What the Multi-state Output Object Is
The Multi-state Output object, defined in Clause 12.19 of ANSI/ASHRAE 135-2024, is how a BACnet device represents a commandable output that has more than two states. Where a Binary Output switches between two states, a Multi-state Output commands one of several: a fan to off, low, medium, or high; a valve to one of a set of named positions.
Present_Value is an unsigned integer from 1 to Number_Of_States, each mapped to a label through the optional State_Text array. Like every commandable BACnet object, it is written through a 16-slot priority array rather than a direct store, so multiple controllers can command the same output without conflict. Multi-state Output objects that support intrinsic reporting apply the CHANGE_OF_STATE event algorithm.
Where You’ll See It
- Multi-speed and multi-position control. Commanding a fan speed, a multi-stage heater, or a positioned damper to a named state.
- Mode commands. Driving a piece of equipment to a selected operating mode over BACnet.
- Sequenced outputs. A single object commanding one of several states in a defined order.
How Commanding Works
A WriteProperty to Present_Value places the requested state number in one of 16 slots of the Priority_Array, chosen by the write’s priority (1 = highest, 16 = lowest). The object drives the highest-priority non-Null slot; writing Null at a priority relinquishes that command, and when every slot is Null the object falls back to Relinquish_Default. Current_Command_Priority reports the active slot. This is the same model the Analog Output uses, applied to a multi-state value.
Required Properties
Clause 12.19 requires every Multi-state Output object to provide these properties. Datatypes are the spec’s names: Unsigned is an integer, BACnetPriorityArray is the 16-slot command array, and the other 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 multi-state-output for this object |
Present_Value | Unsigned | The commanded state, 1 to Number_Of_States. Writable, but through the priority array |
Status_Flags | BACnetStatusFlags | Four health flags: in-alarm, fault, overridden, out-of-service |
Event_State | BACnetEventState | The object’s current event state (normal, off-normal, or fault) |
Out_Of_Service | BOOLEAN | When true, Present_Value is decoupled from the physical output for testing |
Number_Of_States | Unsigned | How many discrete states this object has |
Priority_Array | BACnetPriorityArray | The 16 command slots; the highest-priority non-Null slot wins |
Relinquish_Default | Unsigned | The state the output takes when every priority slot is Null |
Current_Command_Priority | BACnetOptionalUnsigned | Which priority slot (1-16) is currently in control, or none |
Property_List | BACnetARRAY[N] of BACnetPropertyIdentifier | The list of properties this instance actually implements |
Optional Properties
The properties integrators meet most are State_Text (the label for each state number) and Feedback_Value (the sensed actual state, for proving the command took effect). The intrinsic-reporting group generally appears or is absent as a set.
| Property | Datatype | Group |
|---|---|---|
Description | CharacterString | General |
Device_Type | CharacterString | General |
State_Text | BACnetARRAY[N] of CharacterString | State labels |
Feedback_Value | Unsigned | 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 |
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 |
Interface_Value | BACnetOptionalUnsigned | Integration |
Tags | BACnetARRAY[N] of BACnetNameValue | Tagging and profile |
Profile_Location | CharacterString | Tagging and profile |
Profile_Name | CharacterString | Tagging and profile |
Related Objects
- Multi-state Input: the read-only multi-state counterpart.
- Multi-state Value: a multi-state software point.
- Binary Output: the two-state commandable equivalent, with the same priority-array model.
What Chipkin Provides
The Multi-state Output object, including the priority array and the State_Text label model, is a first-class object type in the CAS BACnet Stack. Talk to us about modeling your device’s points.