What the Group Object Is
The Group object, defined in Clause 12.14 of ANSI/ASHRAE 135-2024, is a read shortcut: it names a fixed set of properties across several objects on the same BACnet device, and reading its Present_Value returns all of their current values in one exchange. Instead of issuing a separate request per point, a client reads the Group once and gets the whole collection back.
List_Of_Group_Members defines the membership as a list of read specifications, and Present_Value returns the matching results in the same order. The Group holds no data of its own; every value it returns is fetched live from its member objects at read time. The Group object supports no intrinsic reporting and applies no event algorithm.
Where You’ll See It
- Batched reads. A device gathers a fan coil’s temperature, setpoint, and mode into one Group so a workstation reads the whole unit in a single request.
- Reduced traffic. On slower links such as MS/TP (Master-Slave/Token-Passing, BACnet’s RS-485 serial transport), collapsing many reads into one Group read cuts round trips.
- Logical bundling. Related points that operators always view together, grouped so they travel as a unit.
Required Properties
Clause 12.14 requires every Group object to provide these properties. Datatypes are the spec’s names: CharacterString is text, and the BACnet-prefixed types are structured values defined by the standard. BACnetLIST of ReadAccessResult is the collected set of member values returned on a read.
| Property | Datatype | What it holds |
|---|---|---|
Object_Identifier | BACnetObjectIdentifier | The object’s type and instance number, unique within the device |
Object_Name | CharacterString | The human-readable point name, unique within the device |
Object_Type | BACnetObjectType | Always group for this object |
List_Of_Group_Members | BACnetLIST of ReadAccessSpecification | The member objects and the properties of each that the group returns |
Present_Value | BACnetLIST of ReadAccessResult | The live values of all member properties, fetched at read time |
Property_List | BACnetARRAY[N] of BACnetPropertyIdentifier | The list of properties this instance actually implements |
Optional Properties
The optional set is small. Description is the property integrators meet most.
| Property | Datatype | Group |
|---|---|---|
Description | CharacterString | General |
Audit_Level | BACnetAuditLevel | Auditing |
Auditable_Operations | BACnetAuditOperationFlags | Auditing |
Tags | BACnetARRAY[N] of BACnetNameValue | Tagging and profile |
Profile_Location | CharacterString | Tagging and profile |
Profile_Name | CharacterString | Tagging and profile |
Related Objects
- Global Group: the same read-shortcut idea, but its members can live in other devices across the network.
- Structured View: organizes objects into a hierarchy for navigation, where the Group bundles their values for reading.
- Trend Log Multiple: logs several properties together over time, where the Group returns them once on demand.
What Chipkin Provides
The Group object is a first-class object type in the CAS BACnet Stack, modeled with its member list and combined read. Talk to us about modeling your device’s points.