual automation application. This example will specifically reference the air temperatures in a particular area or “zone” of a building:

Property: Object_Identifier = (Group, Instance 1)
Property: Object_Name = “ZONE1_TEMPS”
Property: Object_Type = GROUP
Property: Description = “Zone 1 Temperature Group”
Property: List_Of_Group_Members = (((Analog Input, Instance 8),(Present_Value, Reliability, Description)), ((Analog Input, Instance 9),(Present_Value, Reliability, Description)), ((Analog Input, Instance 10), (Present_Value, Reliability, Description)),
((Analog Input, Instance 11),(Present_Value, Reliability, Description)),((Analog Input, Instance 12),(Present_Value, Reliability, Description)))
Property: Present_Value = (((Analog Input, Instance 8), Present_Value, 69.7, Reliability, NO_FAULT_DETECTED, Description, “Room 1″), ((Analog Input, Instance 9), Present_Value, 71.2, Reliability, NO_FAULT_DETECTED, Description, “Room 2″),
((Analog Input, Instance 10), Present_Value, -50.0, Reliability, UNRELIABLE_OTHER, Description, “Room 3″), ((Analog Input, Instance 11), Present_Value, 69.7, Reliability,
NO_FAULT_DETECTED, Description, “Room 4″), ((Analog Input, Instance 12), Present_Value, 73.3, Reliability, NO_FAULT_DETECTED, Description, “Room 5″))

Written by: Scott Cosby

© Chipkin Automation Systems 2007

The Event Enrollment object is defined by the standard as information related to the management of events for BACnet systems. Events are essentially changes in the value of any property concerning any object that meets specific criteria.

The central purpose for the Event Enrollment object is to define an event, and offer the developer a connection between the event occurrence and the transmission of notification messages.

The Event Enrollment object contains the following information for effective operation:

• Event Type Description
• Event Occurrence Notification
• Device Notification

The following table will present the actual properties and associated datatypes of the Event Enrollment object from the BACnet standard:

The BACnet standard states that there’s a specific relationship among three properties:

• Event_Type
• Event_State
• Event_Parameters

The Event_Type represents the algorithm used to determine the Event_State. The Event_Parameters property provides certain parameters required by the algorithm. The developer should pay special attention to the relationship of these three properties.

The following table from the standard will examine the three properties and their associated values:

The next two examples from the standard will reveal the Event Enrollment object structure from an actual automation application. Specifically, the two examples will involve temperature control in a certain room of a building:

Property: Object_Identifier = (Analog Input, Instance 2)
Property: Object_Name = "Zone1_Temp"
Property: Object_Type = ANALOG_INPUT
Property: Present_Value = 86.0
Property: Description = "Receptionist Lobby Temp"
Property: Device_Type = "PT 3K RTD"
Property: Status_Flags = {FALSE, FALSE, FALSE, FALSE}
Property: Event_State = NORMAL
Property: Reliability = NO_FAULT_DETECTED
Property: Out_Of_Service = FALSE
Property: Update_Interval = 5
Property: Units = DEGREES_FAHRENHEIT
Property: Min_Pres_Value = 55.0
Property: Max_Pres_Value = 95.0
Property: Resolution = 0.1

The Analog_Input object above is assumed for the following Change_of_Value Event example, and the object is located in Device 12.

Property: Object_Identifier = (Event Enrollment, Instance 2)
Property: Object_Name = "Zone1TempCOV"
Property: Object_Type = EVENT_ENROLLMENT
Property: Description = "Zone 1 Temperature COV"
Property: Event_Type = CHANGE_OF_VALUE
Property: Notify_Type = EVENT
Property: Event_Parameters = (5, 0.25)
Property: Object_Property_Reference = ((Device, Instance 12),(Analog Input, Instance 2), Present_Value)
Property: Event_State = NORMAL
Property: Event_Enable = (TRUE, FALSE, FALSE)
Property: Acked_Transitions = (TRUE, TRUE, TRUE)
Property: Event_Time_Stamps = ((23-MAR-95,18:50:21.2),(*-*-*,*:*:*.*),(23-MAR-95,19:01:34.0))

Written by: Scott Cosby

© Chipkin Automation Systems 2007