What the BitString Value Object Is
The BitString Value object, defined in Clause 12.40 of ANSI/ASHRAE 135-2024, exposes a named bit-string value inside a BACnet device to other devices on the network. It makes any kind of bit-string data readable and, optionally, writable: a packed set of status flags, a group of enable bits, or any collection of individually addressable boolean fields the control logic holds. The optional Bit_Text array names each bit so a reader can map a bit position to its meaning.
A BitString Value can be read-only or commandable. When it is commandable, it carries the same 16-slot priority array as an Analog Output; when it is a simple stored value, it does not. BitString Value objects that support intrinsic reporting apply the CHANGE_OF_BITSTRING event algorithm and deliver the alarm through the event notification services.
Where You’ll See It
- Packed status flags. A word of alarm or status bits exposed as one value other devices read with ReadProperty.
- Enable masks. A set of feature or stage enable bits a technician toggles.
- Grouped booleans. A cluster of related on/off conditions carried together rather than as separate Binary Value objects.
Required Properties
Clause 12.40 requires every BitString Value object to provide these properties. Datatypes are the spec’s names: BitString is a sequence of individually addressable bits, CharacterString a text string, and the BACnet-prefixed types are structured values defined by the standard.
| 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 bitstring-value for this object |
Present_Value | BitString | The bit-string value. Read-only or commandable depending on the instance |
Status_Flags | BACnetStatusFlags | Four health flags: in-alarm, fault, overridden, out-of-service |
Property_List | BACnetARRAY[N] of BACnetPropertyIdentifier | The list of properties this instance actually implements |
When the object is commandable, it additionally implements Priority_Array (the 16 command slots), Relinquish_Default (the fallback when all slots are Null), and Current_Command_Priority. On a simple stored BitString Value these are absent. See How Commanding Works on the Analog Output page for the priority-array model.
Optional Properties
The properties integrators meet most are Bit_Text (the label for each bit position), the alarm group (Alarm_Values, Bit_Mask) that drives the CHANGE_OF_BITSTRING algorithm, and, on commandable instances, the command-tracking group. The intrinsic-reporting group generally appears or is absent as a set.
| Property | Datatype | Group |
|---|---|---|
Description | CharacterString | General |
Out_Of_Service | BOOLEAN | General |
Event_State | BACnetEventState | General |
Bit_Text | BACnetARRAY[N] of CharacterString | General |
Priority_Array | BACnetPriorityArray | Commanding |
Relinquish_Default | BitString | Commanding |
Current_Command_Priority | BACnetOptionalUnsigned | Commanding |
Value_Source | BACnetValueSource | Commanding |
Value_Source_Array | BACnetARRAY[16] of BACnetValueSource | Commanding |
Last_Command_Time | BACnetTimeStamp | Commanding |
Command_Time_Array | BACnetARRAY[16] of BACnetTimeStamp | Commanding |
Alarm_Values | BACnetARRAY[N] of BIT STRING | Intrinsic reporting |
Bit_Mask | BitString | Intrinsic reporting |
Time_Delay | Unsigned | Intrinsic reporting |
Notification_Class | Unsigned | Intrinsic reporting |
Event_Enable | BACnetEventTransitionBits | Intrinsic reporting |
Acked_Transitions | BACnetEventTransitionBits | Intrinsic reporting |
Notify_Type | BACnetNotifyType | Intrinsic reporting |
Event_Time_Stamps | BACnetARRAY[3] of BACnetTimeStamp | Intrinsic reporting |
Event_Message_Texts | BACnetARRAY[3] of CharacterString | Intrinsic reporting |
Event_Message_Texts_Config | BACnetARRAY[3] of CharacterString | Intrinsic reporting |
Event_Detection_Enable | BOOLEAN | Intrinsic reporting |
Event_Algorithm_Inhibit_Ref | BACnetObjectPropertyReference | Intrinsic reporting |
Event_Algorithm_Inhibit | BOOLEAN | Intrinsic reporting |
Time_Delay_Normal | Unsigned | Intrinsic reporting |
Reliability | BACnetReliability | Fault detection |
Reliability_Evaluation_Inhibit | BOOLEAN | Fault detection |
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
- OctetString Value: the equivalent for raw bytes with no per-bit labeling.
- CharacterString Value: the equivalent for readable text.
- Binary Value: a single two-state software point rather than a group of bits.
- Multi-state Value: a single selection from a set of named states.
What Chipkin Provides
The BitString Value object, in both read-only and commandable forms, is a first-class object type in the CAS BACnet Stack. Talk to us about modeling your device’s points.