What the CharacterString Value Object Is
The CharacterString Value object, defined in Clause 12.37 of ANSI/ASHRAE 135-2024, exposes a named text-string value inside a BACnet device to other devices on the network. It makes any kind of character-string data readable and, optionally, writable: a status message, a label, a serial number, or a free-form text field the control logic holds. The mechanisms behind the value are not visible to the client, which sees only the string.
A CharacterString 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. CharacterString Value objects that support intrinsic reporting apply the CHANGE_OF_CHARACTERSTRING event algorithm, and for reliability-evaluation the FAULT_CHARACTERSTRING fault algorithm may be applied; alarms deliver through the event notification services. When the set of strings is fixed and known, a Multi-state Value is often the better fit because its numeric Present_Value is easier for automated consumers to act on.
Where You’ll See It
- Status and label text. A human-readable state string, an equipment label, or a message the logic exposes for a head end (the central BMS server) to display.
- Identifiers. A serial number, firmware string, or asset tag other devices read with ReadProperty.
- Free-form fields. An operator note or configurable caption written to the device.
Required Properties
Clause 12.37 requires every CharacterString Value object to provide these properties. Datatypes are the spec’s names: CharacterString is a text string, BOOLEAN a true/false flag, 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 character-string-value for this object |
Present_Value | CharacterString | The 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 CharacterString 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 Description, the alarm/fault string sets (Alarm_Values, Fault_Values) that drive the CHANGE_OF_CHARACTERSTRING and FAULT_CHARACTERSTRING algorithms, 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 |
Priority_Array | BACnetPriorityArray | Commanding |
Relinquish_Default | CharacterString | 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 BACnetOptionalCharacterString | 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 |
Fault_Values | BACnetARRAY[N] of BACnetOptionalCharacterString | Fault detection |
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 byte-string data rather than readable text.
- BitString Value: the equivalent for a set of individually named bits.
- Multi-state Value: the better fit when the set of strings is fixed and known.
- Analog Value: the numeric software-point equivalent.
What Chipkin Provides
The CharacterString 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.